In the early 21st century, the voices for restraining war are louder than ever, but news headlines illustrate the limits of their influence. The century opened in the middle of a bloody and savage conflict in the Democratic Republic of Congo (DRC) that probably killed over a million people (Pinker 2011, 319), and continued with internationalized civil wars in Iraq and Syria that probably killed about half a million each, with overall civilian victims outnumbering deaths of soldiers, and with accounts of unspeakable brutality widespread. Nevertheless, optimists led by Steven Pinker argue that not only is war in decline, so are violence and cruelty of all kinds. The main reason for this decline, Pinker and his allies contend, is moral progress — norms. The question this essay explores is the degree to which the optimists’ claim is right that both the occurrence and conduct of war are indeed being restrained by the force of norms.
My conclusions are mixed. On the one hand, war is indeed in decline: international wars are surprisingly rare, and the wars (overwhelmingly civil wars) that do occur are smaller in scale than in the past. The reasons for this decline are a mixture of factors: material factors such as industrialization and nuclear weapons; social factors like the rise of international trade, the spread of democratic governments, and the growth of international institutions; and normative changes including the rise of humanitarian norms and of the principles of national self-determination and the stability of borders. On the other hand, there is less progress on restraining violence during wars. Studies find that most belligerents in war — international war and civil war alike — violate the norms of war regarding the treatment of civilians. This picture probably represents some improvement from the past — many states do show restraint — but it is difficult to say how much.
Have Norms Caused a Decrease in War?
This section addresses two questions. First, is there less war going on now than in the past? Second, if there has been a decrease, to what degree have norms been the reason? Though there have been prominent arguments in favor of both propositions, most notably John Mueller’s Retreat from Doomsday and Steven Pinker’s The Better Angels of Our Nature, there are also not-so-prominent objections to their claims that require consideration.
Is War in Decline?
In Jack Levy and William Thompson’s summary of Pinker’s argument, “the most telling statistic since 1945 is zero: zero uses of nuclear weapons, … zero wars between any two great powers, … and zero internationally recognized states disappearing through conquest” (Levy and Thompson 2013, 412). Pinker goes further and claims, “since the end of the Cold War in 1989, wars of all kinds have declined throughout the world” (Pinker 2013, 400). Is this true? Pinker, relying on well-regarded existing data sets, shows that great- power wars and wars in Europe became steadily rarer in recent centuries (Pinker 2011, 229). Furthermore, since 1946, the global death rate from all wars has declined precipitously — both absolutely and relative to population, while international wars have become exceedingly rare (Pinker 2011, 301–2).
There are, however, limits to this trend. First, the claim of a centuries-long trend applies only to great powers — mostly European — since the 18th century. The accounting thus leaves out 19th-century bloodbaths such as China’s Taiping Rebellion. A second point, illustrated in Figure 1 (Palik et al. 2020, 8) below, concerns more recent trends: while the deadliness of war has decreased since World War II, the number of armed conflicts (mostly small ones) has increased dramatically. Importantly, however, these conflicts are almost all civil conflict: international conflicts are rare throughout the period, and colonial wars have disappeared in recent decades. This, then, is the pattern: civil wars have become more widespread but generally smaller in recent decades, while international wars have become rare and wars among great powers (including nuclear wars) nonexistent.
Figure 1: Number of Countries with State-Based
Armed Conflicts, by Conflict Type, 1946 to 2019
Do Norms Explain the Decline of War?
I disaggregate this question into three smaller ones. First, to what degree do norms explain the absence of nuclear war? Second, to what degree do norms explain the decline in international war generally? Third, what is the impact of norms on the frequency of civil war?
The Nuclear Taboo?
