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A powerful new book shows why it's so important to understand war

In his latest book, Why War?, historian Richard Overy grapples with a question that stumped Albert Einstein and Sigmund Freud – why do humans persist in waging war?

By Jeremy Hsu

26 June 2024

The devastated area in Gaza around Al-Shifa hospital in April

AFP via Getty Images

Why War? Richard Overy (Pelican (UK), W. W. Norton (US))

Russia’s invasion of Ukraine , Israel’s military incursions in Gaza, civil wars in Sudan and Myanmar, tens of thousands killed, millions displaced and global military spending standing at $2.4 trillion in 2023 alone.

This persistence of war both defies and demands explanation. Many have tried. Albert Einstein felt driven to seek an (ultimately inconclusive) written exchange with Sigmund Freud on the subject, published as Why War? A number of books have since shared this title, the…

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Why Should We Study War?

In the latter years of World War I, Winston Churchill met with the novelist and poet Siegfried Sassoon. Sassoon was a winner of the Military Cross––he single-handedly routed 60 Germans and captured a trench on the Hindenburg Line––and a fierce pacifist. Sassoon’s reminiscences of that meeting reveal how odd my title question would have struck most people before our time. He recalled that during their conversation, Churchill “gave me an emphatic vindication of militarism as an instrument of policy and stimulator of glorious individual achievements.”

After Sassoon left, he wondered, “Had he been entirely serious . . . when he said that ‘war is the normal occupation of man’? [I]t had been unmistakable that for him war was the finest activity on earth.” Churchill, remember, had served under fire in India, Sudan, Cuba, and South Africa even before his service in the trenches, so his comments were not the braggadocio of the armchair militarist unfamiliar with the horrors of war.

Many of us moderns, of course, find Sassoon’s beliefs, expressed in his poems and novels, about the futility and misery of war more attractive than Churchill’s idealization of it, and consider such enthusiasm untoward, if not sinister. Such attitudes have made war a disreputable topic of study. Once vigorous in the academy, military history programs are rarely found at universities and colleges today, even as  “peace studies” programs have proliferated. Reasons for this change are not hard to find. America’s historically unprecedented military power, its enormous wealth, and since 1865 its freedom from battle on its own soil and from foreign invasion have all insulated Americans from war, and enabled the perception that rather than a foundational and ennobling experience of humanity, war is an unnatural anomaly, a species of barbarism from our benighted past, and hence an unsavory topic of formal study, even as it remains a lucrative (and, to many people, low-brow) subject for books, movies, cable television channels, and video games.

In contrast to the modern disdain for studying war, most people before the twentieth century would have found Churchill’s comments unexceptional, indeed banal, and they would have considered self-evident the answer to the question raised in this essay’s title. The ancient Greeks were one of the most civilized, artistic, and cultured peoples in history. But they never questioned the eternal necessity of war. “War is the father of all,” Heraclitus said of the original “creative destruction.”  Plato in the Laws has Cleinias say, “Peace is only a name; in reality every city is in a natural state of war with every other.” The arch-realist Thucydides in his History of the Peloponnesian War has an Athenian ambassador tell the Spartans that states fight one another because of the constants of human nature such as fear, honor, and self-interest, and invoke higher ideals such as “justice” only when they cannot achieve their aims by force.

All these Greeks agree with Churchill that war is a non-negotiable necessity and a legitimate “instrument of policy,” given the realities of human nature and its perennial passions and interests. In a harsh world of limited resources and violent men, war is as critical for the survival of civilization as agriculture, and as such, it would be as great a folly not to study war, as it would be to ignore the craft and skills of farming.

So too with Churchill’s praise of war as the “stimulator of glorious individual achievements.” From the beginnings of Western literature in Homer’s Iliad , and of history in Herodotus’ Histories , the glorious deeds of warriors, their bravery and self-sacrifice for honor and community, have been celebrated and admired. Who can forget the doomed valor of Hector, when despite knowing he is fated to die at the hands of Achilles, says before his last charge, “But now my death is upon me. Let me at least not die without a struggle, inglorious, but do some big thing first, that men to come shall know of it”?

And even today, in an age of historical amnesia, the last stand of the vastly outnumbered Thespians and Spartans at Thermopylae is still remembered, when, as Herodotus writes, the Greeks, their spears and swords shattered, “defended themselves with knives, if they still had them, and otherwise with their hands and teeth, while the Persians buried them in a hail of missiles.”

Those before us knew that for all its horrors and misery––which our ancestors acknowledged as much as its glories––war is when the best that men are capable of is manifested, and great deeds worthy of memory are achieved. And they understood as well that the commemoration of these deeds by men “who knew their duty and had the courage to do it,” as Pericles said of his fellow Athenians, creates models of virtue and honor for subsequent generations to study and emulate. Only in that way can a civilization survive in a world of limited resources and ruthless aggressors.

Churchill’s comments, then, suggest two reasons for the study of war, one practical, and the other philosophical. If war is an unavoidable and necessary instrument of statecraft, then we should study the origins, conduct, successes, and failures of wars in order to find, as the Roman historian Livy describes the purpose of history, “what to imitate,” and to “mark for avoidance what is shameful in the conception and shameful in the result.” This need is particularly pressing in a democracy, where the military is subordinated to the civilian government, and the voters have the responsibility to debate and deliberate policies, and to choose leaders whose charge is to serve the security and interests of the citizens both in the short and in the long term.

Two historical examples, one ancient, one modern, illustrate the importance of military history for teaching the lessons of the past. In 415 B.C., over ten years into the war against Sparta, the democratic Assembly of Athens voted to send an expeditionary force 800 miles to attack the rich and powerful city of Syracuse. In Thucydides’ telling, this decision was based neither on short-term nor on long-term strategic national interests and security, but on the promise of an expanded empire and the greater revenues that would be available to the citizens through the tribute of subject states.

The charismatic and ambitious Alcibiades was a prime mover of the expedition. He dangled the lure of greater empire, telling the Assembly, “We shall either become masters, as we very easily may, of the whole of Hellas [Greece], or in any case ruin the Syracusans, to the no small advantage of ourselves and our allies.” As for the Assemblymen, Thucydides writes, “The idea of the common people and the soldiery was to earn wages at the moment [the treasury increased the pay for rowers, and the commanders of the ships promised bonuses as well], and make conquests that would supply a never-ending fund of pay for the future.” The expedition sailed, and became one of the most famous military disasters in history. The Athenians lost 6000 men and 200 ships, the whole expeditionary force and a relief fleet as well.

This disaster offers many lessons. First, dispassionate knowledge of the enemy and the logistics of war are critical for success. According to Thucydides, the Athenians were “ignorant of [Sicily’s] size and of the number of its inhabitants, Hellenic and barbarian, and of the fact that they were undertaking a war not much inferior to that against the Peloponnesians.” Thus the Athenians woefully underestimated the power and resources of the Syracusans and the dangers of resupply and relief when 800 miles from home, both factors in the ultimate debacle. Next, parochial self-interest, the selfish desire for personal wealth and glory rather than the safety and well being of the state as a whole, are dangerous motives for undertaking a war, as they obscure the limits and obstacles a more sober consideration might reveal.

Finally, politicians like an Alcibiades––who according to Thucydides was “exceedingly ambitious of a command by which he hoped to reduce Sicily and Carthage, and personally to gain in wealth and reputation by means of his success”­­––will end up sacrificing the state as a whole in order to further their own ambitions. These are all dangers that the citizens should beware when contemplating the use of force to pursue policy, and when deliberating and evaluating the aims which war will achieve.

The modern lesson comes from the origins of World War II. As Winston Churchill said in his famous “Sinews of Peace” speech in Fulton, Missouri in 1946, “There never was a war in all history easier to prevent by timely action than the one which has just desolated such great areas of the globe. It could have been prevented in my belief without the firing of a single shot.” Churchill was referring to the period before 1935, when Germany’s serial violations of the Versailles treaty, particularly its clandestine programs for rebuilding its army and armaments industry, were met with indifference or appeasement. But even later, timely military action could have stopped Nazi Germany at a fraction of the 50 million dead World War II cost.

In 1936, Hitler remilitarized the Rhineland, the territory between the French border and the Rhine River, in violation of the Versailles treaty. His 36,000 policemen and green army recruits faced nearly 100 French and Belgian divisions, who did not fire a shot. Later Hitler would admit that the Germans would have had to “withdraw with our tails between our legs” had the French resisted. Two years later, England and France abandoned their ally Czechoslovakia, and Germany absorbed this strategically critical country. Yet if England and France had fought back with force, an outnumbered Germany would have been defeated, as Poland and the Soviet Union would likely have followed their ally France’s lead. A French advance east from the Maginot Line would have opened a second front and overwhelmed Germany’s manpower and materiel. As historian Williamson Murray writes, “Germany would have faced overwhelming Allied superiority . . . The results would have been inevitable and would have led to the eventual collapse of the Nazi regime at considerably less cost” than the butcher’s bill of World War II.

Once Hitler’s ambitions became obvious even to the appeasers after the debacle of Munich, the French and British announced that they would protect Poland’s territorial integrity should Germany invade. But this was the wrong place and time to draw that particular red line. The occupation of Czechoslovakia had strengthened Germany and put the Wehrmacht on the southern border of Poland, beyond the state-of-the-art fortifications the Czechs had built in their mountainous western region. And Germany now possessed the military hardware of the Czechs and the Skoda works, one of the largest arms manufacturers in Europe. In fact, the Panzer 35(t) and 38(t) tanks used in the invasion of Poland were actually Czech tanks produced by Skoda. Given Germany’s advantages, there simply was not much England and France could do militarily to help the Poles, which explains the 8 months of “phony war” marked by the Allies’ inaction after Hitler invaded Poland.

The lesson we should learn from this sorry history is that preemptive war is a necessity when facing a determined aggressor, and that the time and place of a potential conflict, and the capacity to wage war until its successful conclusion, must be carefully considered and prepared for when making treaty commitments and pledging the nation’s blood and treasure. This means that often a nation cannot merely wait to react to aggression, but must anticipate where the blow will fall.

To use the simile of the great fourth-century Greek orator Demosthenes, when he chastised the Athenians for serially failing to react to Philip of Macedon’s aggression, a nation must not deal with an aggressor the way a barbarian boxes: “The barbarian,” Demosthenes said, “when struck, always clutches the place; hit him on the other side and there go his hands. He neither knows nor cares how to parry a blow or how to watch his adversary.” Given that Hitler had 13 years earlier laid out his plan of conquest in Mein Kampf , the Allies should have anticipated the sequence of aggression that would culminate in the attack on Poland, and resisted the Germans in 1936 in the Rhineland, or in 1938 in Austria or Czechoslovakia.

The larger lesson, however, of this “low dishonest decade,” as W.H. Auden called the thirties, is that success in war depends on morale, not material superiority. Long before 1938, England and France had lost their nerve, and simply did not have the will to fight. Instead they had bought into the illusions of internationalism and collective security, pacifism and disarmament, which had merely fed the alligator of Nazism, to paraphrase Churchill, in the vain hope that they would be eaten last. And this brings us to the philosophical lessons the study of war teaches. Contrary to our modern therapeutic utopianism, the history of war shows us the unchanging, tragic reality of human nature and its irrational passions and interests that will spark state aggression and violence.

The modern world, in contrast, rejects the notion that human nature comprises destructive passions and selfish interests that will start wars only force can stop. On the contrary, to the modern optimist, humans are universally rational and peace loving, if only the external, warping constraints on these qualities––ignorance, poverty, parochial ethnic and nationalist loyalties, the oppression of priestly and aristocratic elites––can be removed. Then people will progress to the realization that their true interests like peace, freedom, and prosperity will be achieved not by force but by international trade, economic development, democracy, and non-lethal transnational institutions that can adjudicate conflict and eliminate the scourge of war.

This influential belief was famously expressed by Immanuel Kant in his 1795 essay “Perpetual Peace.” In it Kant imagined a “federation of free states” that would create a “pacific alliance . . . different from a treaty of peace . . . inasmuch as it would forever terminate all wars, whereas the latter only finishes one.” In his conclusion, Kant expressed the optimism that would become an article of faith in subsequent centuries: “If it is a duty, if the hope can even be conceived, of realizing, though by an endless progress, the reign of public right––perpetual peace, which will succeed to the suspension of hostilities, hitherto named treaties of peace, is not then a chimera, but a problem, of which time, probably abridged by the uniformity of the progress of the human mind, promises us the solution.”

Throughout the nineteenth century international institutions were created to realize this dream and lessen, if not eliminate, the savagery and suffering of war. The First Geneva Convention in 1864 and the Second in 1906 sought to establish laws for the humane treatment of the sick and wounded in war. The first Hague Convention in 1899 established an international Court of Arbitration and codified restrictions on aerial bombardment, poison gas, and exploding bullets.  The preamble to the first Hague Convention explicitly acknowledged its Kantian aims: “the maintenance of the general peace” and the “friendly settlement of international disputes” that both reflected the “solidarity which unites the members of the society of civilized nations” and their shared desire for “extending the empire of law, and of strengthening the appreciation of international justice.” One wonders how such optimism made sense of the Franco-Prussian War three decades earlier, when two of the world’s most “civilized nations” suffered nearly a million casualties, including 170,000 dead.

Even after the industrialized carnage of World War I showed international solidarity and universal progress to be a fantasy, the Versailles treaty established the League of Nations, the transnational institution intended to realize Kant’s dream of a “federation of free states” that would keep the peace and promote global progress. But within a few years the League had been exposed as ineffective, since the same sovereign nations that had fought each other so brutally in the war continued to pursue their zero-sum interests, frequently with force. No more effective has been the United Nations, a “cockpit in the Tower of Babel,” as Churchill feared it might become, that also has failed at its foundational goal of maintaining peace, becoming instead an instrument of the member-states’ nationalist interests, one that frequently supplements and abets, rather than controls or limits state violence.

Familiarity with the history of war should disabuse people of these Kantian illusions. Studying the causes and nature of armed conflict reveals that technological progress, better education and nutrition, global trade, and increased prosperity has not eliminated or reduced wars, but often made them more brutal and destructive. Military history teaches us that war is not a distortion of a peace-loving human nature that not yet has sufficiently progressed beyond such savage barbarism, but rather is a reflection of a flawed human nature, and the necessary instrument for states to protect their security and pursue their interests, whether these are rational and good, or irrational and evil. The study of war, in short, can remind us of the tragic wisdom evident on every page of history: that humans are fallen creatures prone to destructive violence that only righteous violence can check.

The lessons we can learn from studying war, of course, are more numerous than the few discussed here. Our judgment of any war, whether of its origins or its conduct, must be based on the record of history rather than the utopian fantasies of a world that will never exist. From the standard of history, in any conflict we should always expect mistakes, unforeseen consequences, civilian casualties, deaths from friendly fire, barbarism, and cruelty. All of these contingencies can be found in every war, including the so-called “good war,” World War II, from the Market Garden disaster in September 1944 that cost the Allies 16,000 casualties, to the harvesting of gold teeth from the Japanese dead in the South Pacific. These evils are the costs of using violence to defend our security and interests, and should be expected, though never condoned, the moment the decision to go to war has been made.

We also should expect­­––particularly in constitutional states where citizens are responsible for the decision to go to war––impatience, second-guessing, and frustration with these unfortunately perennial evils of armed conflict. And we should not be surprised when the citizens want to punish the politicians and leaders who started and managed the war. After news of the disaster in Sicily reached Athens, Thucydides writes, the people “were angry with the orators who had joined in promoting the expedition, just as if they had not themselves voted it.” We recently experienced the same phenomenon during the Iraq war in 2004, when many of the same Senators who had voted to invade Iraq year earlier, a decision based on the same intelligence the Bush administration had studied, responded to growing criticism of the war by turning against it and attacking the president.

Leon Trotsky allegedly said, “You may not believe in war, but war believes in you.” Though likely a mistranslation, the sentiment is still valuable. War and its horrors will always be with us, along with its unavoidable suffering and cruelty, “such as have occurred and always will occur as long as the nature of mankind remains the same,” as Thucydides writes. And as long as we cherish our way of life, with its freedom and human rights, its prosperity and its opportunity, we will at times have to make the awful decision to send our citizens to fight, kill, and die to defend those goods from those who want to destroy them. The more we know about war, the better equipped we will be to make that choice and see our efforts succeed.

This essay is based on a speech delivered at Hillsdale College. 

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On the Study of War and Warfare

Lt. Gen. HR McMaster | 02.24.17

Editor’s Note: This article, by Lt. Gen H.R. McMaster, appointed national security advisor this week, originally appeared in 2014.

Approaching the Study of War and Warfare

It is hard to improve on the approach to studying war and warfare found in historian Sir Michael Howard’s 1961 seminal essay on how military professionals should develop what Clausewitz described as their own “theory” of war. First, to study in width: to observe how warfare has developed over a long historical period. Next to study in depth: to study campaigns and explore them thoroughly, consulting original sources and applying various theories and interdisciplinary approaches. This is important, Sir Michael observed, because as the “tidy outline dissolves,” we “catch a glimpse of the confusion and horror of real experience.” And lastly to study in context. Wars and warfare must be understood in context of their social, cultural, economic, human, moral, political, and psychological dimensions because “the roots of victory and defeat often have to be sought far from the battlefield.”

To develop understanding in “width, depth, and context,” we must be active learners dedicated to self-study and self-critique. Discussion and debate with others exposes us to different perspectives and helps us consider how what we learn applies to our responsibilities. Participative intellectual activity is critical to the “Self-Development Domain” of our Army’s leader development efforts. And the self-development domain is as important as the Operational Domain (unit training and operational experience) and the Institutional Domain (official Army schools) in helping leaders prepare for the challenges of future war. This is why forums such as the WarCouncil.org are important. Discussions on this site should challenge our assumptions and refine our thinking. [Editor’s note: WarCouncil.org is the domain on which MWI articles were previously published].

Understanding the Context—and the Continuities—of War

Successful American military leaders supplemented their formal learning through active reading, study, and reflection. In 1901, the father of the Army War College, Secretary of War Elihu Root,  commented on  “the great importance of a thorough and broad education for military officers,” due to the “rapid advance of military science; changes of tactics required by the changes in weapons; our own experience in the difficulty of working out problems of transportation, supply, and hygiene; the wide range of responsibilities which we have seen devolving upon officers charged with the civil government of occupied territory; the delicate relations which constantly arise between military and civil authority.” Thus, Root wrote, there was a “manifest necessity that the soldier, above all others, should be familiar with history.”

Self-study and professional discussions help leaders understand the character of  particular conflicts, inform ideas of how armed conflict is likely to evolve, and help leaders understand the complex interactions between military, political, and social factors that influence the situation in war. Because leaders cannot turn back time once war occurs; they must develop an understanding of war and warfare before they enter the field of battle. As  nineteenth-century Prussian philosopher of war  Carl von Clausewitz observed , the study of war and warfare “is meant to educate the mind of the future commander, or, more accurately, to guide him in his self education, not to accompany him to the battlefield; just as a wise teacher guides and stimulates a young man’s intellectual development, but is careful not to lead him by the hand for the rest of his life.” Clausewitz continued, emphasizing that leaders should use their knowledge of military history “to analyze the constituent elements of war, to distinguish precisely what at first sight seems fused, to explain in full the properties of the means employed and to show their probable effects, to define clearly the nature of the ends in view, and to illuminate all phases of warfare in a thorough critical inquiry.”