The most important positive trend in warfare is the total absence of nuclear war since 1945. The most common explanation for this absence is that nuclear deterrence has led to a “nuclear revolution,” characterized by the recognition among leaders of nuclear powers that nuclear war is unwinnable (Jervis 1989). The same phenomenon is also used to explain the absence of great-power war. Since all great powers either are nuclear weapons states or are protected by one, great powers universally recognize that to clash directly with another great power is to risk total and speedy nuclear devastation. The potential consequences are so clear and so costly that no one has dared to challenge a great power’s nuclear deterrent. Decline-of-war enthusiasts like Mueller and Pinker argue against this conclusion, but the direct evidence for it is compelling: when the world came closest to nuclear war, during the Cuban Missile Crisis of 1962, the fear of nuclear devastation was by all accounts uppermost in the minds of the American and Soviet leaders who decided to pull back from the brink. Nikita Khrushchev’s famous Oct. 26, 1962, letter to Kennedy, for example, expressed the need to avoid actions that might “doom the world to the catastrophe of thermonuclear war.”
The deterrence argument, however, does not explain the absence of unilateral nuclear use, for example by the U.S. in the Korean or Vietnam wars. The evidence suggests a normative answer: the nuclear taboo (Tannenwald 2005). In the Korean case, the U.S. military commander in Korea, General Matthew Ridgway, commented in retrospect that nuclear use would have been “the ultimate in immorality” (Tannenwald 2005, 445–46). In the Vietnam case, President Nixon rejected the nuclear taboo, but was nevertheless constrained by it, commenting that if the U.S. used nuclear weapons, “the resulting domestic and international uproar would have damaged our foreign policy on all fronts” (Tannenwald 2005, 450, 456). Later studies have shown that nuclear deterrence and the nuclear taboo actually work together: while most Americans accept the nuclear taboo (Carpenter and Montgomery 2020), even among those who do not, many support nuclear restraint out of concern that U.S. nuclear use might make future nuclear attacks on the U.S. more probable (Sagan and Valentino 2017).
Ukrainian President Volodymyr Zelenskyy addresses the U.N. Security Council on April 05, 2022. (Getty Images)
Norms and the Decline of International War
The case that anti-war norms explain the decline of international war was first brought to prominence by John Mueller. Mueller’s key point is that until World War I, international norms evaluated war favorably: one review concluded that “the intellectual elite of Europe embraced the war not merely as unpleasant necessity … but as spiritual salvation and hope of regeneration” (Mueller 1989, 38). Among those Mueller (1989, 39–45) quotes articulating such views are Oliver Wendell Holmes, Winston Churchill, Theodore Roosevelt, and Emile Zola.
The contrast with post-1945 norms cannot be starker. The new consensus is summarized in the preamble to the U.N. Charter, which begins, “We the peoples of the United Nations, determined to save succeeding generations from the scourge of war, which twice in our lifetime has brought untold sorrow to mankind…” The charter lists first among the purposes of the United Nations “to maintain international peace and security.” According to Mueller, the “historical movement of ideas” exemplified by those statements is the prime cause of the decline of international war. Pinker adds a wide array of additional evidence, including the decline in pro-military ideologies such as fascism and communism, and a well-established norm against violent changes in international borders (Pinker 2011, 259). Finally, one might add evidence from survey data: in a 2010 poll, when asked whether the use of military force was sometimes necessary “to maintain order in the world,” majorities in Germany, Egypt, and Jordan refused to endorse the idea, taking an essentially pacifist position (Pew 2010).
There are, however, also material reasons for the decline in war. First, modern technology has changed the basis of material power. In the pre-industrial world, as Rosecrance (1986, 160) points out, war was efficient: “it was cheaper to seize another state’s territory by force than to … derive benefit from commercial exchange with it.” Since the Industrial Revolution, in contrast, it has been increasingly the case that no matter what a state might want — power, wealth, status, security — the best route to achieving those goals is economic development, which is best achieved in peacetime and through trade as well as domestic technological progress. From this point of view, international war is becoming rare not because its costs are high but because its benefits are low.