Many of the recent difficulties we have encountered in strategic decision-making, operational planning, and force development have stemmed, at least in part, from the neglect of critical continuities in the nature of war. Military professionals, through their study of war and warfare in width, depth, and context as well as through discussion and debate in a variety of forums can help identify changes in the character of conflict as well as underscore important continuities in the nature of war. Consider these continuities, for example:

First, war is political.  As von Clausewitz observed, “war should never be thought of as something autonomous but always as an instrument of policy.” In the aftermath of the 1991 Gulf War, defense thinking was hijacked by a fantastical theory that considered military operations as ends in and of themselves rather than just one of several instruments of power that must be aligned to achieve sustainable strategic goals. Advocates of the “Revolution in Military Affairs” (RMA) predicted that advances in surveillance, communications, and information technologies, along with precision-strike weapons, would overwhelm any opponent. Experience in Afghanistan and Iraq revealed the flawed nature of this thinking. Professionals should be skeptical of ideas and concepts that divorce war from its political nature. And skepticism is particularly appropriate concerning theories that promise fast, cheap, and efficient victories through the application of advanced military technologies.

Second, war is human.  People fight today for the same fundamental reasons that the Greek historian Thucydides identified nearly 2,500 years ago: fear, honor, and interest. Thinking associated with the RMA dehumanized as well as depoliticized the nature of war. The cultural, social, economic, religious, and historical considerations that comprise the human dimension of war must inform wartime planning as well as our preparation for future armed conflict. In Iraq and Afghanistan, gaining an appreciation of the fears, interests, and sense of honor among their internal communities was critical to move those communities toward political accommodations.

Third, war is an uncertain contest of wills.  War’s political and human nature place armed conflict squarely in the realm of uncertainty. The dominant assumption of the RMA, however, was that that knowledge would be the key to victory in future war. Near-perfect intelligence would enable precise military operations within a realm of certainty. In Afghanistan and Iraq, planning was sometimes based on linear projections that did not account for enemy adaptations or the evolution of those conflicts in ways that were difficult to predict at the outset. War remains fundamentally uncertain due to factors that lie outside the reach of information and surveillance technologies. Moreover, war’s uncertainty and non-linearity are results of war’s political and human dimensions as well as the continuous interaction with determined, adaptive enemies. And wars are uncertain because they are contests of wills that unleash unpredictable psychological dynamics.

While a student at the German staff college between the World Wars, Gen. Albert C. Wedemeyer  noted that  “an indomitable will and broad military knowledge, combined with a strong character, are attributes of the successful leader.” Wedermeyer continued, stating that leaders “must have a clear conception of tactical principles and their application,” and “only by continual study of military history and of the conduct of war with careful attention to current developments can the officer acquire the above stated attributes of leadership.” As professional soldiers, we share a moral and professional obligation to read, think critically, and engage in professional discussions. Forums such as WarCouncil.org are critical because junior leaders must develop an appreciation for leadership at the operational, joint, and strategic levels so they can place the actions of their units in context of war aims and develop an ability to advise senior military and civilian leaders on matters of policy and strategy.

To help develop professional expertise, the Maneuver Center of Excellence has developed the Maneuver Leader Self-Study Program (MSSP). The MSSP consists of books, articles, doctrine, film, lectures, practical application exercises, and online discussion forums to educate maneuver leaders about the nature of war and the character of warfare, as well as to emphasize their responsibilities to prepare their soldiers for combat, lead them in battle, and accomplish the mission. The broader intent of the of the MSSP is to enhance understanding of the complex interaction between war and politics in “width, depth, and context.” The program is also meant to foster a commitment to lifelong learning and career-long development to ensure that our leaders are prepared for increased responsibilities. Each MSSP topic contains a brief summary of the chosen topic, its relevance to maneuver leaders, and several study questions for reflection. Topics also contain annotated bibliographies as well as a link to an online discussion forum. In addition to your participation in WarCouncil.org, I invite you to  explore the topics—and the discussions—of the MSSP .

Image credit: Sgt. Michael J. MacLeod, US Army

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Some reject the very idea of the “morality of war”. [ 1 ] Of those, some deny that morality applies at all once the guns strike up; for others, no plausible moral theory could license the exceptional horrors of war. The first group are sometimes called realists. The second group are pacifists. The task of just war theory is to seek a middle path between them: to justify at least some wars, but also to limit them (Ramsey 1961). Although realism undoubtedly has its adherents, few philosophers find it compelling. [ 2 ] The real challenge to just war theory comes from pacifism. And we should remember, from the outset, that this challenge is real. The justified war might well be a chimera.

However, this entry explores the middle path between realism and pacifism. It begins by outlining the central substantive divide in contemporary just war theory, before introducing the methodological schisms underpinning that debate. It then discusses the moral evaluation of wars as a whole, and of individual acts within war (traditionally, though somewhat misleadingly, called jus ad bellum and jus in bello respectively). jus post bellum —is reserved for a separate entry. -->

1. Traditionalists and Revisionists

2.1 historical vs contemporary just war theory, 2.2 institutions and actions, 2.3 overarching disputes in contemporary analytical just war theory, 2.4 dividing up the subject matter, 2.5 the decisive role of necessity and proportionality, 3.1 just cause, 3.2 just peace, 3.3 legitimate authority, 3.4 proportionality, 3.5 last resort (necessity), 4.1 walzer and his critics, 4.2 killing combatants, 4.3 sparing civilians, 4.4 proportionality, 4.5 necessity, 5. the future of just war theory, other internet resources, related entries.

Contemporary just war theory is dominated by two camps: traditionalist and revisionist. [ 3 ] The traditionalists might as readily be called legalists. Their views on the morality of war are substantially led by international law, especially the law of armed conflict. They aim to provide those laws with morally defensible foundations. States (and only states) are permitted to go to war only for national defence, defence of other states, or to intervene to avert “crimes that shock the moral conscience of mankind” (Walzer 2006: 107). Civilians may not be targeted in war, but all combatants, whatever they are fighting for, are morally permitted to target one another, even when doing so foreseeably harms some civilians (so long as it does not do so excessively). [ 4 ]

Revisionists question the moral standing of states and the permissibility of national defence, argue for expanded permissions for humanitarian intervention, problematise civilian immunity, and contend that combatants fighting for wrongful aims cannot do anything right, besides lay down their weapons.

Most revisionists are moral revisionists only: they deny that the contemporary law of armed conflict is intrinsically morally justified, but believe, mostly for pragmatic reasons, that it need not be substantially changed. Some, however, are both morally and legally revisionist. And even moral revisionists’ disagreement with the traditionalists is hardly ersatz: most believe that, faced with a clash between what is morally and what is legally permitted or prohibited, individuals should follow their conscience rather than the law. [ 5 ]

The traditionalist view received its most prominent exposition the same year as it was decisively codified in international law, in the first additional protocol to the Geneva Conventions. Michael Walzer’s Just and Unjust Wars , first published in 1977, has been extraordinarily influential among philosophers, political scientists, international lawyers, and military practitioners. Among its key contributions were its defence of central traditionalist positions on national defence, humanitarian intervention, discrimination, and combatant equality.

Early revisionists challenged Walzer’s views on national defence (Luban 1980a) and humanitarian intervention (Luban 1980b). Revisionist criticism of combatant equality and discrimination followed (Holmes 1989; McMahan 1994; Norman 1995). Since then there has been an explosion of revisionist rebuttals of Walzer (for example Rodin 2002; McMahan 2004b; McPherson 2004; Arneson 2006; Fabre 2009; McMahan 2009; Fabre 2012).

Concurrently, many philosophers welcomed Walzer’s conclusions, but rejected his arguments. They have accordingly sought firmer foundations for broadly traditionalist positions on national defence (Benbaji 2014; Moore 2014), humanitarian intervention (Coady 2002), discrimination (Rodin 2008b; Dill and Shue 2012; Lazar 2015c), and especially combatant equality (Zohar 1993; Kutz 2005; Benbaji 2008; Shue 2008; Steinhoff 2008; Emerton and Handfield 2009; Benbaji 2011).

We will delve deeper into these debates in what follows. First, though, some methodological groundwork. Traditionalists and revisionists alike often rely on methodological or second-order premises, to the extent that one might think that the first-order questions are really just proxy battles through which they work out their deeper disagreements (Lazar and Valentini forthcoming).

2. How Should We Think about the Morality of War?

For the sake of concision this entry discusses only contemporary analytical philosophers working on war. Readers are directed to the excellent work of philosophers and intellectual historians such as Greg Reichberg, Pablo Kalmanovitz, Daniel Schwartz, and Rory Cox to gain further insights about historical just war theory (see, in particular, Cox 2016; Kalmanovitz 2016; Reichberg 2016; Schwartz 2016).

Within contemporary analytical philosophy, there are two different ways in which moral and political philosophers think about war (Lazar and Valentini forthcoming). On the first, institutionalist , approach, philosophers’ primary goal is to establish what the institutions regulating war should be. In particular, we should prescribe morally justified laws of war. We then tell individuals and groups that they ought to follow those laws. On the second approach, we should focus first on the moral reasons that apply directly to individual and group actions, without the mediating factor of institutions. We tell individuals and groups to act as their moral reasons dictate. Since this approach focuses not on the institutions that govern our interactions, but on those interactions themselves, we will call it the “interactional” approach. [ 6 ]

In general, the institutionalist approach is favoured by indirect consequentialists and contractualists. Indirect consequentialists believe these institutions are justified just in case they will in fact have better long-run results than any feasible alternative institutions (see Mavrodes 1975; Dill and Shue 2012; Shue 2013; Waldron 2016). Contractualists believe these institutions ground or reflect either an actual or a hypothetical contract among states and/or their citizens, which specifies the terms of their interaction in war (see Benbaji 2008, 2011, 2014; Statman 2014).

Non-contractualist deontologists and direct- or act-consequentialists tend to prefer the interactional approach. Their central question is: what moral reasons bear directly on the permissibility of killing in war? This focus on killing might seem myopic—war involves much more violence and destruction than the killing alone. However, typically this is just a heuristic device; since we typically think of killing as the most presumptively wrongful kind of harm, whatever arguments one identifies that justify killing are likely also to justify lesser wrongs. And if the killing that war involves cannot be justified, then we should endorse pacifism.

Any normative theory of war should pay attention both to what the laws of war should be, and to what we morally ought to do. These are two distinct but equally important questions. And they entail the importance of a third: what ought we to do all things considered, for example when law and morality conflict? Too much recent just war theory has focused on arguing that philosophical attention should be reserved to one of the first two of these questions (Buchanan 2006; Shue 2008, 2010; Rodin 2011b). Not enough has concentrated on the third (though see McMahan 2008; Lazar 2012a).

Although this entry touches on the first question, it focuses on the second. Addressing the first requires detailed empirical research and pragmatic political speculation, both of which are beyond my remit here. Addressing the third takes us too deep into the minutiae of contemporary just war theory for an encyclopaedia entry.

What’s more, even institutionalists need some answer to the second question—and so some account of the interactional morality of war. Rule-consequentialists need an account of the good (bad) that they are hoping that the ideal laws of war will maximise (minimise) in the long run. This means, for example, deciding whether to aim to minimise all harm, or only to minimise wrongful harm. The latter course is much more plausible—we wouldn’t want laws of war that, for example, licensed genocide just in case doing so leads to fewer deaths overall. But to follow this course, we need to know which harms are (extra-institutionally) wrongful. Similarly, contractualists typically acknowledge various constraints on the kinds of rules that could form the basis of a legitimate contract, which, again, we cannot work out without thinking about the extra-institutional morality of war (Benbaji 2011).

Even within interactional just war theory, several second-order disagreements underscore first-order disputes. First: when thinking about the ethics of war, what kinds of cases should we use to test our intuitions and our principles? We can start by thinking about actual wars and realistic wartime scenarios, paying attention to international affairs and military history. Or, more clinically, we can construct hypothetical cases to isolate variables and test their impact on our intuitions.

Some early revisionists relied heavily on highly artificial cases (e.g., McMahan 1994; Rodin 2002). They were criticized for this by traditionalists, who generally use more empirically-informed examples (Walzer 2006). But one’s standpoint on the substantive questions at issue between traditionalists and revisionists need not be predetermined by one’s methodology. Revisionists can pay close attention to actual conflicts (e.g., Fabre 2012). Traditionalists can use artificial hypotheticals (e.g., Emerton and Handfield 2009; Lazar 2013).

Abstraction forestalls unhelpful disputes over historical details. It also reduces bias—we are inclined to view actual conflicts through the lens of our own political allegiances. But it also has costs. We should be proportionately less confident of our intuitions the more removed the test case is from our lived experience. Philosophers’ scenarios involving mind-control, armed pedestrians, trolleys, meteorites, and incredibly complicated causal sequences are pure exercises in imagination. How can we trust our judgements about such cases more than we trust our views on actual, realistic scenarios? What’s more, abandoning the harrowing experience of war for sanitized hypothetical cases might be not merely epistemically unsound, but also disrespectful of the victims of war. Lastly, cleaned-up examples often omit morally relevant details—for instance, assuming that everyone has all the information relevant to their choice, rather than acknowledging the “fog of war”, and making no allowances for fear or trauma.

Artificial hypotheticals have their place, but any conclusions they support must be tested against the messy reality of war. What’s more, our intuitive judgements should be the starting-point for investigation, rather than its end.

The second divide is related to the first. Reductivists think that killing in war must be justified by the same properties that justify killing outside of war. Non-reductivists , sometimes called exceptionalists , think that some properties justify killing in war that do not justify killing outside of war. [ 7 ] Most exceptionalists think that specific features of killing in war make it morally different from killing in ordinary life—for example, the scale of the conflict, widespread and egregious non-compliance with fundamental moral norms, the political interests at stake, the acute uncertainty, the existence of the law of armed conflict, or the fact that the parties to the conflict are organized groups. A paradigm reductivist, by contrast, might argue that justified wars are mere aggregates of justified acts of individual self- and other-defence (see Rodin 2002; McMahan 2004a).

Reductivists are much more likely to use far-fetched hypothetical cases, since they think there is nothing special about warfare. The opposite is true for exceptionalists. Walzer’s first critics relied on reductivist premises to undermine the principles of national defence (Luban 1980a; Rodin 2002), discrimination (Holmes 1989; McMahan 1994), and combatant equality (Holmes 1989; McMahan 1994). Many traditionalists replied by rejecting reductivism, arguing that there is something special about war that justifies a divergence from the kinds of judgements that are appropriate to other kinds of conflict (Zohar 1993; Kutz 2005; Benbaji 2008; Dill and Shue 2012). Again, some philosophers buck these overarching trends (for reductivist traditionalist arguments, see e.g., Emerton and Handfield 2009; Lazar 2015c; Haque 2017; for non-reductivist revisionist arguments, see e.g., Ryan 2016).

The debate between reductivism and exceptionalism is overblown—the concept of “war” is vague, and while typical wars involve properties that are not instantiated in typical conflicts outside of war, we can always come up with far-fetched hypotheticals that don’t involve those properties, which we wouldn’t call “wars”. But this masks a deeper methodological disagreement: when thinking about the morality of war, should we start by thinking about war, or by thinking about the permissible use of force outside of war? Should we model justified killing in war on justified killing outside of war? Or, in focusing on the justification of killing in war, might we then discover that there are some non-canonical cases of permissible killing outside of war? My own view is that thinking about justified killing outside of war has its place, but must be complemented by thinking about war directly.

Next, we can distinguish between individualists and collectivists; and we can subdivide them further into evaluative and descriptive categories. Evaluative individualists think that a collective’s moral significance is wholly reducible to its contribution to the well-being of the individuals who compose it. Evaluative collectivists think that collectives can matter independently of how they contribute to individual well-being. Descriptive individualists think that any act that might appear to be collective is reducible to component acts by individuals. Descriptive collectivists deny this, thinking that some acts are irreducibly collective. [ 8 ]

Again, the dialectic of contemporary just war theory involves revisionists first arguing that we cannot vindicate traditionalist positions on descriptively and evaluatively individualist grounds, with some traditionalists then responding by rejecting descriptive (Kutz 2005; Walzer 2006; Lazar 2012b) and evaluative individualism (Zohar 1993). And again there are outliers—individualist traditionalists (e.g., Emerton and Handfield 2009) and collectivist revisionists (e.g., Bazargan 2013).

Unlike the reductivist/exceptionalist divide, the individualist/collectivist split cannot be resolved by thinking about the morality of war on its own. War is a useful test case for theories of collective action and the value of collectives, but no more than that. Intuitions about war are no substitute for a theory of collective action. Perhaps some collectives have value beyond their contribution to the well-being of their members. For example, they might instantiate justice, or solidarity, which can be impersonally valuable (Temkin 1993). It is doubtful, however, that groups have interests independent from the well-being of their members. On the descriptive side, even if we can reduce collective actions to the actions of individual members, this probably involves such complicated contortions that we should seriously question whether it is worth doing (Lazar 2012b).

Traditionally, just war theorists divide their enquiry into reflection on the resort to war— jus ad bellum —and conduct in war— jus in bello . More recently, they have added an account of permissible action post-war, or jus post bellum . Others suggest an independent focus on war exit, which they have variously called jus ex bello and jus terminatio (Moellendorf 2008; Rodin 2008a). These Latin labels, though unfortunately obscurantist, serve as a useful shorthand. When we refer to ad bellum justice, we mean to evaluate the permissibility of the war as a whole. This is particularly salient when deciding to launch the war. But it is also crucial for the decision to continue fighting. Jus ex bello , then, fits within jus ad bellum . The jus in bello denotes the permissibility of particular actions that compose the war, short of the war as a whole.

Traditional just war theory construes jus ad bellum and jus in bello as sets of principles, satisfying which is necessary and sufficient for a war’s being permissible. Jus ad bellum typically comprises the following six principles:

• Just Cause: the war is an attempt to avert the right kind of injury.
• Legitimate Authority: the war is fought by an entity that has the authority to fight such wars.
• Right Intention: that entity intends to achieve the just cause, rather than using it as an excuse to achieve some wrongful end.
• Reasonable Prospects of Success: the war is sufficiently likely to achieve its aims.
• Proportionality: the morally weighted goods achieved by the war outweigh the morally weighted bads that it will cause.
• Last Resort (Necessity): there is no other less harmful way to achieve the just cause.

Typically the jus in bello list comprises:

• Discrimination: belligerents must always distinguish between military objectives and civilians, and intentionally attack only military objectives.
• Proportionality: foreseen but unintended harms must be proportionate to the military advantage achieved.
• Necessity: the least harmful means feasible must be used.

These all matter to the ethics of war, and will be addressed below. However, it is unhelpful to view them as a checklist of necessary and sufficient conditions. When they are properly understood, only proportionality and necessity (in the guise of last resort in the jus ad bellum ) are necessary conditions for a war, or an act in a war, to be permissible, since no matter how badly going to war fails the other ad bellum criteria (for example) it might still be permissible because it is the least awful of one’s alternatives, and so satisfies the necessity and proportionality constraints.