Conquest does not pay in the long run. The most recent great-power attempt to make it pay was Soviet control over its Eastern European empire, but by the 1980s it became clear that maintaining that empire cost more than it yielded, as demonstrated in a study tellingly entitled “The Empire Strikes Back” (Bunce 1985). Call this argument the “industrial peace.”
Another explanation for the decline of war has to do with social organization: the notion of the “Kantian triad.” According to this argument, it is the combination of the spread of democracy, the rise of international commerce, and the growth of international organizations that accounts for the absence of war among democracies (Oneal and Russett 1999). Supplementing this insight is another: that the democratic peace is essentially a liberal peace, requiring not only liberal institutions to work but also liberal values (Owen 1994). The key point is that these factors work together. The Europe of 1914 featured industrialized economies tightly linked by international trade, and by the standards of the time the leading powers (including Germany) were democracies. The missing elements were liberal values — especially a high valuation on peace itself — and a network of international institutions to facilitate the peaceful settlement of international disputes.
In sum, the decline of great-power war seems to have four necessary elements. First, industrialization provided a path to national power and security that was superior to war. Second, the spread of liberal norms and the decline of militaristic ones made the peaceful alternative desirable. Third, the growth of international institutions and international commerce made strategies of peaceful rise practical, while the growth of democracy, especially in the great powers in the West, constrained leaders of democracies to consider popular preferences on the subject. Finally, nuclear deterrence and the nuclear taboo made the avoidance of great-power war necessary.
The factors that make for great-power peace also explain the broader decline in international war. For small and middling powers, as for large ones, nationalism makes foreign conquest unprofitable, and trade and economic development offer a superior route to progress. Additionally, as Saddam Hussein learned, liberal norms and institutions like the U.N. help make aggressive war infeasible, as the aggressor is likely to face opposing great-power intervention coordinated by the U.N. or other international security institutions like NATO.
Effects on Civil War
Civil war, unlike international war, is not clearly in decline. Indeed, as Figure 1 shows, armed conflicts inside states have actually increased in frequency in recent decades. While most of these armed conflicts are small, the increase in global battle deaths in the 2010s is mostly the result of the civil wars in Afghanistan, Syria, and Iraq (Palik et al. 2020). The worst cases tend to be internationalized civil wars in which intervention by foreign armies lengthens wars and makes them more violent. The United States is the most prolific intervener, but by no means the only one. The most violent civil wars include cases in which U.S. intervention worsened the violence (Iraq 2003–2010 and Afghanistan 2001–2021) and cases in which the U.S. did little while other states intervened (Syria and DRC). However, foreign intervention sometimes has pacifying effects: NATO military intervention ended Bosnia’s agony in 1995, for example, while tacit collaboration between the U.S. and Iran helped Iraq defeat the ISIS uprising after 2015. The sum of these effects is clear, however: civil wars still kill very large numbers of people.
All of this is true, however, primarily in places where the factors driving international peace are weak. The most important such factors are institutions, especially those of the state. It is widely agreed that state weakness is a key permissive factor in causing civil war (Fearon and Laitin 2003). Also vital are nationalist sentiments: when ethnic or nationalist narratives pit groups in a state against each other instead of fostering national unity, civil war can result (Kaufman 2015). Poverty is also a key factor: civil war is most probable in poor states (Collier and Hoeffler 2004), not in states where the industrial peace is most advanced. Finally, liberal norms also matter: liberal democracies, whether rich or poor, are not commonly victims of civil war. The effects of such normative change, however, seems to be limited primarily to wealthier parts of the globe.
Do Norms Temper Belligerent Behavior During War?
Given the prominence of evidence that wartime atrocities remain distressingly widespread, it may seem improbable that jus in bello norms have had an appreciable effect on the actual conduct of recent wars. Nevertheless, this is a great deal of evidence for the widespread acceptance of jus in bello norms, and some evidence for their efficacy. I will review them in that order.
The Spread of Norms of Restraint
The first set of evidence for the spread of norms of restraint in war comes from international law: the fact that virtually all countries have signed a series of Geneva Conventions encoding such restraint. If the original Geneva Conventions were mere lip service, they would have been ignored and allowed to atrophy after their signature in 1949, but they have not. Instead, they have been updated and tightened with the articulation and signature of a series of additional protocols and follow-on conventions (Fazal 2018, 20–21). It is universally acknowledged among sovereign states that atrocities in war, including the killing or targeting of civilians, rape, and ethnic cleansing, are wrong.
Pinker provides copious evidence that the spread of these norms is merely one part of a broader global rise in humanitarian values. Torture in the past was not only common, but also provided sources of amusement (Pinker 2011, 67); now even judicial torture is banned virtually everywhere (Pinker 2011, 149). Similarly, capital punishment was once a universal practice, but now virtually every European country has abolished it, and most countries, like the U.S., that still allow the practice rarely employ it (Pinker 2011, 150–51). Slavery, too, has been eliminated on moral grounds, with England and the U.S. paying high prices in the nineteenth century to achieve that goal (Kaufmann and Pape 1999).
Most importantly, the norms of jus in bello have come to be coded in military ethics, at least in some places. Pinker (2011, 264–65) mentions the example of the code of the “Ethical [U.S.] Marine Warrior,” whose mantra is: “The Ethical Warrior is a protector of life. Whose life? Self and others. … All others.” Similarly, the U.S. Army War College organizes an annual mock trial during which U.S. “Air Force General Carl A. Spaatz is indicted … for crimes against humanity for the indiscriminate aerial bombing of Dresden, Germany, during World War II” (Swift 2001, 13). Additionally, jus in bello rules are starting to be enforced at the international level, most prominently in special tribunals such as the International Criminal Tribunals for the Former Yugoslavia and for Rwanda. This normative change gives reason to expect that wartime atrocities might as a result be in decline in recent years. Are they?
Observance of Humanitarian Norms in Warfare
On the surface, evidence that humanitarian norms are not observed in warfare seems overwhelming. In the Darfur conflict, for example, the Sudanese military used bombs that were “terror weapons aimed solely at civilians,” and together with Janjaweed militiamen, would attack villages and “shoot all those who could not run away. Small children, being light, were often tossed back in the burning houses” (Prunier 2005, 99–100). The overall strategy was one of ethnic cleansing that drove more than 2 million from their homes (Human Rights Watch 2007, 5) and killed 200,000 or more (“Bashir Charged”). In the war in the DRC, according to Human Rights Watch, “Indiscriminate attacks, extrajudicial executions of civilians, rape, and large-scale destruction of civilian property characterized the conduct of the belligerents” (“D.R.C. Human Rights”). A flare-up in fighting in 2017 involved “boys being forced to rape their mothers, … militia, some of whom sported female genitals (clitorises and vaginas) as medals,” and “people cutting up, cooking and eating human flesh” (“Mass Rape”).
What the cases of Darfur and DRC prove is that there are still places where humanitarian norms of jus in bello are utterly ineffective, restraining neither governments nor rebel groups. On the other hand, while the U.S. has gone to war repeatedly in recent decades, it has gone far in its efforts to reduce casualties, especially among civilians. In Afghanistan in 2008, Pinker notes, “the U.S. Air Force followed a set of humanitarian protocols … that Human Rights Watch praised for its ‘very good record of minimizing harm to civilians’” (Pinker 2011, 266). These efforts are highly imperfect, however, failing to prevent repeated instances of attacks on civilian targets including medical facilities and wedding parties. U.S. use of drone aircraft presents a similarly mixed picture. Pinker similarly notes, quoting Joshua Goldstein, that the U.S. tactic of drone strikes is far more discriminate than were previous tactics, such as ground assaults on villages. On the other hand, the fact that drone strikes are so low in cost — including, comparatively, their human costs — means that they are used more prolifically than other tactics were. Similarly, the precision of their aim points is little help when flawed U.S. intelligence mistakes innocent civilian targets for terrorist ones.