To get an intuitive grasp on necessity and proportionality, note that if someone threatens my life, then killing her would be proportionate; but if I could stop her by knocking her out, then killing her would be unnecessary, and so impermissible. The necessity and proportionality constraints have the same root: with few exceptions (perhaps when it is deserved), harm is intrinsically bad. Harms (and indeed all bads) that we cause must therefore be justified by some positive reason that counts in their favour—such as good achieved or evil averted (Lazar 2012a). Both the necessity and proportionality constraints involve comparing the bads caused by an action with the goods that it achieves. They differ only in the kinds of options they compare.

The use of force is proportionate when the harm done is counterbalanced by the good achieved in averting a threat. To determine this, we typically compare the candidate course of action with what would happen if we allowed the threat to eventuate.

Of course, in most cases we will have more than one means of averting or mitigating the threat. And a harmful option can be permissible only if all the harm that it involves is justified by a corresponding good achieved. If some alternative would as successfully avert the threat, but cause less harm, then the more harmful option is impermissible, because it involves unnecessary harm.

Where an option O aims to avert a threat T , we determine O ’s necessity by comparing it with all the other options that will either mitigate or avert T . We determine its proportionality by comparing it with the harm suffered if T should come about. The only difference between the proportionality and necessity constraints is that the former involves comparing one’s action with a very specific counterfactual scenario—in which we don’t act to avert the threat—while the latter involves comparing it with all your available options that have some prospect of averting or mitigating the threat. In my view, we should simply expand this so that the necessity constraint compares all your available options bar none. Then proportionality would essentially involve comparing each option with the alternative of doing nothing, while necessity would involve comparing all options (including doing nothing) in terms of their respective balances of goods and bads. On this approach, necessity would subsume proportionality. But this is a technical point with little substantive payoff.

More substantively, necessity and proportionality judgements concern consequences, and yet they are typically made ex ante , before we know what the results of our actions will be. They must therefore be modified to take this uncertainty into account. The most obvious solution is simply to refer to expected threats and expected harms, where the expected harm of an option O is the probability-weighted average of the harms that might result if I take O , and the expected threat is the probability-weighted average of the consequences of doing nothing to prevent the threat—allowing for the possibility that the threat might not eventuate at all (Lazar 2012b). We would also have to factor in the options’ probability of averting the threat. This simple move obscures a number of important and undertheorised issues that we cannot discuss in detail here. For now, we must simply note that proportionality and necessity must be appropriately indexed to the agent’s uncertainty.

Necessity and proportionality judgements involve weighing harms inflicted and threats averted, indeed all relevant goods and bads. The simplest way to proceed would be to aggregate the harms to individual people on each side, and call the act proportionate just in case it averts more harm than it causes, and necessary just in case no alternative involves less harm. But few deontologists, and indeed few non-philosophers, think in this naively aggregative way. Instead we should weight harms (etc.) according to factors such as whether the agent is directly responsible for them, and whether they are intended or merely foreseen. [ 9 ] Many also think that we can, perhaps even must, give greater importance in our deliberations to our loved ones (for example) than to those tied to us only by the common bonds of humanity (Hurka 2007; Lazar 2013; for criticism, see Lefkowitz 2009). Similarly, we might justifiably prioritise defending our own state’s sovereignty and territorial integrity, even when doing so would not be impartially best. [ 10 ] Only when all these and other factors (many of which are discussed below) are taken into consideration can we form defensible conclusions about which options are necessary and proportionate.

The other elements of the ethics of war contribute to the evaluation of proportionality and necessity, in one (or more) of three ways: identifying positive reasons in favour of fighting; delineating the negative reasons against fighting; or as staging-posts on the way to judgements of necessity and proportionality.

Given the gravity of the decision to go to war, only very serious threats can give us just cause to fight. So if just cause is satisfied, then you have weighty positive reasons to fight. Lacking just cause does not in itself aggravate the badness of fighting, but does make it less likely that the people killed in pursuing one’s war aims will be liable to be killed (more on this below, and see McMahan 2005a), which makes killing very hard to justify. Even if having a just cause is not strictly speaking a necessary condition for warfare to be permissible, the absence of a just cause makes it very difficult for a war to satisfy proportionality.

If legitimate authority is satisfied then additional positive reasons count in favour of fighting (see below). If it is not satisfied, then this adds an additional reason against fighting, which must be overcome for fighting to be proportionate.

The “reasonable prospects of success” criterion is a surmountable hurdle on the way to proportionality. Typically, if a war lacks reasonable prospects of success, then it will be disproportionate, since wars always involve causing significant harms, and if those harms are likely to be pointless then they are unlikely to be justified. But of course sometimes one’s likelihood of victory is very low, and yet fighting is still the best available option, and so necessary and proportionate. Having reasonable prospects of success matters only for the same reasons that necessity and proportionality matter. If necessity and proportionality are satisfied, then the reasonable prospects of success standard is irrelevant.

Right intention may also be irrelevant, but insofar as it matters its absence would be a reason against fighting; having the right intention does not give a positive reason to fight.

Lastly, discrimination is crucial to establishing proportionality and necessity, because it tells us how to weigh the lives taken in war.

3. Jus ad Bellum

Wars destroy lives and environments. In the eight years following the Iraq invasion in 2003, half a million deaths were either directly or indirectly caused by the war (Hagopian et al. 2013). Direct casualties from the Second World War numbered over 60 million, about 3 per cent of the world’s population. War’s environmental costs are less commonly researched, but are obviously also extraordinary (Austin and Bruch 2000). Armed forces use fuels in Olympian quantities: in the years from 2000–2013, the US Department of Defense accounted for around 80% of US federal government energy usage, between 0.75 and 1 quadrillion BTUs per year—a little less than all the energy use that year in Denmark and Bulgaria, a little more than Slovakia and Serbia (Energy Information Administration 2015a,b). They also directly and indirectly destroy habitats and natural resources—consider, for example, the Gulf war oil spill (El-Baz and Makharita 1994). For both our planet and its inhabitants, wars are truly among the very worst things we can do.

War can be necessary and proportionate only if it serves an end worth all this death and destruction. Hence the importance of having a just cause. And hence too the widespread belief that just causes are few and far between. Indeed, traditional just war theory recognizes only two kinds of justification for war: national defence (of one’s own state or of an ally) and humanitarian intervention. What’s more, humanitarian intervention is permissible only to avert the very gravest of tragedies—“crimes that shock the moral conscience of mankind” (Walzer 2006: 107).

Walzer argued that states’ claims to sovereignty and territorial integrity are grounded in the human rights of their citizens, in three ways. First, states ensure individual security. Rights to life and liberty have value “only if they also have dimension” (Walzer 2006: 58), which they derive from states’ borders—“within that world, men and women… are safe from attack; once the lines are crossed, safety is gone” (Walzer 2006: 57). Second, states protect a common life, made by their citizens over centuries of interaction. If the common life of a political community is valued by its citizens, then it is worth fighting for. Third, they have also formed a political association, an organic social contract, whereby individuals have, over time and in informal ways, conceded aspects of their liberty to the community, to secure greater freedom for all.

These arguments for national defence are double-edged. They helped explain why wars of national defence are permissible, but also make justifying humanitarian intervention harder. One can in principle successfully conclude a war in defence of oneself or one's allies without any lasting damage to the political sovereignty or territorial integrity of any of the contending parties. In Walzer’s view, humanitarian interventions, in which one typically defends people against their own state, necessarily undermine political sovereignty and territorial integrity. So they must meet a higher burden of justification.

Walzer’s traditionalist stances on national defence and humanitarian intervention met heavy criticism. Early sceptics (Doppelt 1978; Beitz 1980; Luban 1980a) challenged Walzer’s appeal to the value of collective freedom, noting that in diverse political communities freedom for the majority can mean oppression for the minority (see also Caney 2006). In modern states, can we even speak of a single common life? Even if we can, do wars really threaten it, besides in extreme cases? And even if our common life and culture were threatened, would their defence really justify killing innocent people?

Critics also excoriated Walzer’s appeal to individual rights (see especially Wasserstrom 1978; Luban 1980b). They questioned the normative purchase of his metaphor of the organic social contract (if hypothetical contracts aren’t worth the paper they’re not written on, then what are metaphorical contracts worth?). They challenged his claim that states guarantee individual security: most obviously, when humanitarian intervention seems warranted, the state is typically the greatest threat to its members.

David Rodin (2002) advanced the quintessentially reductivist critique of Walzer, showing that his attempt to ground state defence in individual defensive rights could not succeed. He popularized the “bloodless invasion objection” to this argument for national defensive rights. Suppose an unjustly aggressing army would secure its objectives without loss of life if only the victim state offers no resistance (the 2001 invasion of Afghanistan, and the 2003 invasion of Iraq, arguably meet this description, as might some of Russia’s territorial expansions). If the right of national defence is grounded in states’ members’ rights to security, then in these cases there would be no right of national defence, because their lives are at risk only if the victim state fights back. And yet we typically do think it permissible to fight against annexation and regime change.

By undermining the value of sovereignty, revisionists lowered the bar against intervening militarily in other states. Often these arguments were directly linked: some think that if states cannot protect the security of their members, then they lack any rights to sovereignty that a military intervention could undermine (Shue 1997). [ 11 ] Caney (2005) argues that military intervention could be permissible were it to serve individual human rights better than non-intervention. Others countenance so-called “redistributive wars”, fought on behalf of the global poor to force rich states to address the widespread violations of fundamental human rights caused by their economic policies (Luban 1980b; Fabre 2012; Lippert-Rasmussen 2013; Øverland 2013).

Other philosophers, equally unpersuaded by Walzer’s arguments, nonetheless reject a substantively revisionist take on just cause. If the individual self-defence-based view of jus ad bellum cannot justify lethal defence against “lesser aggression”, then we could follow Rodin (2014), and argue for radically revisionist conclusions about just cause; or we could instead reject the individual self-defence-based approach to justifying killing in war (Emerton and Handfield 2014; Lazar 2014).

Some think we can solve the “problem of lesser aggression” by invoking the importance of deterrence, as well as the impossibility of knowing for sure that aggression will be bloodless (Fabre 2014). Others think that we must take proper account of people’s interest in having a democratically elected, or at least home-grown, government, to justify national defence. On one popular account, although no individual could permissibly kill to protect her own “political interests”, when enough people are threatened, their aggregated interests justify going to war (Hurka 2007; Frowe 2014). Counterintuitively, this means that more populous states have, other things equal, more expansive rights of national defence. However, perhaps states have a group right to national defence, which requires only that a sufficient number of individuals have the relevant political interests—any excess over the threshold is morally irrelevant. Many already think about national self-determination in this way: the population of the group seeking independence has to be sufficiently large before we take their claim seriously, but differences above that threshold matter much less (Margalit and Raz 1990).

The revisionist take on humanitarian intervention might also have some troubling results. If sovereignty and territorial integrity matter little, then shouldn’t we use military force more often? As Kutz (2014) has argued, revisionist views on national defence might license the kind of military adventurism that went so badly wrong in Iraq, where states have so little regard for sovereignty that they go to war to improve the domestic political institutions of their adversaries.

We can resolve this worry in one of two ways. First, recall just how infrequently military intervention succeeds. Since it so often not only fails, but actually makes things worse, we should use it only when the ongoing crimes are so severe that we would take any risk to try to stop them.

Second, perhaps the political interests underpinning the state’s right to national defence are not simply interests in being part of an ideal liberal democracy, but in being governed by, very broadly, members of one’s own nation, or perhaps even an interest in collective self-determination. This may take us back to Walzer’s “romance of the nation-state”, but people clearly do care about something like this. Unless we want to restrict rights of national defence to liberal democracies alone (bearing in mind how few of them there are in the world), we have to recognize that our political interests are not all exclusively liberal-democratic.

What of redistributive wars? Too often arguments on this topic artfully distinguish between just cause and other conditions of jus ad bellum (Fabre 2012). Even when used by powerful states against weak adversaries, military force is rarely a moral triumph. It tends to cause more problems than it solves. Redistributive wars, as fought on behalf of the “global poor” against the “global rich”, would obviously fail to achieve their objectives, indeed they would radically exacerbate the suffering of those they aim to help. So they would be disproportionate, and cannot satisfy the necessity constraint. The theoretical point that, in principle, not only national defence and humanitarian intervention could give just causes for war is sound. But this example is in practice irrelevant (for a robust critique of redistributive wars, see Benbaji 2014).

And yet, given the likely path of climate change, the future might see resource wars grow in salience. As powerful states find themselves lacking crucial resources, held by other states, we might find that military attack is the best available means to secure these resources, and save lives. Perhaps in some such circumstances resource wars could be a realistic option.

The goods and bads relevant to ad bellum proportionality and necessity extend far beyond the armistice. This is obvious, but has recently received much-needed emphasis, both among philosophers and in the broader public debate sparked by the conflicts in Iraq and Afghanistan (Bass 2004; Coady 2008; May 2012). Achieving your just cause is not enough. The aftermath of the war must also be sufficiently tolerable if the war is to be proportionate, all things considered. It is an open question how far into the future we have to look to assess the morally relevant consequences of conflict.

Historically, just war theory has been dominated by statists. Most branches of the tradition have had some version of a “legitimate”, “proper” or “right” authority constraint, construed as a necessary condition for a war to be ad bellum just. [ 12 ] In practice, this means that sovereigns and states have rights that non-state actors lack. International law gives only states rights of national defence and bestows “combatant rights” primarily on the soldiers of states. Although Walzer said little about legitimate authority, his arguments all assume that states have a special moral standing that non-state actors lack.

The traditionalist, then, says it matters that the body fighting the war have the appropriate authority to do so. Some think that authority is grounded in the overall legitimacy of the state. Others think that overall legitimacy is irrelevant—what matters is whether the body fighting the war is authorized to do so by the polity that it represents (Lazar forthcoming-b). Either way, states are much more likely to satisfy the legitimate authority condition than non-state actors.

Revisionists push back: relying on reductivist premises, they argue that killing in war is justified by the protection of individual rights, and our licence to defend our rights need not be mediated through state institutions. Either we should disregard the legitimate authority condition or we should see it as something that non-state actors can, in fact, fulfil (Fabre 2008; Finlay 2010; Schwenkenbecher 2013).

Overall, state legitimacy definitely seems relevant for some questions in war (Estlund 2007; Renzo 2013). But authorization is more fundamental. Ideally, the body fighting the war should be authorized to do so by the institutions of a constitutional democracy. Looser forms of authorization are clearly possible; even a state that is not overall legitimate might nonetheless be authorized by its polity to fight wars of national defence.

Authorization of this kind matters to jus ad bellum in two ways. First, fighting a war without authorization constitutes an additional wrong, which has to be weighed against the goods that fighting will bring about, and must pass the proportionality and necessity tests. When a government involves its polity in a war, it uses the resources of the community at large, as well as its name, and exposes it to both moral and prudential risks (Lazar forthcoming-b). Doing this unauthorized is obviously deeply morally problematic. Any form of undemocratic decision-making by governments is objectionable; taking decisions of this magnitude without the population’s granting you the right to do so is especially wrong.

Second, authorization can allow the government to act on positive reasons for fighting that would otherwise be unavailable. Consider the claim that wars of national defence are in part justified by the political interests of the citizens of the defending state—interests, for example, in democratic participation or in collective self-determination. A government may defend these aggregated political interests only if it is authorized to do so. Otherwise fighting would contravene the very interests in self-determination that it is supposed to protect. But if it is authorized, then that additional set of reasons supports fighting.

As a result, democratic states enjoy somewhat more expansive war rights than non-democratic states and non-state movements. The latter two groups cannot often claim the same degree of authorization as democratic states. Although this might not vindicate the current bias in international law towards states, it does suggest that it corresponds to something more than the naked self-interest of the framers of international law—which were, of course, states. This obviously has significant implications for civil wars (see Parry 2016).

The central task of the proportionality constraint, recall, is to identify reasons that tell in favour of fighting and those that tell against it. Much of the latter task is reserved for the discussion of jus in bello below, since it concerns weighing lives in war.

Among the goods that help make a war proportionate, we have already considered those in the just cause and others connected to just peace and legitimate authority. Additionally, many also think that proportionality can be swayed by reasonable partiality towards one’s own state and co-citizens. Think back to the political interests that help justify national defence. If we were wholly impartial, then we should choose the course that will best realise people’s political interests overall. So if fighting the defensive war would undermine the political interests of the adversary state’s citizens more than it would undermine our own, then we should refuse to fight. But this is not how we typically think about the permission to resort to war: we are typically entitled to be somewhat partial towards the political interests of our co-citizens.

Some propose further constraints on what goods can count towards the proportionality of a war. McMahan and McKim (1993) argued that benefits like economic progress cannot make an otherwise disproportionate war proportionate. This is probably true in practice, but perhaps not in principle—that would require a kind of lexical priority between lives taken and economic benefits, and lexical priorities are notoriously hard to defend. After all, economic progress saves lives.

Some goods lack weight in ad bellum proportionality, not because they are lexically inferior to other values at stake, but because they are conditional in particular ways. Soldiers have conditional obligations to fulfil their roles, grounded in their contracts, oaths, and their co-citizens’ legitimate expectations. That carrying out an operation fulfils my oath gives me a reason to perform that operation, which has to be weighed in the proportionality calculation (Lazar 2015b). But these reasons cannot contribute to ad bellum proportionality in the same way, because they are conditional on the war as a whole being fought. Political leaders cannot plausibly say: “were it not for all the oaths that would be fulfilled by fighting, this war would be disproportionate”. This is because fighting counts as fulfilling those oaths only if the political leader decides to take her armed forces to war.

Another reason to differentiate between proportionality ad bellum and in bello is that the relevant comparators change for the two kinds of assessment. In a loose sense, we determine proportionality by asking whether some option is better than doing nothing. The comparator for assessing the war as a whole, then, is not fighting at all , ending the war as a whole. That option is not available when considering particular actions within the war—one can only decide whether or not to perform this particular action.

Are pre-emptive wars, fought in anticipation of an imminent enemy attack, permissible? What of preventive wars, in which the assault occurs prior to the enemy having any realistic plan of attack (see, in general, Shue and Rodin 2007)? Neoconservatives have recently argued, superficially plausibly, that the criterion of last resort can be satisfied long before the enemy finally launches an attack (see President 2002). The right answer here is boringly familiar. In principle, of course this is possible. But, in practice, we almost always overestimate the likelihood of success from military means and overlook the unintended consequences of our actions. International law must therefore retain its restrictions, to deter the kind of overzealous implementation of the last-resort principle that we saw in the 2003 invasion of Iraq (Buchanan and Keohane 2004; Luban 2004).

Another frequently discussed question: what does the “last” in last resort really mean? The idea is simple, and is identical to in bello necessity. Going to war must be compared with the alternative available strategies for dealing with the enemy (which also includes the various ways in which we could submit). Going to war is literally a last resort when no other available means has any prospect of averting the threat. But our circumstances are not often this straitened. Other options always have some chance of success. So if you have a diplomatic alternative to war, which is less harmful than going to war, and is at least as likely to avert the threat, then going to war is not a last resort. If the diplomatic alternative is less harmful, as well as less likely to avert the threat, then the question is whether the reduction in expected harm is great enough for us to be required to accept the reduction in likelihood of averting the threat. If not, then war is your last resort. [ 13 ]

4. Jus in Bello

The traditionalist jus in bello , as reflected in international law, holds that conduct in war must satisfy three principles:

• Discrimination: Targeting noncombatants is impermissible. [ 14 ]
• Proportionality: Collaterally harming noncombatants (that is, harming them foreseeably, but unintendedly) is permissible only if the harms are proportionate to the goals the attack is intended to achieve. [ 15 ]
• Necessity: Collaterally harming noncombatants is permissible only if, in the pursuit of one’s military objectives, the least harmful means feasible are chosen. [ 16 ]

These principles divide the possible victims of war into two classes: combatants and noncombatants. They place no constraints on killing combatants. [ 17 ] But—outside of “supreme emergencies”, rare circumstances in which intentionally killing noncombatants is necessary to avert an unconscionable threat—noncombatants may be killed only unintendedly and, even then, only if the harm they suffer is necessary and proportionate to the intended goals of the attack. [ 18 ] Obviously, then, much hangs on what makes one a combatant. This entry adopts a conservative definition. Combatants are (most) members of the organized armed forces of a group that is at war, as well as others who directly participate in hostilities or have a continuous combat function (for discussion, see Haque 2017). Noncombatants are not combatants. There are, of course, many hard cases, especially in asymmetric wars, but they are not considered here. “Soldier” is used interchangeably with “combatant” and “civilian” interchangeably with “noncombatant”.

Both traditionalist just war theory and international law explicitly license fighting in accordance with these constraints, regardless of one’s objectives. In other words, they endorse:

Combatant Equality: Soldiers who satisfy Discrimination, Proportionality, and Necessity fight permissibly, regardless of what they are fighting for. [ 19 ]

We discuss Proportionality and Necessity below; for now let us concentrate on Michael Walzer’s influential argument for Discrimination and Combatant Equality, which has proved very controversial.

Individual human beings enjoy fundamental rights to life and liberty, which prohibit others from harming them in certain ways. Since fighting wars obviously involves depriving others of life and liberty, according to Walzer, it can be permissible only if each of the victims has, “through some act of his own … surrendered or lost his rights” (Walzer 2006: 135). He then claims that, “simply by fighting”, all combatants “have lost their title to life and liberty” (Walzer 2006: 136). First, merely by posing a threat to me, a person alienates himself from me, and from our common humanity, and so himself becomes a legitimate target of lethal force (Walzer 2006: 142). Second, by participating in the armed forces, a combatant has “allowed himself to be made into a dangerous man” (Walzer 2006: 145), and thus surrendered his rights. By contrast, noncombatants are “men and women with rights, and… they cannot be used for some military purpose, even if it is a legitimate purpose” (Walzer 2006: 137). This introduces the concept of liability into the debate, which we need to define carefully. On most accounts, that a person is liable to be killed means that she is not wronged by being killed. Often this is understood, as it was in Walzer, in terms of rights: everyone starts out with a right to life, but that right can be forfeited or lost, such that one can be killed without that right being violated or infringed. Walzer and his critics all agreed that killing a person intentionally is permissible only if either she has lost the protection of her right to life, or if the good achieved thereby is very great indeed, enough that, though she is wronged, it is not all things considered wrong to kill her. Her right is permissibly infringed. Walzer and his critics believe that such cases are very rare in war, arising only when the alternative to intentionally violating people’s right to life is an imminent catastrophe on the order of Nazi victory in Europe (this is an example of a supreme emergency).

These simple building blocks give us both Discrimination and Combatant Equality—the former, because noncombatants, in virtue of retaining their rights, are not legitimate objects of attack; the latter, because all combatants lose their rights, regardless of what they are fighting for: hence, as long as they attack only enemy combatants, they fight legitimately, because they do not violate anyone’s rights.

These arguments have faced withering criticism. The simplest objection against Combatant Equality brings it into conflict with Proportionality (McMahan 1994; Rodin 2002; Hurka 2005). Unintended noncombatant deaths are permissible only if proportionate to the military objective sought. This means the objective is worth that much innocent suffering. But military objectives are merely means to an end. Their worth depends on how valuable the end is. How many innocent deaths would be proportionate to Al Shabab’s successfully gaining control of Mogadishu now or to Iraq’s capturing Kuwaiti territory and oil reserves in 1991? In each case the answer is obvious: none.

Proportionality is about weighing the evil inflicted against the evil averted (Lee 2012). But the military success of unjust combatants does not avert evil, it is itself evil. Evil intentionally inflicted can only add to, not counterbalance, unintended evils. Combatant Equality cannot be true.

Other arguments against Combatant Equality focus on Walzer’s account of how one loses the right to life. They typically start by accepting his premise that permissible killing in war does not violate the rights of the victims against being killed, at least for intentional killing. [ 20 ] This contrasts with the view that sometimes people’s rights to life can be overridden, so war can be permissible despite infringing people’s rights. Walzer’s critics then show that his account of how we lose our right to life is simply not plausible. Merely posing a threat to others—even a lethal threat—is not sufficient to warrant the loss of one’s fundamental rights, because sometimes one threatens others’ lives for very good reasons (McMahan 1994). The soldiers of the Kurdish Peshmerga, heroically fighting to rescue Yazidis from ISIL’s genocidal attacks, do not thereby lose their rights not to be killed by their adversaries. Posing threats to others in the pursuit of a just aim, where those others are actively trying to thwart that just aim, cannot void or vitiate one’s fundamental natural rights against being harmed by those very people. The consent-based argument is equally implausible as a general defence for Combatant Equality. Unjust combatants have something to gain from waiving their rights against lethal attack, if doing so causes just combatants to effect the same waiver. And on most views, many unjust combatants have nothing to lose, since by participating in an unjust war they have at least weakened if not lost those rights already. Just combatants, by contrast, have something to lose, and nothing to gain. So why would combatants fighting for a just cause consent to be harmed by their adversaries, in the pursuit of an unjust end?

Walzer’s case for Combatant Equality rests on showing that just combatants lose their rights to life. His critics have shown that his arguments to this end fail. So Combatant Equality is false. But they have shown more than this. Inspired by Walzer to look at the conditions under which we lose our rights to life, his critics have made theoretical advances that place other central tenets of jus in bello in jeopardy. They argued, contra Walzer, that posing a threat is not sufficient for liability to be killed (McMahan 1994, 2009). But they also showed that posing the threat oneself is not necessary for liability either. This is more controversial, but revisionists have long argued that liability is grounded, in war as elsewhere, in one’s responsibility for contributing to a wrongful threat. The US president, for example, is responsible for a drone strike she orders, even though she does not fire the weapon herself.

As many have noted, this argument undermines Discrimination (McMahan 1994; Arneson 2006; Fabre 2012; Frowe 2014). In many states, noncombatants play an important role in the resort to military force. In modern industrialized countries, as much as 25 per cent of the population works in war-related industries (Downes 2006: 157–8; see also Gross 2010: 159; Valentino et al. 2010: 351); we provide the belligerents with crucial financial and other services; we support and sustain the soldiers who do the fighting; we pay our taxes and in democracies we vote. Our contributions to the state’s capacity over time give it the strength and support to concentrate on war. [ 21 ] If the state’s war is unjust, then many noncombatants are responsible for contributing to wrongful threats. If that is enough for them to lose their rights to life, then they are permissible targets.

McMahan (2011a) has sought to avert this troubling implication of his arguments by contending that almost all noncombatants on the unjust side (unjust noncombatants) are less responsible than all unjust combatants. But this involves applying a double standard, talking up the responsibility of combatants, while talking down that of noncombatants, and mistakes a central element in his account of liability to be killed. On his view, a person is liable to be killed in self- or other-defence in virtue of being, of those able to bear an unavoidable and indivisible harm, the one who is most responsible for this situation coming about (McMahan 2002, 2005b). Even if noncombatants are only minimally responsible for their states’ unjust wars—that is, they are not blameworthy, they merely voluntarily acted in a way that foreseeably contributed to this result—on McMahan’s view this is enough to make them liable to be killed, if doing so is necessary to save the lives of wholly innocent combatants and noncombatants on the just side (see especially McMahan 2009: 225).

One response is to reject this comparative account of how responsibility determines liability, and argue for a non-comparative approach, according to which one’s degree of responsibility must be great enough to warrant such a severe derogation from one’s fundamental rights. But if we do this, we must surely concede that many combatants on the unjust side are not sufficiently responsible for unjustified threats to be liable to be killed. Whether through fear, disgust, principle or ineptitude, many combatants are wholly ineffective in war, and contribute little or nothing to threats posed by their side. The much-cited research of S. L. A. Marshall claimed that only 15–25 per cent of Allied soldiers in the Second World War who could have fired their weapons did so (Marshall 1978). Most soldiers have a natural aversion to killing, which even intensive psychological training may not overcome (Grossman 1995). Many contribute no more to unjustified threats than do noncombatants. They also lack the “ mens rea ” that might make liability appropriate in the absence of a significant causal contribution. They are not often blameworthy. The loss of their right to life is not a fitting response to their conduct.

If Walzer is right that in war, outside of supreme emergencies, we may intentionally kill only people who are liable to be killed, and if a significant proportion of unjust combatants and noncombatants are responsible to the same degree as one another for unjustified threats, and if liability is determined by responsibility, then we must decide between two unpalatable alternatives. If we set a high threshold of responsibility for liability, to ensure that noncombatants are not liable to be killed, then we will also exempt many combatants from liability. In ordinary wars, which do not involve supreme emergencies, intentionally killing such non-liable combatants would be impermissible. This moves us towards a kind of pacifism—though warfare can in principle be justified, it is so hard to fight without intentionally killing the non-liable that in practice we must be pacifists (May 2015). But if we set the threshold of responsibility low, ensuring that all unjust combatants are liable, then many noncombatants will be liable too, thus rendering them permissible targets and seriously undermining Discrimination. We are torn between pacifism on the one hand, and realism on the other. This is the “responsibility dilemma” for just war theory (Lazar 2010).

Just war theory has meaning only if we can explain why killing some combatants in war is allowed, but we are not thereby licensed to kill everyone in the enemy state. Here the competing forces of realism and pacifism are at their most compelling. It is unsurprising, therefore, that so much recent work has focused on this topic. We cannot do justice to all the arguments here, but will instead consider three kinds of response: all-out revisionist; moderate traditionalist; and all-out traditionalist.

The first camp faces two challenges: to justify intentionally killing apparently non-liable unjust combatants; but to do this without reopening the door to Combatant Equality, or indeed further undermining Discrimination. Their main move is to argue that, despite appearances, all and only unjust combatants are in fact liable to be killed.

McMahan argues that liability to be killed need not, in fact, presuppose responsibility for an unjustified threat. Instead, unjust combatants’ responsibility for just combatants’ reasonable beliefs that they are liable may be enough to ground forfeiture of their rights (McMahan 2011a). Some argue that combatants’ responsibility for being in the wrong place at the wrong time is enough (likening them to voluntary human shields). [ 22 ] More radically still, some philosophers abandon the insistence on individual responsibility, arguing that unjust combatants are collectively responsible for contributing to unjustified threats, even if they are individually ineffective (or even counterproductive) (Kamm 2004; Bazargan 2013).

Lazar (forthcoming-a) suggests these arguments are unpersuasive. Complicity might be relevant to the costs one is required to bear in war, but most liberals will baulk at the idea of losing one’s right to life in virtue of things that other people did. And if combatants can be complicitously liable for what their comrades-in-arms did, then why shouldn’t noncombatants be complicitously liable also?

Blameworthy responsibility for other people’s false beliefs does seem relevant to the ethics of self- and other-defence. That said, consider an idiot who pretends to be a suicide bomber as a prank, and is shot by a police officer (Ferzan 2005; McMahan 2005c). Is killing him objectively permissible? It seems doubtful. The officer’s justified belief that the prankster posed a threat clearly diminishes the wrongfulness of killing him (Lazar 2015a). And certainly the prankster’s fault excuses the officer of any guilt. But killing the prankster still seems objectively wrong. Even if someone’s blameworthy responsibility for false beliefs could make killing him objectively permissible, most philosophers agree that many unjust combatants are not to blame for the injustice of their wars (McMahan 1994; Lazar 2010). And it is much less plausible that blameless responsibility for beliefs can make one a permissible target. Even if it did, this would count in favour of moderate Combatant Equality, since most just combatants are also blamelessly responsible for unjust combatants’ reasonable beliefs that they are liable to be killed.

Moderate traditionalists think we can avoid the realist and pacifist horns of the responsibility dilemma only by conceding a moderate form of Combatant Equality. The argument proceeds in three stages. First, endorse a non-comparative, high threshold of responsibility for liability, such that most noncombatants in most conflicts are not responsible enough to be liable to be killed. This helps explain why killing civilians in war is so hard to justify. Of course, it also entails that many combatants will be innocent too. The second step, then, is to defend the principle of Moral Distinction , according to which killing civilians is worse than killing soldiers. This is obviously true if the soldiers are liable and the civilians are not. But the challenge is to show that killing non-liable civilians is worse than killing non-liable soldiers. If we can do that, then the permissibility of intentionally killing non-liable soldiers does not entail that intentionally killing non-liable noncombatants is permissible. Of course, one might still argue that, even if Moral Distinction is true, we should endorse pacifism. But, and this is the third stage, the less seriously wrongful some act is, the lesser the good that must be realised by performing that act, for it to be all things considered permissible. If intentionally killing innocent combatants is not the worst kind of killing one can do, then the good that must be realised for it to be all things considered permissible is less than is the case for, for example, intentionally killing innocent civilians, which philosophers tend to think can be permissible only in a supreme emergency. This could mean that intentionally killing innocent soldiers is permissible even in the ordinary circumstances of war.

Warfare can be justified, then, by a combination of liability and lesser evil grounds. Some unjust combatants lose their rights not to be killed. Others’ rights can be overridden without that implying that unjust noncombatants’ rights may be overridden too. We can reject the pacifist horn of the responsibility dilemma. But a moderate Combatant Equality is likely to be true: since killing innocent combatants is not the worst kind of killing, it is correspondingly easier for unjust combatants to justify using lethal force (at least against just combatants). This increases the range of cases in which they can satisfy Discrimination, Proportionality, and Necessity, and so fight permissibly.

Much hangs, then, on the arguments for Moral Distinction. Some focus on why killing innocent noncombatants is especially wrongful; others on why killing innocent combatants is not so bad. This section considers the second kind of argument, returning to the first in the next section.

The revisionists’ arguments mentioned above might not ground liability, but do perhaps justify some reason to prefer harming combatants. Combatants can better avoid harm than noncombatants. Combatants surely do have somewhat greater responsibilities to bear costs to avert the wrongful actions of their comrades-in-arms than do noncombatants. And the readiness of most combatants to fight—regardless of whether their cause is just—likely means that even just combatants have somewhat muddied status relative to noncombatants. They conform to their opponents’ rights only by accident. They have weaker grounds for complaint when they are wrongfully killed than do noncombatants, who more robustly respect the rights of others (on robustness and respect, see Pettit 2015).

Additionally, when combatants kill other combatants, they typically believe that they are doing so permissibly. Most often they believe that their cause is just, and that this is a legitimate means to bring it about. But, insofar as they are lawful combatants, they will also believe that international law constrains their actions, so that by fighting in accordance with it they are acting permissibly. Lazar (2015c) argues that killing people when you know that doing so is objectively wrong is more seriously objectionable than doing so when you reasonably believe that you are acting permissibly.

The consent-based argument for Combatant Equality fails because of its empirical, not its normative premise. If combatants in fact waived their rights not to be killed by their adversaries, even when fighting a just war, then that would clearly affect their adversaries’ reasons for action, reducing the wrongfulness of killing anyone who had waived that right. The problem is that they have not waived their rights not to be killed. However, they often do offer a more limited implicit waiver of their rights. The purpose of having armed forces, and the intention of many who serve in them, is to protect civilians from the predations of war. This means both countering threats to and drawing fire away from them. Combatants interpose themselves between the enemy and their civilian compatriots, and fight on their compatriots’ behalf. If they abide by the laws of war, they clearly distinguish themselves from the civilian population, wearing a uniform and carrying their weapons openly. They implicitly say to their adversaries: “you ought to put down your weapons. But if you are going to fight, then fight us ”. This constitutes a limited waiver of their rights against harm. Like a full waiver, it alters the reasons confronting their adversaries—under these circumstances, other things equal it is worse to kill the noncombatants. Of course, in most cases unjust combatants ought simply to stop fighting. But this conditional waiver of their opponents’ rights means that, if they are not going to put down arms, they do better to target combatants than noncombatants.

Of course, one might think that in virtue of their altruistic self-sacrifice, just combatants are actually the least deserving of the harms of war (Tadros 2014). But, first, warfare is not a means for ensuring that people get their just deserts. More importantly, given that their altruism is specifically intended to draw fire away from their compatriot noncombatants, it would be perverse to treat this as a reason to do precisely what they are trying to prevent.

These arguments and others suggest that killing innocent combatants is not the worst kind of killing one can do. It might therefore be all things considered permissible in the ordinary circumstances of war, provided enough good is achieved thereby. If unjust combatants attack only just combatants, and if they achieve some valuable objective by doing so—defence of their comrades, their co-citizens, or their territory—they therefore might fight permissibly, even though they violate the just combatants’ rights (Kamm 2004; Hurka 2005; Kamm 2005; Steinhoff 2008; Lazar 2013). At least, it is more plausible that they can fight permissibly than if we regarded every just combatant’s death as equivalent to the worst kind of murder. This does not vindicate Combatant Equality—it simply shows that, more often than one might think, unjust combatants can fight permissibly. Add to that the fact that all wars are morally heterogeneous, involving just and unjust phases (Bazargan 2013), and we quickly see that even if Combatant Equality in the laws of war lacks fundamental moral foundations, it is a sensible approximation of the truth.

Some philosophers, however, seek a more robust defence of Combatant Equality. The three most prominent lines are institutionalist. A contractualist argument (Benbaji 2008, 2011) starts by observing that states (and their populations) need disciplined armies for the purposes of national defence. If soldiers always had to decide for themselves whether a particular war was just, many states could not raise armies when they need to. They would be unable to deter aggression. All states, and all people, benefit from an arrangement whereby individual combatants waive their rights not to be killed by one another—allowing them to obey their state’s commands without second-guessing every deployment. Combatants tacitly consent to waive their rights in this way, given common knowledge that fighting in accordance with the laws of war involves such a waiver. Moreover, their assent is “morally effective” because it is consistent with a fair and optimal contract among states.

International law does appear to change the moral standing of combatants. If you join the armed forces of a state, you know that, at international law, you thereby become a legitimate target in armed conflict. This has to be relevant to the wrongfulness of harming you, even if you are fighting for a just cause. But Benbaji’s argument is more ambitious than this. He thinks that soldiers waive their rights not to be killed by one another—not the limited, conditional waiver described above, but an outright waiver, that absolves their adversaries of any wrongdoing (though it does not so absolve their military and political leaders).