Going beyond anecdotal evidence, a few scholars have compiled data sets assessing compliance with the laws of war, finding that the picture is mixed. One study, led by James Morrow (2007, 562), considered the behavior of each belligerent toward each of their enemies in 48 international wars between 1899 and 1991. It found that about two-thirds of these cases involved “non-compliance” or “low compliance” with the laws of war regarding the targeting of civilians. There was no clear trend of increasing or decreasing compliance over the course of the century studied (Morrow and Jo 2006, 108–9).
In a separate study, Jessica Stanton compiled a data set of every civil war in the world from 1989 to 2010, identifying those parties to the conflicts that showed restraint, and distinguishing them from those who systematically violated the norms of war. She finds, in sum, that among rebel groups, 30.1 percent massacred civilians and 29.1 percent committed other major violations, while 41.8 percent generally observed the key norms of war. Among governments, 24.5 percent massacred civilians and 26.5 percent carried out other major violations, while 49 percent generally observed the norms of war (Stanton 2016, 4–5).
However, the largest civil wars all feature large-scale violations of the norms of war. Inspecting Stanton’s (2016, 84–88) data, I compared her set of conflicts to separate estimates of casualties, and identified 15 where estimates of total deaths reach at least 100,000, including the wars in Darfur, Iraq, and DRC. Among these 15 largest civil wars, seven featured restraint by the government side, but none featured restraint by both sides according to Stanton’s codings.
In sum, the norms of jus in bello have had some effect, but that effect is spotty. Certainly the U.S. has made an effort to avoid civilian casualties in its recent wars.
Stanton’s data suggests U.S. allies such as the regimes in Iraq and Afghanistan (after the U.S. invasions of those countries) made similar efforts, as have 49 percent of the states in her sample. However, as the examples from Darfur and DRC illustrate, atrocities remain distressingly common: most of the time, either governments or rebels engage in large-scale violations of humanitarian norms in war regarding the targeting of civilians, if not both. Historically, most belligerents in international wars have done the same.
Evidence indicates that the norms of war do matter: belligerents and potential belligerents are more restrained than they were in past centuries. This restraint is most marked in the rising disinclination of states and peoples to resort to war. War has become exceptionally uncommon across large swathes of the world, and international wars are vanishingly rare everywhere. Anti-war norms such as those encoded in the U.N. Charter have played an important role in this decline of war, though material and social factors have also made necessary contributions. On the other hand, these developments have not prevented an increase in the frequency of civil wars since 1946.
When war does break out, normative restraints have less effect. The largest wars all feature atrocities: most civil war rebels engage in them, as do most governments. Nevertheless, in recent civil wars, nearly half of governments have made sustained efforts to avoid inflicting civilian casualties. This is probably an improvement compared to past centuries, when atrocities were apparently more routine, but it is difficult to know how much improvement there has been.
Future prospects are not necessarily rosy. Populist political forces, most importantly in the United States, are undermining commitment to all of the factors driving the democratic peace — democratic institutions and practices at home, participation in international trade and international institutions, and liberal norms and values. At the same time, the profound irrationality of these movements casts doubt on their willingness to recognize the costs of war and the benefits of peace. George Bush’s magnification of American nationalism to motivate the war in Iraq illustrates how these dynamics can work. In the longer run, climate change will exacerbate key factors leading to war, such as poverty, state weakness, and destabilizing migration flows, and their effects will be concentrated in already-fragile areas such as Africa and the Middle East. Norms of war will be increasingly challenged in these conditions.
 Peter Liberman (Does Conquest Pay?) argues the opposite about the Soviet case, but this conclusion primarily applies to East Germany and primarily to the short run. Liberman (1998, 150) concedes that returns decline over time.