The first problem with this proposal is that it rests on contentious empirical speculation about whether soldiers in fact consent in this way. But setting that aside, second, it is radically statist, implying that international law simply doesn’t apply to asymmetric conflicts between states and non-state actors, since the latter are not part of the appropriate conventions. This gives international law shallow foundations, which fail to support the visceral outrage that breaches of international law typically evoke. It also suggests that states that either don’t ratify major articles of international law, or that withdraw from agreements, can escape its strictures. This seems mistaken. Third, we typically regard waivers of fundamental rights as reversible when new information comes to light. Why shouldn’t just combatants be allowed to withdraw their rights-waiver when they are fighting a just war? Many regard the right to life as inalienable; even if we deny this, we must surely doubt whether you can alienate it once and for all, under conditions of inadequate information. Additionally, suppose that you want to join the armed forces only to fight a specific just war (McMahan 2011b). Why should you waive your rights against harm in this case, given that you plan only to fight now? Fourth, and most seriously, even if Benbaji’s argument explained why killing combatants in war is permissible regardless of the cause you are serving, it cannot explain why unintentionally killing noncombatants as a side-effect of one’s actions is permissible. By joining the armed forces of their state, soldiers at least do something that implies their consent to the regime of international law that structures that role. But noncombatants do not consent to this regime. Soldiers fighting for unjust causes will inevitably kill many innocent civilians. If those deaths cannot be rendered proportionate, then Combatant Equality does not hold.

The second institutionalist argument starts from the belief that we have a duty to obey the law of our legitimate state. This gives unjust combatants, ordered to fight an unjust war, some reason to obey those orders. We can ground this in different ways. Estlund (2007) argues that the duty to obey orders derives from the epistemic authority of the state—it is more likely than an individual soldier to know whether this war is just (see Renzo 2013 for criticism); Cheyney Ryan (2011) emphasizes the democratic source of the state’s authority, as well as the crucial importance of maintaining civilian control of the military. These are genuine moral reasons that should weigh in soldiers’ deliberations. But are they really weighty enough to ground Combatant Equality? It seems doubtful. They cannot systematically override unjust combatants’ obligations not to kill innocent people. This point stands regardless of whether these reasons weigh in the balance, or are exclusionary reasons that block others from being considered (Raz 1985). The rights of innocent people not to be killed are the weightiest, most fundamental rights around. For some other reason to outweigh them, or exclude them from deliberation, it would have to be extremely powerful. Combatants’ obligations to obey orders simply are not weighty enough—as everyone recognises with respect to obedience to unlawful in bello commands (McMahan 2009: 66ff).

Like the first argument, the third institutionalist argument grounds Combatant Equality in its long-term results. But instead of focusing on states’ ability to defend themselves, it emphasizes the importance of limiting the horrors of war, given that we know that people deceive themselves about the justice of their cause (Shue 2008, 2010; Dill and Shue 2012; Shue 2013; Waldron 2016). Since combatants and their leaders almost always believe themselves to be in the right, any injunction to unjust combatants to lay down their arms would simply be ignored, while any additional permissions to harm noncombatants would be abused by both sides. In almost all wars, it is sufficient to achieve military victory that you target only combatants. If doing this will minimize wrongful deaths in the long run, we should enjoin that all sides, regardless of their aims, respect Discrimination. Additionally, while it is extremely difficult to secure international agreement even about what in fact constitutes a just cause for war (witness the controversy over the Rome statute on crimes of aggression, which took many years of negotiation before diplomats agreed an uneasy compromise), the traditionalist principles of jus in bello already have broad international support. They are hard-won concessions that we should abandon only if we are sure that the new regime will be an improvement (Roberts 2008).

Although this argument is plausible, it doesn’t address the same question as the act-focused arguments that preceded it. One thing we can ask is: given a particular situation, what ought we to do? How ought soldiers to act in Afghanistan, or Mali, or Syria, or Somalia? And when we ask this question, we shouldn’t start by assuming that we or they will obviously fail to comply with any exacting moral standards that we might propose (Lazar 2012a; Lazar and Valentini forthcoming). When considering our own actions, and those of people over whom we have influence, we should select from all the available options, not rule some out because we know ourselves to be too immoral to take them. When designing institutions and laws, on the other hand, of course we should think about how people are likely to respond to them. We need to answer both kinds of questions: what really ought I to do? And what should the laws be, given my and others’ predictable frailty?

A moderate Combatant Equality, then, is the likely consequence of avoiding the pacifist horn of the responsibility dilemma. To show that killing in war is permissible, we need to show that intentionally killing innocent combatants is not as seriously wrongful as intentionally killing innocent noncombatants. And if killing innocent combatants is not the worst kind of killing, it can more plausibly be justified by the goods achieved in ordinary wars, outside of supreme emergencies. On this view, contrary to the views of both Walzer and his critics, much of the intended killing in justified wars is permissible not because the targets are liable to be killed, but because infringing their rights is a permissible lesser evil. But this principle applies regardless of whether you are on the just or the unjust side. This in turn increases the range of cases in which combatants fighting on the unjust side will be able to fight permissibly: instead of needing to achieve some good comparable to averting a supreme emergency in order to justify infringing the rights of just combatants, they need only achieve more prosaic kinds of goods, since these are not the worst kinds of rights infringements. So unjust combatants’ associative duties to protect one another and their compatriots, their duties to obey their legitimate governments, and other such considerations, can sometimes make intentionally killing just combatants a permissible lesser evil, and unintentionally killing noncombatants proportionate. This means that the existing laws of war are a closer approximation of combatants’ true moral obligations than many revisionists think. Nonetheless, much of the killing done by unjust combatants in war is still objectively wrong.

The middle path in just war theory depends on showing that killing civilians is worse than killing soldiers. This section discusses arguments to explain why killing civilians is distinctly objectionable. We discuss the significance of intentional killing when considering proportionality, below.

These arguments are discussed at great length in Lazar (2015c), and are presented only briefly here. They rest on a key point: Moral Distinction says that killing civilians is worse than killing soldiers. It does not say that killing civilians is worse than killing soldiers, other things equal . Lazar holds that stronger principle but does not think that the intrinsic differences between killing civilians and killing soldiers—the properties that are necessarily instantiated in those two kinds of killings—are weighty enough to provide Moral Distinction with the kind of normative force needed to protect noncombatants in war. That protection depends on mobilising multiple foundations for Moral Distinction, which include many properties that are contingently but consistently instantiated in acts that kill civilians and kill soldiers, which make killing civilians worse. We cannot ground Moral Distinction in any one of these properties alone, since each is susceptible to counterexamples. But when they are all taken together, they justify a relatively sharp line between harming noncombatants and harming combatants. There are, of course, hard cases, but these must be decided by appealing to the salient underlying properties rather than to the mere fact of membership in one group or the other.

First, at least deliberately killing civilians in war usually fails even the most relaxed interpretation of the necessity constraint. This is not always true—killing is necessary if it is effective at achieving your objective, and no other effective options are available. Killing civilians sometimes meets this description. It is often effective: the blockade of Germany helped end the first world war, though it may have caused as many as half a million civilian deaths; Russian targeting of civilians in Chechnya reduced Russian combatant casualties (Lyall 2009); Taliban anti-civilian tactics have been effective in Afghanistan. And these attacks are often the last recourse of groups at war (Valentino 2004); when all other options have failed or become too costly, targeting civilians is relatively easy to do. Indeed, as recent terrorist attacks have shown (Mumbai and Paris, for example), fewer than ten motivated gunmen with basic weaponry can bring the world’s most vibrant cities screeching to a halt. So, killing civilians can satisfy the necessity constraint. Nonetheless, attacks on civilians are often wholly wanton, and there is a special contempt expressed in killing innocent people either wantonly or for its own sake. At least if you have some strategic goal in sight, you might believe that something is at stake that outweighs the innocent lives taken. Those who kill civilians pointlessly express their total disregard for their victims in doing so.

Second, even when killing civilians is effective, it is usually so opportunistically (Quinn 1989; Frowe 2008; Quong 2009; Tadros 2011). That is, the civilians’ suffering is used as a means to compel their compatriots and their leaders to end their war. Sieges and aerial bombardments of civilian population centres seek to break the will of the population and of their government. Combatants, by contrast, are almost always killed eliminatively —their deaths are not used to derive a benefit that could not be had without using them in this way; instead they are killed to solve a problem that they themselves pose. This too seems relevant to the relative wrongfulness of these kinds of attacks. Of course, at the strategic level every death is intended as a message to the enemy leadership, that the costs of continuing to fight outweigh the benefits. But at the tactical level, where the actual killing takes place, soldiers typically kill soldiers eliminatively, while they kill civilians opportunistically. If this difference is morally important, as many think, and if acts that kill civilians are opportunistic much more often than are acts that kill soldiers, then acts that kill civilians are, in general, worse than acts that kill soldiers. This lends further support to Moral Distinction.

Third, as already noted above, the agent’s beliefs can affect the objective seriousness of her act of killing. Killing someone when you have solid grounds to think that doing so is objectively permissible wrongs that person less seriously than when your epistemic basis for harming them is weaker. More precisely, killing an innocent person is more seriously wrongful the more reason the killer had to believe that she was not liable to be killed (Lazar 2015a).

Last, in ordinary thinking about the morality of war, the two properties most commonly cited to explain the distinctive wrongfulness of harming civilians, after their innocence, are their vulnerability and their defencelessness. Lazar (2015c) suspects that the duties to protect the vulnerable and not to harm the defenceless are almost as basic as the duty not to harm the innocent. (Note that these duties apply only when their object is morally innocent.) Obviously, on any plausible analysis, civilians are more vulnerable and defenceless than soldiers, so if killing innocent people who are more vulnerable and defenceless is worse than killing those who are less so, then killing civilians is worse than killing soldiers.

Undoubtedly soldiers are also often vulnerable too—one thinks of the “Highway of Death”, in Iraq 1991, when American forces destroyed multiple armoured divisions of the Iraqi army, which were completely unprotected (many of the personnel in those divisions escaped into the desert). But this example just shows that killing soldiers, when they are vulnerable and defenceless, is harder to justify than when they are not. Provided the empirical claim that soldiers are less vulnerable and defenceless than civilians is true, this simply supports the case for Moral Distinction.

Holding the principle of Moral Distinction allows one to escape the realist and pacifist horns of the responsibility dilemma, while still giving responsibility its due. Even revisionists who deny moderate Combatant Equality could endorse Moral Distinction, and thereby retain the very plausible insight that it is worse to kill just noncombatants than to kill just combatants. And, if they are to account for most people’s considered judgements about war, even pacifists need some account of why killing civilians is worse than killing soldiers.

However, Moral Distinction is not Discrimination. It is a comparative claim, and it says nothing about intentions. Discrimination, by contrast, prohibits intentionally attacking noncombatants, except in supreme emergencies. It is the counterpart of Proportionality, which places a much weaker bar on unintentionally killing noncombatants. Only a terrible crisis could make it permissible to intentionally attack noncombatants. But the ordinary goods achieved in individual battles can justify unintentional killing. What justifies this radical distinction?

This is one of the oldest questions in normative ethics (though for the recent debate, see Quinn 1989; Rickless 1997; McIntyre 2001; Delaney 2006; Thomson 2008; Tadros 2015). On most accounts, those who intend harm to their victims show them a more objectionable kind of disrespect than those who unavoidably harm them as a side-effect. Perhaps the best case for the significance of intentions is, first, in a general argument that mental states are relevant to objective permissibility (Christopher 1998; see also Tadros 2011). And second, we need a rich and unified theoretical account of the specific mental states that matter in this way, into which intentions fit. It may be that the special prohibition of intentional attacks on civilians overstates the moral truth. Intentions do matter. Other things equal, intentional killings are worse than unintended killings (though some unintended killings that are wholly negligent or indifferent to the victim are nearly as bad as intentional killings). But the difference between them is not categorical. It cannot sustain the contrast between a near-absolute prohibition on one hand, and a sweeping permission on the other.

Of course, this is precisely the kind of nuance that would be disastrous if implemented in international law or if internalized as a norm by combatants. Weighing lives in war is informationally incredibly demanding. Soldiers need a principle they can apply. Discrimination is that principle. It is not merely a rule of thumb, since it entails something that is morally grounded—killing civilians is worse than killing soldiers. But it is also a rule of thumb, because it draws a starker contrast between intended and unintended killing than is intrinsically morally justified.

As already noted, proportionality and necessity contain within them almost every other question in the ethics of war; we now consider two further points.

First, proportionality in international law is markedly different from the version of the principle that first-order moral theory supports. At law, an act of war is proportionate insofar as the harm to civilians is not excessive in relation to the concrete and direct military advantage realized thereby. As noted above, in first-order moral terms, this is unintelligible. But there might be a better institutional argument for this neutral conception of proportionality. Proportionality calculations involve many substantive value judgements—for example, about the significance of moral status, intentions, risk, vulnerability, defencelessness, and so on. These are all highly controversial topics. Reasonable disagreement abounds. Many liberals think that coercive laws should be justified in terms that others can reasonably accept, rather than depending on controversial elements of one’s overarching moral theory (Rawls 1996: 217). The law of armed conflict is coercive; violation constitutes a war crime, for which one can be punished. Of course, a more complex law would not be justiciable, but we also have principled grounds for not basing international law on controversial contemporary disputes in just war theory. Perhaps the current standard can be endorsed from within a wider range of overarching moral theories than could anything closer to the truth.

Second, setting aside the law and focusing again on morality, many think that responsibility is crucial to thinking about proportionality, in the following way. Suppose the Free Syrian Army (FSA) launches an assault on Raqqa, stronghold of ISIL. They predict that they will cause a number of civilian casualties in their assault, but that this is only because ISIL has chosen to operate from within a civilian area, forcing people to be “involuntary human shields”. Some think that ISIL’s responsibility for putting those civilians at risk allows the FSA to give those civilians’ lives less weight in their deliberations than would be appropriate if ISIL had not used them as human shields (Walzer 2009; Keinon 2014).

But one could also consider the following: Even if ISIL is primarily at fault for using civilians as cover, why should this mean that those civilians enjoy weaker protections against being harmed? We typically think that one should only lose or forfeit one’s rights through one’s own actions. But on this argument, civilians enjoy weaker protections against being killed through no fault or choice of their own. Some might think that more permissive standards apply for involuntary human shields because of the additional value of deterring people from taking advantage of morality in this kind of way (Smilansky 2010; Keinon 2014). But that argument seems oddly circular: we punish people for taking advantage of our moral restraint by not showing moral restraint. What’s more, this changes the act from one that foreseeably kills civilians as an unavoidable side-effect of countering the military threat to one that kills those civilians as a means to deter future abuses. This instrumentalizes them in a way that makes harming them still harder to justify.

The foregoing considerations are all also relevant to necessity. They allow us to weigh the harms at stake, so that we can determine whether the morally weighted harm inflicted can be reduced at a reasonable cost to the agents. The basic structure of necessity is the same in bello as it is ad bellum , though obviously the same differences in substance arise as for proportionality. Some reasons apply only to in bello necessity judgements, not to ad bellum ones, because they are conditional on the background assumption that the war as a whole will continue. This means that we cannot reach judgements of the necessity of the war as a whole by simply aggregating our judgements about the individual actions that together constitute the war.

For example, in bello one of the central questions when applying the necessity principle is: how much risk to our own troops are we required to bear in order to minimize harms to the innocent? Some option can be necessary simply in virtue of the fact that it saves some of our combatants’ lives. Ad bellum , evaluating the war as a whole, we must of course consider the risk to our own combatants. But we do so in a different way—we ask whether the goods achieved by the war as a whole will justify putting our combatants at risk. We don’t then count among the goods achieved by the war the fact that multiple actions within the war will save the lives of individual combatants. We cannot count averting threats that will arise only if we decide to go to war among the goods that justify the decision to go to war.

This relates directly to the largely ignored requirement in international law that combatants must

take all feasible precautions in the choice of means and methods of attack with a view to avoiding, and in any event to minimizing, incidental loss of civilian life, injury to civilians and damage to civilian objects. (Geneva Convention, Article 57, 2(a)(ii))

This has deep moral foundations: combatants in war are morally required to reduce the risk to innocents until doing so further would involve an unreasonably high cost to them, which they cannot be required to bear. Working out when that point is reached involves thinking through: soldiers’ role-obligations to assume risks; the difference between doing harm to civilians and allowing it to happen to oneself or one’s comrades-in-arms; the importance of associative duties to protect one’s comrades; and all the considerations already adduced in favour of Moral Distinction. This calculus is very hard to perform. My own view is that combatants ought to give significant priority to the lives of civilians (Walzer and Margalit 2009; McMahan 2010b). This is in stark contrast to existing practice (Luban 2014).

Much recent work has used either traditionalist or revisionist just war theory to consider new developments in the practice of warfare, especially the use of drones, and the possible development of autonomous weapons systems. Others have focused on the ethics of non-state conflicts, and asymmetric wars. Very few contemporary wars fit the nation-state model of the mid-twentieth century, and conflicts involving non-state actors raise interesting questions for legitimate authority and the principle of Discrimination in particular (Parry 2016). A third development, provoked by the terrible failure to plan ahead in Iraq and Afghanistan, is the wave of reflection on the aftermath of war. This topic, jus post bellum , is addressed separately.

As to the philosophical foundations of just war theory: the traditionalist and revisionist positions are now well staked out. But the really interesting questions that remain to be answered should be approached without thinking in terms of that split. Most notably, political philosophers may have something more to contribute to the just war theory debate. It would be interesting, too, to think with a more open mind about the institutions of international law (nobody has yet vindicated the claim that the law of armed conflict has authority, for example), and also about the role of the military within nation-states, outside of wartime (Ryan 2016).

The collective dimensions of warfare could be more fully explored. Several philosophers have considered how soldiers “act together” when they fight (Zohar 1993; Kutz 2005; Bazargan 2013). But few have reflected on whether group agency is present and morally relevant in war. And yet it is superficially very natural to discuss wars in these terms, especially in evaluating the war as a whole. When the British parliament debated in late 2015 whether to join the war against ISIL in Syria and Iraq, undoubtedly each MP was thinking also about what she ought to do. But most of them were asking themselves what the United Kingdom ought to do. This group action might be wholly reducible to the individual actions of which it is composed. But this still raises interesting questions: in particular, how should I justify my actions, as an individual who is acting on behalf of the group? Must I appeal only to reasons that apply to me? Or can I act on reasons that apply to the group’s other members or to the group as a whole? And can I assess the permissibility of my actions without assessing the group action of which they are part? Despite the prominence of collectivist thinking in war, discussion of war’s group morality is very much in its infancy.

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How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

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• The International Committee of the Red Cross on War and Law
• Rome Statute of the International Criminal Court (1998)
• Ethical War Blog
• The Stockholm Centre for the Ethics of War and Peace
• Orend, Brian, “War,” Stanford Encyclopedia of Philosophy (Spring 2016 Edition), Edward N. Zalta (ed.), URL = < https://plato.stanford.edu/archives/spr2016/entries/war/ >. [This was the previous entry on War in the Stanford Encyclopedia of Philosophy — see the version history .]

authority | justice: global | justice: transitional | pacifism | political realism: in international relations | responsibility: collective | rights: human | sovereignty | world government

Acknowledgments

Many thanks to Thomas Pogge for his comments on this entry, which were a great benefit throughout. This entry draws on all my work in just war theory, and so I owe a great debt to the many philosophers who have contributed so much to my understanding of these issues, both in their published work and in conversation. Most of the people in the bibliography deserve a mention, but I reserve particular thanks for Henry Shue, Jeff McMahan, and David Rodin, for setting me on this path.

Copyright © 2016 by Seth Lazar < seth . lazar @ anu . edu . au >

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Article contents

The conduct and consequences of war.

• Alyssa K. Prorok Alyssa K. Prorok Department of Political Science, University of Illinois at Urbana-Champaign
•  and  Paul K. Huth Paul K. Huth Department of Government and Politics, University of Maryland
• https://doi.org/10.1093/acrefore/9780190846626.013.72
• Published in print: 01 March 2010
• Published online: 22 December 2017
• This version: 25 June 2019
• Previous version

The academic study of warfare has expanded considerably over the past 15 years. Whereas research used to focus almost exclusively on the onset of interstate war, more recent scholarship has shifted the focus from wars between states to civil conflict, and from war onset to questions of how combatants wage and terminate war. Questioned as well are the longer-term consequences of warfare for countries and their populations. Scholarship has also shifted away from country-conflict-year units of analysis to micro-level studies that are attentive to individual-level motives and explanations of spatial variation in wartime behavior by civilians and combatants within a country or armed conflict. Today, research focuses on variations in how states and rebel groups wage war, particularly regarding when and how wars expand, whether combatants comply with the laws of war, when and why conflicts terminate, and whether conflicts end with a clear military victory or with a political settlement through negotiations. Recent research also recognizes that strategic behavior continues into the post-conflict period, with important implications for the stability of the post-conflict peace. Finally, the consequences of warfare are wide-ranging and complex, affecting everything from political stability to public health, often long after the fighting stops.

• interstate war
• laws of war
• civilian victimization
• war termination
• war severity
• post-conflict peace

Updated in this version

Updated introduction, subheadings, references, and substantial revision throughout.

Introduction

Over the past 15 years, research by social scientists on the conduct and consequences of war has expanded considerably. Previously, scholarly research had been heavily oriented towards the analysis of the causes of interstate war and its onset. Three simultaneous trends, however, have characterized scholarship on war since the early 2000s. First, studies of the dynamics of civil war have proliferated. Second, war is conceptualized as a series of inter-related stages in which the onset, conduct, and termination of wars as well as post-war relations are analyzed theoretically and empirically in a more integrated fashion. Third, studies have shifted away from country-conflict-year units of analysis to micro-level studies that are sensitive to spatial variation in behavior within a country or conflict.

This article reviews and assesses this body of recent scholarship, which has shifted the focus from war onset to questions of how combatants wage war and what are the longer-term consequences of warfare for countries and their populations. Scholarly research examines the conduct and consequences of both interstate and civil wars.

The analysis is organized into three main sections. It begins with research on how states and rebel groups wage war, with particular attention given to questions regarding war expansion, compliance with the laws of war, and war severity. Section two turns to the literature on war duration, termination, and outcomes. Different explanations are discussed, for when and why wars come to an end; then, the question of how war’s end influences the prospects for a stable post-war peace is considered. In section three, recent scholarship is examined on the consequences of war for post-war trends in political stability and public health. The concluding discussion addresses some of the important contributions associated with recent scholarship on the conduct and consequences of war as well as promising directions for future research.

The Waging of Civil and International Wars

What accounts for the nature of the wars we see? This broad question drives a new research tradition in conflict studies that compliments traditional analyses of war onset by shifting the focus to state behavior during war. This research goes beyond understandings of why states fight one another to engaging questions of why states join ongoing wars, when and why they follow the laws of war, and what explains the severity of wars. Taken together, these questions open the black box of wartime behavior.

Intervention and the Expansion of Interstate Wars

Research on war expansion developed as a natural outgrowth of analyses of war onset: scholars studying why states initiate conflict shifted focus to understand why third parties join ongoing wars. The link between alliances and joining behavior has been central to studies of war expansion, spawning a broad research tradition that focuses on alliances and geography, differences among types of alliances, and the characteristics of alliance members. Siverson and Starr ( 1991 ), for example, find a strong interaction effect between geography and alliances, in that a warring neighbor who is an ally increases the likelihood of a state joining an existing conflict. Leeds, Long, and Mitchell ( 2000 ) also find that the specific content of alliance obligations is critical to understanding when states choose to intervene, and that states uphold the terms of their alliance commitments nearly 75% of the time. Most recently, Vasquez and Rundlett ( 2016 ) found that alliances are essentially a necessary condition for war expansion, highlighting the importance of this factor in explaining joining behavior.

Alliance behavior is also an important topic in the study of democratic wartime behavior. While Choi ( 2004 ) presents findings suggesting that democracies are particularly likely to align with one another, Reiter and Stam ( 2002 ) provide counter-evidence that democracies are willing to align with non-democracies when it serves their strategic interests. Given the tendency to uphold alliance obligations, and empirical evidence showing that war initiators are more successful when their adversary does not receive third-party assistance (Gartner & Siverson, 1996 ), recent theoretical research suggests that states, understanding joining dynamics, might manipulate war aims to reduce the likelihood of outside intervention (Werner, 2000 ).

These studies suggest that war expansion should be understood as the consequence of a decision calculus undertaken by potential joiners. While much of the contemporary literature focuses on alliance behavior, this only indirectly gets at the question of who will join ongoing conflicts. A full explanation of war expansion from this perspective would also require that we explain when states form alliances in the first place. Further, the analyses of Gartner and Siverson ( 1996 ) and of Werner ( 2000 ) suggest that strategic thinking must be the focus of future research on war expansion. Recent research begins to address this issue: DiLorenzo and Rooney ( 2018 ) examine how uncertainty over estimates of third party resolve influence war-making decisions of states, finding that rival states are more likely to initiate conflict when domestic power shifts in potential joiner states (i.e., allies) increase uncertainty over the strength of that alliance commitment. Future research should continue to investigate the links between expectations of third-party behavior and initial war initiation decisions, as this research highlights important selection processes that empirical research has not yet fully explored.

Finally, recent research goes further to connect war initiation and expansion by arguing that commitment problems—one of the key bargaining failures leading to war initiation—also helps explain war expansion. Shirkey ( 2018 ) finds that wars caused by commitment rather than information problems are more likely to expand, as they are generally fought over greater war aims, are more severe, and last longer. These factors generate risks and rewards for intervention that encourage expansion.

The literature on interstate war expansion has made progress in the last decade with much closer attention to modeling strategic calculations by combatants and potential interveners. The result has been a better understanding of the interrelationship between onset and joining behavior and the realization that the timing and the sequence in which sides intervene is critical to war expansion (Joyce, Ghosn, & Bayer, 2014 ).

Expansion of Civil Wars

The analog to studies of war expansion in the interstate context has traditionally been the study of intervention in the civil war context. Research in this field treats the decision to intervene in much the same way as the war expansion literature treats the potential joiner’s decision calculus. That is, intervention is the result of a rational, utility-maximizing decision calculus in which potential interveners consider the costs and benefits of intervention as well as the potential for achieving desired outcomes. Understood in these terms, both domestic and international strategic considerations affect the decision to intervene, with the Cold War geopolitical climate much more conducive to countervailing interventions than the post-Cold War era has been (Regan, 2002a ), and peacekeeping-oriented interventions most likely in states with ethnic, trade, military, or colonial ties to the intervening state (Rost & Greig, 2011 ).

Whether states are most likely to intervene in easy or hard cases is a central question. While Aydin ( 2010 ) showed that states will delay intervention when previous interventions by other states have failed to influence the conflict, Rost and Grieg ( 2011 ) showed that state-based interventions for peacekeeping purposes are most likely in tough cases—long ethnic wars and conflicts that kill and displace large numbers of civilians. Finally, Gent ( 2008 ) shows that the likelihood of success may not affect the intervention decision equally for government and opposition-targeted interventions. He finds that both types of intervention are more likely when governments face stronger rebel groups, thus implying that intervention in support of rebel groups occurs when the likelihood of success is highest, but intervention supporting governments is most likely when states face their most intense challenges.

There are two likely sources of the discrepancies in this literature. First, most analyses have focused exclusively on the intervener’s decision calculus, or the supply side, failing to account for variation in the demand for intervention. Second, there is significant inconsistency in the literature’s treatment of the goals of interveners. Some analyses assume that states intervene to end conflicts, while others don’t make this limiting assumption but still fail to distinguish among interventions for different purposes.

Newer research takes important strides to address these issues. First, Salehyan, Skrede Gleditsch, and Cunningham ( 2011 ) developed a theory of third party support for insurgent groups that explicitly modeled both supply-side and demand-side factors driving the intervention decision. They found that demand is greatest among weak rebel groups, but supply is greatest for strong groups. Second, research by Cunningham ( 2010 ) explicitly measured whether third party states intervene with independent goals, and Stojek and Chacha ( 2015 ) theorized that intervention behavior is driven by economic motivations. Trade ties increase the likelihood of intervention on the side of the government.

Finally, Kathman ( 2010 ) focused on contiguous state interveners in examining motives for intervention. He developed a measure of conflict infection risk that predicts the likelihood of conflict spreading to each contiguous state. Empirically, he finds that, as the risk of contagion increases, so does the probability of intervention by at-risk neighbors. This research develops a convincing mechanism and empirical test to explain a subset of interventions and provides a clear link from intervention research to recent research on civil conflict contagion. While the contagion literature is too broad to review here, mechanisms posited for civil war expansion across borders range from refugee flows (Salehyan & Gleditsch, 2006 ), to ethnic kinship ties (Forsberg, 2014 ), to increased military expenditures in neighboring states (Phillips, 2015 ).

The literature on intervention into civil wars has grown significantly over the past decade as internationalization of civil conflicts has become common and often results in escalatory dynamics that are of deep concern to analysts and policymakers.

Compliance With the Laws of War

Scholars have recently begun studying the conditions under which compliance with the laws of war is most likely and the mechanisms most important in determining compliance. This research shifts the focus toward understanding state behavior during war and the strategic and normative considerations that influence decision-making processes of states. Two key questions drive scholarship in this tradition; first, does international law constrain state behavior, even when the state is threatened by severe conflict, and second, can observed compliance be attributed to ratification status, or is it instead a result of strategic decision making?

Scholars have yet to provide conclusive answers to these questions; while compliance is observed in many circumstances, most scholars attribute observed restraint to factors other than international law. Legro ( 1995 ), for example, found that international agreements had limited impact on Britain and Germany’s use of unrestricted submarine warfare, strategic bombing of civilian targets, and chemical weapons during WWII. In analyses of civilian targeting during interstate war, Downes ( 2006 ) and Valentino, Huth, and Croco ( 2006 ) also found that international law itself has little impact on a state’s propensity for civilian targeting. Downes argued that civilian targeting occurs most often when states are fighting protracted wars of attrition and desire to save lives on their own side, or when they intend to annex enemy territory with potentially hostile civilians. Valentino et al. ( 2006 ) similarly found that the decision to target civilians is driven by strategic considerations and is unconstrained by treaty obligations relating to the laws of war. Finally, Fazal and Greene ( 2015 ) found that observed compliance is explained by identity rather than law; violations are much more common in European vs. non-European dyads than in other types of dyads.

While these analyses suggest that international law has little effect on state behavior and that observed compliance is incidental, Price ( 1997 ) and Morrow ( 2014 ) argued that law does exert some influence on compliance behavior. Price attributed variation in the use of chemical weapons to the terms of international agreements, arguing that complete bans are more effective than partial bans. Morrow ( 2014 ), however, demonstrated that law’s impact varies depending upon issue area, regime characteristics, and adversary identity. Of eight issue areas, he found the worst compliance records on civilian targeting and prisoners of war, which perhaps accounts for the largely negative conclusions drawn by Downes ( 2006 ) and Valentino et al. ( 2006 ). Additionally, Morrow found, unlike Valentino et al., that democratic states are more likely to comply after ratification than before, suggesting that obligations under international law do affect state behavior, at least in democracies. Finally, he demonstrated that compliance increases significantly when an adversary has also ratified a given treaty, arguing this effect is due to reciprocity.

More recent scholarship expands this research, showing that law may affect state behavior through additional mechanisms that previous research had not considered. For example, Kreps and Wallace ( 2016 ) and Wallace ( 2015 ) found that public support for state policies as diverse as drone strikes and torture of prisoners of war are critically influenced by international law. International condemnation of U.S. policies reduces public support most when such condemnation focuses on legal critiques. This suggests that international law influences state behavior in democracies through its effect on public opinion, not through liberal norms of nonviolence. Additionally, Appel and Prorok ( 2018 ) and Jo and Thompson ( 2014 ) showed that external constraints influence states’ compliance behavior. Specifically, Appel and Prorok showed that states target fewer civilians in interstate war when they are embedded in alliance and trade networks dominated by third party states who have ratified international treaties prohibiting the abuse of non-combatants during war. Jo and Thompson showed that states are more likely to grant international observers access to detention centers when they are more reliant upon foreign aid. These findings suggest that international law can influence state behavior indirectly, through pressure exerted by international donors and backers.

Scholarship on compliance with the laws of war in interstate wars has made considerable progress over the past decade. We now know much more about the contingent support of democratic state leaders and publics for compliance with the laws of war. This key finding opens up new areas of research on the strategic efforts of political and military leaders to convince publics of their commitment to international law and whether those strategies are likely to be successful.

Civilian Targeting in Civil War

The mistreatment and deliberate targeting of civilian populations is an active area of research by scholars who study civil wars (Hultman, 2007 ; Humphreys & Weinstein, 2006 ; Kalyvas, 2006 ; Valentino et al., 2004 ; Weinstein, 2007 ; Wickham-Crowley, 1990 ). Most research on this topic treats the use of violence against civilians as a strategic choice; that is, combatants target civilians to induce their compliance, signal resolve, weaken an opponent’s support base, or extract resources from the population. In his seminal work on the topic, Kalyvas ( 2006 ) demonstrated that combatants resort to the use of indiscriminate violence to coerce civilian populations when they lack the information and control necessary to target defectors selectively. Similarly, Valentino ( 2005 ) and Valentino et al. ( 2004 ) found that incumbents are more likely to resort to mass killing of civilians when faced with strong insurgent opponents that they are unable to defeat through more conventional tactics.

More recent analyses have built upon these earlier works, adding levels of complexity to the central theories developed previously and examining new forms of violence that previous studies did not. Balcells ( 2011 ) brought political considerations back in, finding that direct violence is most likely in areas where pre-conflict political power between state and rebel supporters was at parity, while indirect violence is most likely in locations where the adversary’s pre-war political support was highest. Wood ( 2010 ) accounted for the impact of relative strength and adversary strategy, finding that weak rebel groups, lacking the capacity to protect civilian populations, will increase their use of violence in response to state violence, while strong rebel groups display the opposite pattern of behavior. Lyall ( 2010a ) also found conditionalities in the relationship between state behavior and insurgent reactions, demonstrating that government “sweep” operations are much more effective at preventing and delaying insurgent violence when carried out by forces of the same ethnicity as the insurgent group. Finally, Cohen ( 2016 ) advanced research by focusing on wartime sexual violence. She found that rape, like other forms of violence, is used strategically in civil war. Specifically, armed groups use rape as a socialization tactic: groups that recruit through abduction engage in rape at higher rates, to generate loyalty and trust between soldiers.

This large body of research provides many insights into the strategic use of violence against civilians during civil war. However, until recently, little research addressed questions of compliance with legal obligations. With the recent formation of the International Criminal Court, however, states and rebel groups are now subject to legal investigation for failure to comply with basic principles of the laws of war.

Emerging research suggests that the International Criminal Court (ICC) and international law more generally do affect the behavior of civil war combatants. For example, Hillebrecht ( 2016 ) found that ICC actions during the Libyan civil war reduced the level of mass atrocities committed in the conflict, while Jo and Simmons ( 2016 ) found that the ICC reduces civilian targeting by governments and rebel groups that are seeking legitimacy, suggesting international legal institutions can reduce violations of humanitarian law during civil war. These findings should be tempered, however, by recent research suggesting that ICC involvement in civil wars can, under certain conditions, extend ongoing conflicts (Prorok, 2017 ).

Finally, beyond the ICC, Stanton ( 2016 ) and Jo ( 2015 ) both demonstrated that international law constrains civil war actors by establishing standards against which domestic and international constituencies judge the behavior of governments and rebel groups. Particularly when rebels are seeking legitimacy, Jo argues, they are more likely to comply with international legal standards in a variety of areas, from protection of civilian populations to child soldiering. This research suggests that even without direct intervention by the ICC, international law can influence the behavior of governments and rebels engaged in civil war.

While recent research has shown that the laws of war can influence civilian targeting in civil wars, the large loss of civilian life in the Syrian civil war highlights how fragile the commitment to international law can be. It points to important future research questions about when threats of various sanctions by the international community against non-compliance are actually credible and which actors can apply effective coercive pressure.

Losses Suffered in Wars

Recent scholarship has taken up the issue of war severity. Empirical research suggests that the tactics and strategies used by states during war, and the political pressures that compel them to adopt those policies, affect the severity of conflict. Biddle ( 2004 ), for instance, argued that war-fighting strategies influence the magnitude of losses sustained during war, and found that states employing the modern system of force reduce their exposure to lethal firepower, thus limiting losses. Valentino, Huth, and Croco ( 2010 ) examined the reasons behind different strategic choices, arguing that democratic sensitivity to the costs of war pressure democratic leaders to adopt military policies designed to limit fatalities. They found that increasing military capabilities decreases civilian and military fatalities, while reliance on guerrilla or attrition strategies, as well as fighting on or near one’s own territory, increases fatalities. They reported that democracies are significantly more likely to join powerful alliances and less likely to use attrition or guerrilla strategies, or to fight on their own territory.

Speaking to the conventional wisdom that interstate warfare is on the decline, recent research by Fazal ( 2014 ) suggests that modern medical advances mean that, while war has become less fatal, it has not necessarily become less severe. This raises questions about common understandings of broad trends in conflict frequency and severity as well as questions about best practices for measuring conflict severity. Future research should grapple with both of these issues.

Civil war studies have recently begun to focus more on conflict severity as an outcome in need of explanation. Many key explanatory factors in early research mirrored those in interstate war research, making comparison possible. For example, like interstate war, civil war scholarship consistently finds that democracies suffer less severe conflicts than nondemocracies (Heger & Salehyan, 2007 ; Lacina, 2006 ; Lujala, 2009 ). Regarding state military strength, research by Lujala ( 2009 ) demonstrated that relative equality between government and rebel forces leads to the deadliest conflicts, as rebels with the strength to fight back will likely inflict more losses than those without the ability to sustain heavy engagement with government forces. Finally, recent research by Balcells and Kalyvas ( 2014 ) mirrored work on interstate war by focusing on how the military strategies adopted by combatants affect conflict intensity. They found that civil conflicts fought via conventional means tend to be more lethal than irregular or symmetric nonconventional (SNC) wars, as only the former involve direct confrontations with heavy weaponry. While research on conflict severity is still developing, these studies suggest that democracy, military strength, and strategy are consistent predictors of conflict severity, although the mechanisms posited for the effects of these variables sometimes differ between civil and interstate war.

What this research does not provide clear answers on is how battle losses trend throughout the course of conflict, as most factors examined in the above research are static throughout a conflict. As our ability to measure conflict severity at a more micro temporal and spatial level has improved, emerging research is beginning to address these questions. For example, Hultman, Kathman, and Shannon ( 2014 ) find that increasing UN troop presence decreases battlefield deaths by increasing the costs of perpetrating violence. Dasgupta Gawande, and Kapur ( 2017 ) also found reductions in insurgent violence associated with implementation of development programs, though the pacifying effects of such programs are conditional upon local state capacity. Additional research shows that trends in violence in Islamist insurgencies vary predictably, with violence suppressed due to anticipated social disapproval during important Islamic holidays (Reese, Ruby, & Pape, 2017 ). Recent research also suggests local variation in cell-phone coverage affects local levels of insurgent violence, as increasing cell-phone communication improves the state’s ability to gather information and monitor insurgent behavior, thereby reducing insurgent violence (Shapiro & Weidmann, 2015 ). These recent studies represent an important trend in conflict severity research that more carefully examines the dynamics of escalation and de-escalation within given conflicts, both spatially and temporally. We encourage additional research in this vein.

The Duration, Termination, and Outcome of War

What accounts for the duration, termination, and outcomes of interstate and civil wars, and the durability of the peace that follows these conflicts? These questions represent a central focus of contemporary conflict studies, and are closely linked in terms of their explanations. A major innovation in this literature in the past 10 to 15 years has been the extension of the bargaining model of war from its original application in the context of war onset (Blainey, 1973 ; Fearon, 1995 ) to its use in the context of war duration, termination, and outcome.

The turn to bargaining models has placed relative military capabilities and battlefield developments at the center of much of the theoretical literature in this area. This focus, however, has spawned a backlash in recent years, as patterns that contradict the implications of bargaining models are detected and theorized. The bargaining approach and its critiques are discussed in the following sections.

Duration of Wars

Understood within the bargaining framework, war duration is closely linked to factors that influence the relative strength of combatants. Theoretical and empirical research suggests that longer wars occur when opponents of relatively equal strength cannot achieve breakthroughs on the battlefield (Bennett & Stam, 1996 ; Filson & Werner, 2007b ; Slantchev, 2004 ), although this pattern does not hold for wars involving non-state actors where a large asymmetry in power increases war duration (Sullivan, 2008 ).

Additional research suggests, however, that relative military strength may not be the best predictor of war duration. Bennett and Stam ( 1996 ), for example, demonstrated that military strategy has a large impact on war duration, independent of military strength, with attrition and punishment strategies leading to longer wars than maneuver strategies. The type of political objectives sought by a war initiator may also offset the impact of military strength, as war aims that require significant target compliance generally lead to longer wars (Sullivan, 2008 ). Still others argue that domestic political sensitivity to concessions-making increases conflict duration, while domestic cost sensitivity leads to shorter wars (Filson & Werner, 2007a ; Mattes & Morgan, 2004 ). Thus, democracies are expected to fight shorter wars (Filson & Werner, 2007b ), whereas mixed regimes will fight longer wars as they gamble for resurrection in the face of high domestic costs for war losses (Goemans, 2000 ). Research by Lyall ( 2010b ), however, suggests that this relationship is conditional upon conflict type, as he found no relationship between democracy and war duration in the context of counterinsurgency wars.

Biddle ( 2004 ) more directly challenged bargaining models of war duration by comparing the predictive power of models including traditional measures of relative military capabilities to those accounting for combatants’ methods of force employment. Biddle demonstrated that models taking force employment into account generate more accurate predictions of war duration than those assuming an unconditional relationship between military power and war duration. A second important challenge to traditional applications of bargaining models comes from Reiter ( 2009 ). He demonstrated that the argument that decisive battlefield outcomes promote quick termination is conditional upon the absence of commitment problems. When compliance fears dominate information asymmetries, battle losses and the expectation of future losses may not be sufficient to end conflict, as belligerents will continue fighting in pursuit of absolute victory to eliminate the threat of the losing state defecting from post-war settlements. Reiter thus demonstrates that commitment problems and information asymmetries have varying effects on war duration, and both must be accounted for in models of conflict duration and termination.

Despite these critiques, more recent research continues to approach the question of war duration from the bargaining perspective. Shirkey ( 2012 ), for example, argued that late third-party joiners to interstate conflicts lengthen those disputes by complicating the bargaining process. Joiners add new issues to the war and increase uncertainty about relative power among combatants, thus requiring additional fighting to reveal information and find a bargained solution. Weisiger ( 2016 ) similarly focused on information problems, but attempts to unpack the mechanism by focusing on more specific characteristics of battlefield events. Using new data on the timing of battle deaths for specific war participants, Weisiger found that settlement is more likely after more extensive fighting, and that states are more likely to make concessions after their battle results have deteriorated. Finally, recent research has also begun to problematize resolve, considering how variation in actors’ resolve affects their willingness to stay in a fight or cut losses (Kertzer, 2017 ). This represents a fruitful area for future research, as conceptually and empirically unpacking resolve will shed new light on costs of war and how they relate to war onset, duration, and termination.

Scholars studying the duration of civil wars also commonly apply a rationalist perspective. Factors that increase the costs of sustaining the fight generally shorten wars, while those that raise the costs of making concessions tend to lengthen conflicts. Along these lines, research suggests that the availability of contraband funding for rebel groups lengthens conflicts by providing rebels with the economic resources to sustain their campaigns (Fearon, 2004 ). However, additional research demonstrates that the influence of contraband is mitigated by fluctuations in its market value (Collier, Hoeffler, & Söderbom, 2004 ), by how rebels earn funding from resources (through smuggling versus extortion; Conrad, Greene, Igoe Walsh, & Whitaker, 2018 ), and by the composition of state institutions (Wiegand & Keels, 2018 ).

Research suggests that structural conditions also affect civil war duration, such as the stakes of war, ethnic divisions, and the number of combatants involved. For example, ethnic conflicts over control of territory are generally longer than those fought over control of the central government (Balch-Lindsay & Enterline, 2000 ; Collier et al., 2004 ; Fearon, 2004 ). Regarding the role of ethnicity, Wucherpfennig, Metternich, Cederman, and Skrede Gleditsch ( 2012 ) demonstrated that the effect of ethnic cleavages is conditional on their relationship to political institutions. Regarding the complexity of the conflict, Cunningham ( 2011 ) found that civil wars with a greater number of combatants on each side are longer than those with fewer combatants. Findley ( 2013 ), however, showed that the number of conflict actors has varying effects across different stages of conflict, encouraging cooperation early on while impeding lasting settlement.

Third party intervention has also received significant attention in the civil war duration literature, with scholars generally arguing that intervention affects duration by augmenting the military strength of combatants. Empirical findings in early studies are mixed, however; while results consistently show that unbiased intervention or simultaneous intervention on both sides of a conflict increase war duration (Balch-Lindsay & Enterline, 2000 ; Balch-Lindsay, Enterline, & Joyce, 2008 ; Regan, 2002b ), biased interventions generate more inconsistent results.

In a valuable study addressing limitations of earlier research, Cunningham ( 2010 ) focused on the goals of third parties, and found that when interveners pursue agendas that are independent of those of the internal combatants, wars are more difficult to terminate due to decreased incentives to negotiate and a higher likelihood that commitment problems stymie settlements. This suggests that the empirical finding that intervention lengthens war may be driven by a subset of cases in which third parties intervene with specific goals. Ultimately, analyses focused on intervention do not account for the potential selection effect that influences when states will intervene. If Gent ( 2008 ) is correct, biased intervention should be most likely when the power ratio between government and rebel forces is close to parity, a factor which, if ignored, may bias the results of these analyses.

More recent studies have continued to unpack intervention, demonstrating that there are important distinctions beyond the biased versus balanced debate. Sawyer, Cunningham, and Reed ( 2015 ), for example, showed that different types of external support affect rebel fighting capacity differently. Specifically, fungible types of support like financial and arms transfers are particularly likely to lengthen conflict because they increase uncertainty over relative power. Similarly, Narang ( 2015 ) also focused on the uncertainty induced by external support. He showed that humanitarian assistance inadvertently increases both actors’ uncertainty over relative power, thereby prolonging civil war.

Until recently, this literature suffered from a major weakness in that it relied empirically on state-level variables that did not fully capture the dyadic nature of its theoretical propositions. Cunningham, Skrede Gleditsch, and Salehyan ( 2013 ) new dyadic data represents an important contribution to the field, as it explicitly measures the relative strength, mobilization capacity, and fighting capacity of rebel groups and applies a truly dyadic empirical approach. New research in this field should continue to approach questions of war duration and outcome with dyadic data and theory along with more micro-level studies that seek to explain variation in rebel and state fighting across different geographic locations and over time (e.g., Greig, 2015 ).

Ending Wars as a Bargaining Process

Interstate wars rarely end in the complete destruction of the defeated party’s military forces. Instead, new information is revealed through combat operations and negotiating behavior which enables belligerents to converge on a mutually agreeable settlement short of total war. Wittman ( 1979 ) provided the first formal articulation of the bargaining model in the context of war termination. He argued theoretically that war continues until both adversaries believe they can be made better off through settlement. Subsequent analyses have focused on both the battlefield conditions and strategies of negotiations leading states to believe settlement is the better option.

These analyses show that, as a state’s resources are depleted from battle losses, it has incentives to negotiate a settlement more acceptable to its adversary rather than suffer total defeat (Filson & Werner, 2002 ; Smith & Stam, 2004 ). Further, fighting battles reduces uncertainty by revealing information about resolve, military effectiveness, and the true balance of power between adversaries, causing expectations on the likely outcome of the war to converge, and making settlement possible (Wagner, 2000 ). Wartime negotiations provide adversaries with additional information, which Slantchev ( 2011 ) argued makes war termination more likely.

Challenging traditional notions regarding the likelihood of termination in the face of large asymmetries in capabilities, Slantchev ( 2011 ) argued that war termination depends upon states’ abilities to both impose and bear the costs of fighting. If a weaker state can minimize the costs it bears while forcing its adversary to expand its war effort, the benefits of fighting relative to its costs are reduced, and the stronger state may choose termination. The implication of this argument relates closely to Biddle’s ( 2004 ) empirical critique of the bargaining literature, which finds modern methods of force employment can mitigate losses during war, thereby shifting the balance of costs and benefits independent of relative military capabilities. Reiter’s ( 2009 ) critique of bargaining approaches also has implications for war termination. While traditional approaches argue that fighting battles reveals information and increases the likelihood of termination, Reiter suggested that this is only the case if belligerents expect their opponent to comply with the post-war status quo. If commitment problems are severe, information revealed during battles and war-time negotiations will have little effect on termination.

Biddle’s argument that country-year measures of military capabilities are inexact and crude proxies for the concepts advanced in theoretical models is a strong one that should be taken seriously by scholars. We therefore appreciate the contributions of Ramsay ( 2008 ) and Weisiger ( 2016 ), which use more fine-grained battle trend data rather than country-level measures of military capabilities to empirically test the implications of bargaining theories of war termination, and advocate future research adopting this strategy for testing the implications of bargaining theories.

Much of the literature on civil war termination also focuses on how battlefield developments affect the termination of civil wars. Collier et al. ( 2004 ) built on the idea of war as an information revelation mechanism, arguing that the probability of settlement should increase as war duration increases and more information is revealed regarding the relative strength of each side. Others focus on the costs of battle, with research showing that settlements are more likely when the costs of battle are high and the relative payoffs from victory decrease (Walter, 2002 ). Also, a relatively equal balance of power between combatants creates a mutually hurting stalemate, in which neither side can achieve victory, and settlement becomes more likely (Walter, 2002 ).

Empirical results support many of these theoretical predictions. Several scholars show that the longer a civil war lasts, the more likely it is to terminate (Collier et al., 2004 ; Fearon,, 2004 ; Regan, 2002b ), and that the probability of negotiated settlement increases as conflict duration increases (Mason, Weingarten, & Fett, 1999 ). The magnitude of conflict, measured as total war deaths, also correlates positively with the probability of adversaries initiating negotiations (Walter, 2002 ). Finally, Walter ( 2002 ) found that military stalemates significantly increase the likelihood of negotiations as well as the implementation of a ceasefire.

While these results support the theoretical predictions surrounding “hurting stalemates,” Walter’s coding of stalemates does not account for the timing of the stalemate or the number of stalemates that occur throughout the course of conflict. We therefore see great value in more recent research that uses new micro-level data to more closely capture actual battle dynamics and incorporate more information at the conflict and group-level. For example, Hultquist ( 2013 ) used a novel troop strength measure to better capture relative strength between rebel and government forces. He found that relative power parity increases the likelihood of negotiated settlement, while power imbalances extend civil war. Making use of fine-grained data on battle event dates and locations, Greig ( 2015 ) showed that the location, and changes in location over time, of battle events relays information to combatants that, in turn, affects their willingness to negotiate and settle their conflicts. We encourage additional research in this vein moving forward.

Domestic-Level Factors and War Termination

Recent research suggests that domestic political conditions influence war termination. Specifically, domestic political accountability, the domestic audience’s expectations, and cost-sensitivity affect leaders’ decisions to continue fighting versus settling on specific terms (Mattes & Morgan, 2004 ). Along these lines, Goemans ( 2000 ) argued that the postwar fate of leaders influences their choice between terminating and continuing a war. The threat of severe punishment by domestic actors increases the costs of war losses for leaders of semi-repressive regimes, leading them to continue fighting a war they are losing in the hope of achieving victory. Thus, war termination does not follow strictly from battle trends.

Empirically, Goemans ( 2000 ) found that losing mixed regimes suffer significantly more battle deaths than democratic or autocratic losers, and that wars fought against losing mixed regimes last, on average, almost twice as long as those fought against either democratic or autocratic losers. Taken together, these results suggest that mixed regime leaders are likely to sustain rather than terminate a losing war, and more generally, that regime type significantly influences war termination. Croco ( 2015 ) refined Goemans’s work by arguing that the individual responsibility of leaders for involving their country in a war has important effects on war termination patterns, with culpable leaders more likely to fight for victory in order to avoid being punished domestically for poor wartime performance. Croco and Weeks ( 2013 ) refined this logic further, showing that only culpable leaders from democracies and vulnerable nondemocracies face increased punishment risk from war losses. Koch and Sullivan ( 2010 ) provide another take on the relationship between domestic politics and war termination, demonstrating that partisanship significantly affects democratic states’ war termination decisions. Faced with declining approval for military interventions, their results demonstrate, right-leaning governments will continue the fight, while left-leaning executives will be more likely to end their military engagements.

The analog to studying domestic-level factors in interstate conflict would be to examine the effect of internal state and rebel characteristics on civil war termination. Traditionally, civil war studies have focused only on state characteristics, as data on rebel groups’ organization and internal characteristics has been unavailable. Early research argued that state capacity, regime characteristics, and ethnic/religious divisions influenced war termination by influencing the balance of power, accountability of leaders, and stakes of conflict, but empirical results provided mixed support for these theories (e.g., DeRouen & Sobek, 2004 ; Svensson, 2007 ; Walter, 2002 ).

More recent research has made significant strides in understanding how internal characteristics of combatants affect civil conflict termination by using new data to explore how the composition and practices (i.e., leader characteristics, governance, and internal cohesion) of rebel groups influence civil conflict dynamics. This research demonstrates that some of the same leader-accountability mechanisms that affect interstate war termination also influence civil conflict. For example, Prorok ( 2016 ) used novel data on rebel group leaders to show that culpable leaders are less willing to terminate or settle for compromise outcomes than their non-culpable counterparts in civil wars, just like in interstate conflicts. Heger and Jung ( 2017 ) also advanced existing research by using novel data on rebel service provision to civilian populations to explore how good rebel governance affects conflict negotiations. They found that service-providing rebels are more likely to engage in negotiations and to achieve favorable results, arguing that this reflects the lower risk of spoilers from groups with broad support and centralized power structures. Finally, Findley and Rudloff ( 2012 ) examined rebel group fragmentation’s effects on conflict termination and outcomes. Using computational modeling, they find that fragmentation only sometimes increases war duration (on fragmentation, also see Cunningham, 2014 ).

These studies underscore the value of exploring rebel group internal structures and practices in greater detail in future research, as they have an important impact on how, and when, civil wars end.

Victory/Defeat in Wars

Recent scholarship on victory and defeat in war suggests, as in the duration and termination literatures, that domestic politics, strategies of force employment, military mechanization, and war aims mediate the basic relationship between military strength and victory. Empirical results show that strategy choices and methods of force employment have a greater impact on war outcomes than relative military capabilities (Biddle, 2004 ; Stam, 1996 ), that high levels of mechanization within state militaries actually increase the probability of state defeat in counterinsurgency wars (Lyall & Wilson, 2009 ), and that weak states win more often when they employ an opposite-strategy approach in asymmetric conflicts (Arreguin-Toft, 2006 ) or when the stronger party’s war aims require high levels of target compliance (Sullivan, 2007 ). High relative losses and increasing war duration also decrease the likelihood of victory for war initiators, even if prewar capabilities favored the aggressor (Slantchev, 2004 ).

More recent research focuses on counter-insurgent conflicts, using new micro-level data and modeling techniques to address questions of counterinsurgent effectiveness in these complex conflicts. For example, Toft and Zhukov ( 2012 ) evaluated the effectiveness of denial versus punishment strategies, finding that denial (i.e., increasing the costs of expanding insurgent violence) is most effective, while punishment is counterproductive. Relatedly, Weidmann and Salehyan ( 2013 ) used an agent-based model applied to the U.S. surge in Baghdad to understand the mechanisms behind the surge’s success. They found that ethnic homogenization, rather than increased counterinsurgent capacity, best accounts for the surge’s success. Finally, Quackenbush and Murdie ( 2015 ) found that, counter to conventional wisdom, past experiences with counterinsurgency or conventional warfare have little effect on future success in conflict. States are not simply fighting the last war.

An important area of research that has fostered significant debate among scholars focuses on explaining the historical pattern of high rates of victory by democracies in interstate wars. The strongest explanations for the winning record of democracies center on their superior battlefield initiative and leadership, cooperative civil-military relations, and careful selection into wars they have a high probability of winning (Reiter & Stam, 2002 ). Challenging these results both theoretically and empirically, however, Desch ( 2002 ) argues that “democracy hardly matters,” that relative power plays a more important role in explaining victory. This debate essentially comes down to the relative importance of realist-type power variables versus regime type variables in explaining military victory; while scholars such as Lake ( 1992 ) and Reiter and Stam ( 2002 ) argued that regime type matters more, Desch asserted that relative power is the more important determinant of military victory.

Ultimately, we find Desch’s objections to the relevance of democracy to be overstated and his theoretical and empirical justifications to be largely unconvincing. First, Desch’s analysis is biased against Reiter and Stam’s argument because it is limited to dyads that Desch labels “fair fights,” that is, dyads with relatively equal military capabilities. This does not allow Desch to test the selection effect that Reiter and Stam discuss. Second, Desch failed to recognize that many of the realist variables he attributes the greatest explanatory power to are actually influenced by the foreign and military policies adopted by democratic leaders (Valentino et al., 2010 ). Democracy thus has both a direct and an indirect effect on war outcomes, and because Desch ignores the latter, he underestimates democracy’s total impact. Finally, the impacts of power variables may be overstated, as recent research demonstrates that military power’s influence is conditional upon method of force employment and military mechanization (Biddle, 2004 ; Lyall & Wilson, 2009 ).

More recent research examines some of the mechanisms suggested for the unique war-time behavior of democracies, raising some questions about existing mechanisms and suggesting alternatives to explain democratic exceptionalism. For example, Gibler and Miller ( 2013 ) argued that democracies tend to fight short, victorious wars because they have fewer territorial (i.e., high salience) issues over which to fight, rather than because of their leaders’ political accountability. Once controlling for issue salience, they find no relationship between democracy and victory. Similarly, using novel statistical techniques that allow them to account for the latent abilities of states, Renshon and Spirling ( 2015 ) showed that democracy only increases military effectiveness under certain conditions, and is actually counterproductive in others. Finally, new research by Bausch ( 2017 ) using laboratory experiments to test the mechanisms behind democracy and victory suggested that only some of these mechanisms hold up. Specifically, Bausch found that democratic leaders are actually more likely to select into conflict and do not mobilize more resources for war once involved, contrary to the selection and war fighting stories developed by Reiter and Stam ( 2002 ). He did find, however, that democratic leaders are less likely to accept settlement and more likely to fight to decisive victory once conflict is underway, and that democratic leaders are more likely to be punished than autocrats for losing a war. Thus, the debate over the democratic advantage in winning interstate wars continues to progress in productive directions.

Theoretical arguments regarding civil war outcomes focus on state/rebel strength, positing that factors such as natural resource wealth, state military capacity, and third-party assistance influence relative combatant strength and war outcomes. Empirical studies find that increasing state military strength decreases the likelihood of negotiated settlement and increases the probability of government victory (Mason et al., 1999 ). Characteristics of the war itself also affect outcomes, with the probability of negotiated settlement increasing as war duration increases (Mason et al., 1999 ; Walter, 2002 ), and high casualty rates increasing the likelihood of rebel victory (Mason et al., 1999 ).

Debate remains over how third-party interventions affect civil war outcomes. UN intervention decreases the likelihood of victory by either side while increasing the probability of negotiated war terminations (DeRouen & Sobek, 2004 ). This impact is time sensitive, however (Mason et al., 1999 ). Further, the impact of unilateral interventions is less clear. While Regan ( 1996 ) found intervention supporting the government to increase the likelihood of war termination, Gent ( 2008 ) found military intervention in support of rebels to increase their chance of victory but that in support of governments to have no significant impact. More recent research by Sullivan and Karreth ( 2015 ) helps explain this discrepancy. They argued that biased intervention only alters the chances for victory by the supported side if that side’s key deficiency is conventional war-fighting capacity. Empirically, they show that because rebels are generally weaker, military intervention on their behalf increases their chance of victory. For states, however, military intervention only increases their odds of victory if the state is militarily weaker than or at parity with the rebels.

Additional new research by Jones ( 2017 ) also represents an important step forward in understanding the effects of intervention in civil war. By examining both the timing and strategy of intervention, Jones demonstrated that the effects of intervention on conflict outcomes are much more complex than previous research suggests.

Post-War Peace Durability

As with studies on war duration, termination, and outcomes, much of the literature on the stability of post-war peace grows from extensions of the bargaining model of war. For these scholars, recurrence is most likely under conditions that encourage the renegotiation of the terms of settlement, including postwar changes in the balance of power (Werner, 1999 ) and externally forced ceasefires that artificially terminate fighting before both sides agree on the proper allocation of the spoils of war (Werner & Yuen, 2005 ). Building off of commitment problem models, Fortna ( 2004b ) argued that strong peace agreements that enhance monitoring, incorporate punishment for defection, and reward cooperation help sustain peace. Specific measures within agreements, however, affect the durability of peace differently. For example, troop withdrawals and the establishment of demilitarized zones decrease the likelihood of war resumption, while arms control measures have no significant impact (Fortna, 2004b , p. 176).

Postwar intervention is also expected to increase peace duration by ameliorating commitment problems, as peacekeepers act as a physical barrier and reduce security fears, uncertainty, and misperceptions between former adversaries (Fortna, 2004a ). Empirical results support this theoretical prediction, and while the size of the force is not significant, both monitoring and armed forces missions increase the durability of post-war peace (Fortna, 2004a ).

The debate that remains in this literature is whether or not peace agreements can effectively mitigate the influence of relative power variables. Recent research by Lo, Hashimoto, and Reiter ( 2008 ) suggests that they cannot. They demonstrated that cease-fire agreement strength has almost no significant impact on post-war peace duration, while factors encouraging renegotiation receive partial support. While discrepancies in results may be in part attributable to differences in time periods covered, this result essentially confirms Warner and Yuen’s ( 2005 ) finding that externally imposed war termination invites resumption of conflict, regardless of the presence of strong cease-fire agreements.

If, at the end of a civil conflict, each side maintains its ability to wage war, issues of credibility can undermine the peace and cause the conflict to resume. Thus, wars ending in negotiated settlements are more likely to recur than those ending with a decisive victory because both sides have the ability to resume fighting to gain greater concessions and neither can credibly commit to the peace (Licklider,, 1995 ; Walter, 2002 ). More recent research confirms that conflicts ending in military victory are less likely to recur than those ending in settlement (Caplan & Hoeffler, 2017 ; Toft, 2009 ), though Toft suggested that this is particularly true for rebel victories.

This understanding of post-war peace in terms of the bargaining model’s commitment problem has led scholars to examine three primary avenues through which commitment problems might be overcome and peace maintained. First, partition has been advanced as a possible solution to post-war instability. The separation of warring factions is expected to reduce security fears by creating demographically separate, militarily defensible regions (Kaufmann, 1996 ). Empirical evidence generally supports this strategy. Partitions that successfully separate warring ethnic groups significantly reduce the risk of renewed conflict (Johnson, 2008 ), while those that do not achieve demographic separation increase the risk of renewed hostilities (Tir, 2005 ). Further, relative to de facto separation, autonomy arrangements, or maintenance of a unitary state, partition is significantly less likely to lead to war recurrence (Chapman & Roeder, 2007 ).

Second, third-party intervention is expected to play a role in ameliorating the security dilemma arising from commitment problems in post-conflict states (Fearon, 2004 ; Walter, 2002 ). Empirical results confirm that third-party security guarantees are critical to the signing and durability of peace settlements (Walter, 2002 ). Once settlement has been reached, third-party guarantees and international peacekeeping establish punishments for defection (Fortna, 2008 ; Walter, 2002 ), thereby reducing incentives for and increasing costs of renewed conflict. More recent research that employs more fine-grained data on the size and composition of UN peacekeeping forces suggests, however, that this type of third-party guarantee is most effective when it has the military power to enforce the peace. Specifically, Hultman, Kathman, and Shannon ( 2016 ) found that increasing UN troop presence increases peace durability, but the presence of other types of UN monitors has little effect on peace duration. By using more fine-grained data, this study makes an important contribution by allowing us to parse the mechanisms driving the role of third party guarantees in promoting peace.

Third, the incorporation of power-sharing arrangements that guarantee the survival of each side into the postwar settlement is also expected to solve post-civil war commitment problems (Walter, 2002 ). These arrangements allow adversaries to generate costly signals of their resolve to preserve the peace, thus ameliorating security fears (Hartzell & Hoddie, 2007 ). Empirical results indicate that given a negotiated settlement, the agreement’s ability to ameliorate security concerns is positively associated with the preservation of peace. Thus, the more regulation of coercive and political power included in an agreement, and the greater the number of dimensions (political, territorial, military, economic) of power sharing specified, the more likely agreements are to endure (Hartzell & Hoddie, 2007 ).

More recently, scholars have begun to extend this research by focusing more broadly on settlement design. Whereas previous research tended to simply count the number of power-sharing dimensions, newer analyses focus on issues such as the quality of the agreement (Badran, 2014 ) and equality in the terms of settlement (Albin & Druckman, 2012 ). Martin ( 2013 ), for example, found that provisions that share power at the executive level are less effective than those that regulate power at the level of rank-and-file or the public, as elite-level power-sharing is relatively easy for insincere actors to engage in at a relatively low cost. Cammett and Malesky ( 2012 ) found that proportional representation provisions are particularly effective at stabilizing post-conflict peace because of their ability to promote good governance and service provision, while Joshi and Mason ( 2011 ) similarly found that power-sharing provisions that expand the size of the governing coalition result in more stable peace. These analyses suggest that delving further into the design and content of settlement agreements is a positive avenue for future research. Future research should also examine how implementation of peace agreements proceeds, and how the timing and sequencing of implementation affects the durability of peace (e.g., Langer & Brown, 2016 ).

Finally, emerging research on civil war recurrence also shifts focus toward rebel groups and how their composition and integration affect post-conflict peace. For example, new research finds that rebel group fragmentation hastens the recurrence of civil war (Rudloff & Findley, 2016 ), while greater inclusion of former rebels in government improves prospects for post-conflict peace (Call, 2012 ; Marshall & Ishiyama, 2016 ). Emerging research on post-conflict elections also represents an important area for further study, as debate remains over how elections affect conflict recurrence. While some argue that they destabilize the peace (Flores & Nooruddin, 2012 ), others suggest they actually reduce the risk of conflict recurrence (Matanock, 2017 ).

The Longer-Term Consequences of Wars

What are the political, economic, and social consequences of interstate and civil wars, and what explains these postwar conditions? As Rasler and Thompson ( 1992 ) recognized, the consequences of war are often far-reaching and complex. Given this complexity, much of the literature varies significantly in quality and coverage; while post-war political change has received significant attention from political scientists, the social and health-related consequences of war are less well-known.

Post-War Domestic Political Stability and Change

Scholarship on post-war political stability focuses on both regime and leadership change, positing political accountability as a central mechanism in both cases. Interstate war has been theorized to induce internal revolution both indirectly (Skocpol, 1979 ) and directly (Bueno De Mesquita et al., 2003 ; Goemans, 2000 ). Empirical results support the accountability argument, as war losses and increasing costs of war increase the likelihood of post-war leadership turnover (Bueno De Mesquita & Siverson, 1995 ) as well as violent regime overthrow (Bueno De Mesquita, Siverson, & Woller, 1992 ). Related work shows that accountable leaders are also more likely to face foreign-imposed regime change at the hands of war victors (Bueno De Mesquita et al., 2003 ).

A central focus of recent research has been the conditional relationship between war outcomes and regime type. In his seminal study, Goemans, 2000 ) found that leaders of mixed and democratic regimes are more likely to be removed from office as a result of moderate losses in war than are leaders of autocracies. These findings, however, have been challenged by recent scholarship. Colaresi ( 2004 ) finds no difference in leadership turnover rates across all regimes types under conditions of moderate war losses, and Chiozza and Goemans ( 2004 ), employing a different measure of war outcomes and discounting the impact of termination over time, find that defeat in war is most costly for autocratic leaders and has no significant impact on tenure for democratic leaders.

Recently, research in the civil war literature has begun to focus more on post-war democratization, elections, and how groups transition from fighting forces to political parties. Much of the early work in this area focused on the link between war outcomes and the development of democratic institutions in the post-war period, specifically arguing that negotiated settlements facilitate democratization by requiring the inclusion of opposition groups in the decision-making process (Doyle & Sambanis, 2006 ; Gurses & Mason, 2008 ). More recent research, however, challenges this conventional wisdom, showing that the benefits of negotiated settlement are limited to the short-term and that economic factors are better predictors of post-war democratization (Fortna & Huang, 2012 ).

Recognizing that not all negotiated settlements are created equal, scholars have also begun to examine how variation in power-sharing provisions influences democratization. Debate remains on this topic as well, however. While some argue that power-sharing facilitates democratization by generating costly signals that create the stability necessary for democratization (Hoddie & Hartzell, 2005 ), others argue that they undermine democratization by reifying wartime cleavages, incentivizing political parties to seek support only from their own wartime constituencies, and undermining public confidence in governmental institutions (Jung, 2012 ). However, after accounting for non-random selection into power-sharing, Hartzell and Hoddie ( 2015 ) found that the inclusion of multiple power-sharing provisions in peace agreements increases post-civil war democratization. Future research should delve further into this debate, and consider more carefully whether specific types of provisions or institutional designs vary in their ability to promote democracy. Joshi ( 2013 ) represents an important first step in this direction, finding that institutional designs that favor inclusivity (e.g., parliamentary systems and proportional representation) are more successful at producing democracy.

Debate also continues over the effects of international intervention on post-conflict democratization. While some scholars expect intervention to facilitate postwar democratization by mitigating commitment problems and raising the costs of defection (Doyle & Sambanis, 2006 ), others suggest it is used as a tool by interveners to impose amenable, generally non-democratic, institutions in the target country (Bueno De Mesquita & Downs, 2006 ). Doyle and Sambanis ( 2006 ) found multidimensional UN missions incorporating economic reconstruction, institutional reform, and election oversight, to be significantly and positively correlated with the development of postwar democracy. However, Gurses and Mason ( 2008 ) and Fortna and Huang ( 2012 ) challenged this finding, reporting no significant relationship between UN presence and postwar democratization, and Paris ( 2004 ) and Bueno de Mesquita and Downs ( 2006 ) showed that peacebuilding missions and UN interventions actually decrease levels of democracy.

Future research should attempt to reconcile many of these open debates in both the interstate and civil conflict literatures. It should also build upon emerging research on post-conflict elections (Flores & Nooruddin, 2012 ; Matanock, 2017 ) and rebel governance (Huang, 2016 ). Huang’s work on rebel governance, in particular, shows that how rebels interact with civilian populations during conflict has important implications for post-conflict democratization.

Public Health Conditions in the Aftermath of Wars

Social scientists have recently begun to study the consequences of war for the postwar health and well-being of civilian populations. Theoretical arguments developed in this literature generally do not distinguish between interstate and civil war, instead developing mechanisms that apply to both types of conflict. The most direct public health consequence of war, of course, results from the killing and wounding of civilian populations. Scholars argue, however, that more indirect mechanisms cause longer-term public health problems as well. War, for example, is expected to undermine long-term public health by exposing populations to hazardous conditions through the movement of refugees and soldiers as vectors for disease (Ghobarah, Huth, & Russett, 2003 ; Iqbal, 2006 ), damaging health-related facilities and basic infrastructure (Li & Wen, 2005 ; Plümper & Neumayer, 2006 ), and reducing government spending and private investment on public health (Ghobarah et al., 2003 ).

Many empirical analyses, unfortunately, do not directly address the mechanisms outlined above. Overall, findings indicate that both civil and interstate war increase adult mortality in the short and long term (Li & Wen, 2005 ) and decrease health-adjusted life-expectancy in the short term (Iqbal, 2006 ). Conflict severity is also influential; while low-level conflict has no significant effect on mortality rates, severe conflict increases mortality and decreases life-expectancy in the long run (Li & Wen, 2005 ; Hoddie & Smith, 2009 ; Iqbal, 2006 ). Comparing the health impacts of interstate and civil wars, analysts have found interstate conflict to exert a stronger, negative impact on long-term mortality rates than civil war, despite the finding that civil war’s immediate impact is more severe (Li & Wen, 2005 ). Finally, many analysts have found that the negative, long-term effects of war are consistently stronger for women and children (Ghobarah, et al., 2003 ; Plümper & Neumayer ( 2006 ) than for men.

This developing field provides important new insights into the civilian consequences of war, but remains underdeveloped in many respects. First, while some evidence suggests that civil and interstate war might affect public health differently, the mechanisms behind these differences require further elaboration. Research by Hoddie and Smith, represented an important contribution in this respect, as it distinguishes between different conflict strategies, finding that conflicts involving extensive violence against noncombatants have more severe health consequences than those in which most fatalities are combat-related. Second, theoretical models are generally much more developed and sophisticated than the data used to test them. While data availability is limited, efforts should be made to more closely match theory and empirics.

Third, analyses that employ disaggregated measures of health consequences (Ghobarah et al., 2003 ) provided a more thorough understanding of the specific consequences of war and represent an important avenue for additional theoretical and empirical development. Iqbal and Zorn ( 2010 ) thus focus specifically on conflict’s detrimental impact on the transmission of HIV/AIDS, while Iqbal ( 2010 ) examines the impact of conflict on many different health-based metrics, including infant mortality, health-associated life expectancy, fertility rates, and even measles and diphtheria vaccination rates. These studies represent important advances in the literature, which should be explored further in future research to disentangle the potentially complex health effects of civil and interstate conflict.

Finally, recent research has begun to conceptualize health more broadly, accounting for the psychological consequences of wartime violence. Building upon research in psychology, Koos ( 2018 ) finds that exposure to conflict-related sexual violence in Sierra Leone generates resilience: affected households display greater cooperation and altruism than those unaffected by such violence during conflict. Bauer et al. ( 2016 ) similarly find that conflict fosters greater social cohesion and civic engagement in the aftermath of war. This is an important area for future research. As conceptions of conflict-related violence broaden, our conceptualizations of the consequences of violence should also expand to include notions of how conflict affects psychological health, community cohesion, and other less direct indicators of public health.

This final section highlights some of the contributions generated by scholarship on the conduct and consequences of war, as well as some of the gaps that remain to be addressed. First, this body of scholarship usefully compliments the large and more traditional work of military historians who study international wars, as well as the work of contemporary defense analysts who conduct careful policy analyses on relevant issues such as wartime military tactics and strategy as well as weapon system performance. The bargaining model of war has also proven a useful theoretical framework in which to structure and integrate theoretical analyses across different stages in the evolution of war.

Second, a number of studies in this body of work have contributed to the further development and testing of the democratic peace literature by extending the logic of political accountability models from questions of war onset to democratic wartime behavior. New dependent variables, including civilian targeting, imposition of regime change, the waging of war in ways designed to reduce military and civilian losses, and victory versus defeat in war have been analyzed. As a result, a number of new arguments and empirical findings have improved our understanding of how major security policy decisions by democratic leaders are influenced by domestic politics.

Third, this literature has advanced scholarship on international law and institutions by examining questions about compliance with the laws of war and the role played by the UN in terminating wars and maintaining a durable post-war peace. The impact of international law and institutions is much better understood on issues relating to international political economy, human rights, and international environmental governance than it is on international security affairs. As a result, studies of compliance with the laws of war, the design of ceasefire agreements, or international peace-building efforts address major gaps in existing literature.

Fourth, this new body of research has explicitly focused on the consequences of war for civilian populations, a relatively neglected topic in academic research. Research on questions such as the deliberate targeting of civilians during wars and the longer-term health consequences of war begin to address this surprising gap in research. As such, this new literature subjects the study of terrorism to more systematic social science methods and also challenges the common practice of restricting terrorism to non-state actors and groups when, in fact, governments have resorted to terrorist attacks on many occasions in the waging of war.

While this literature has advanced scholarship in many ways, there remain several theoretical and empirical gaps that future research should aim to address, two of which are highlighted here. First, while research on interstate war duration and termination is more theoretically unified than its civil war counterpart, the dominance of the bargaining model in this literature is currently being challenged. Recent research on asymmetric conflict suggests that the basic tenants of the bargaining model may not hold for non-symmetric conflict, while research on force employment and mechanization suggest that traditional power measures exert a conditional impact at best. Additional research is needed to determine the conditions under which bargaining logic applies and its relative importance in explaining wartime behavior and war outcomes.

Second, the accumulation of knowledge on civil war’s conduct and consequences has lagged behind that on interstate war, partially because the civil war literature is younger, and partially because sub-national level data is only now becoming more readily available. While bargaining logic is often applied to civil war, we have little cross-national information on relative capabilities and battle trends, and thus a very limited understanding of the way in which these variables affect civil war duration and outcomes. New micro-level data and studies that are beginning to address these problems represent a promising direction forward for civil conflict research.

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