As the headlines daily show, wars are terrible occurrences. They see human beings behaving at their worst, maiming and slaughtering each other in large numbers; achieving their ends, even if they do so, at enormous economic and social cost, with an appalling loss of life. Yet the use of armed force is only one of the four ways by which humans can acquire whatever material or ideal resources they may desire. I have defined these as the four sources of social power—military, ideological, political and economic—traceable across human history.footnote1 Why do humans so often use military power, not cooperative norms, economic exchange or political diplomacy to attain foreign-policy goals?

The dominant theorizations of the causes of war come from the realist school of international relations. This tradition posits two major concepts. The first is the anarchic nature of international space. In contrast to the rule of law within states, there is no world arbiter above them wielding international law. Thus, states are always anxious about other states’ intentions; they reason that the greater their own power, the less likely they are to be attacked, so they all build up their military forces. This however leads to ‘security dilemmas’, as the build-up alarms their rivals into escalating their military preparedness, too.footnote2 Moreover, insecurity means that all protagonists can claim to be acting in legitimate self-defence. This is a powerful argument, but it needs to be qualified. It is true that geopolitical relations are on the whole less rule-governed than social relations within states, but we should treat international ‘anarchy’ as a variable, historically present to differing degrees. As realists also acknowledge, a hegemonic state can knock heads together to achieve geopolitical order and peace; the model cases are Britain in the 19th century and the us since 1945. Hegemons have been uncommon, however, since other states may form ‘balancing’ alliances against a superordinate power. The notion of international anarchy as a cause of war is useful, then, but variably so. Moreover, it tends to block out the possibility of domestic causes of war.

Realism’s second core thesis is that states are rational, unitary actors, using carefully calculated means to maximize the chance of achieving their goals. John Mearsheimer puts the case succinctly:

Great powers are rational actors. In particular, they consider the preferences of other states and how their own behaviour is likely to affect the behaviour of those other states, and how the behaviour of those other states is likely to affect their own strategy for survival. Moreover, states pay attention to the long term as well as the immediate consequences of their actions.footnote3

Yet decisions for war or peace are usually made in highly fraught environments of growing domestic and overseas tension. Anarchy breeds fear of others, which rises as the possibility of war looms; these are conditions conducive to angry or panicky behaviour, rather than calm calculation. Thus not all realists stress calculative efficiency. Kenneth Waltz, for example, argues that states often act in reckless, non-strategic ways, but when they do so they are punished by the system, whereas states that act rationally are rewarded. Here rationality lies not with the individual state actor, but with the hidden hand of the system.footnote4

In what follows I will question these assumptions about the rationality of war. Drawing on the results of a broad historical survey that spans ancient Rome, imperial China, medieval Japan, Europe, Latin America and the wars of the 20th and 21st centuries, I examine the motives for war and the extent to which means were rationally calculated against ends.footnote5 I then go on to ask: if war has not been as rational as realists make out, why is that? Situating wars in their historical and environmental contexts, I identify actors and examine their motives. Why—and by whom—were these lethal conflicts chosen, or stumbled into, ahead of immeasurably less destructive alternatives?

1. deciding on war

War submits rulers, generals and soldiers to the fickle fortunes of battle. When given the order to prepare, generals draw up campaign plans and mobilize resources—during this highly calculative phase, quartermasters’ logistics predominate. But once battle with the enemy is joined, all hell breaks loose. Soldiers experience warfare as fearful chaos, from the ferocious body-on-body slashing of earlier periods to the callousness of modern warfare, in which gunners and infantry blaze away at a distant enemy, still vulnerable to random death inflicted without warning from the skies. Moreover, carefully laid plans can rarely be implemented because of the enemy’s unexpected behaviour or unanticipated battlefield terrain. These were Clausewitz’s ‘frictions’ of battle, Ibn Khaldun’s ‘hidden causes’ of outcomes, the grounds for Napoleon’s adage that no plan of operations extends with any certainty beyond the first contact with the main hostile force. During the 14th–15th century Hundred Years’ War between France and England, six of the seven biggest battles were decided by unexpected terrain or enemy dispositions. The small engagements of American units in World War Two and Vietnam were often settled by terrain, mistakes, good fortune or unexpected bravery.footnote6 Today, warfare’s unpredictable nature is evident in Ethiopia, Somalia, Yemen, Ukraine—and, in a different way, Palestine.

Ancient Rome to Vietnam

But if battle is conducted in a fog, what about the initial decisions for war? Could they be described as strictly rational? The Roman Senate debated war and peace decisions at length. Yet the arguments focused on anticipated economic profit—ignoring the cost in lives—while domestic political rivalries were often a subtext. War was usually approved, unless a jealous senator intervened to block a rival’s chance at military command. When over-confidence led to defeat, the Senate’s usual response was to dig deeper into its manpower resources, until Rome emerged victorious. War for Romans was not really a ‘choice’, it was what they did, historically baked into their militarized institutions and culture. They were willing to sacrifice more than their enemies to win. By contrast, the Carthaginians placed greater value on economic well-being and refused to sacrifice as much as Rome. So they lost the Punic Wars—and Carthage was destroyed. There are many other examples of militarized cultures: the rulers of the ex-barbarian Yuan and Qing dynasties of China, the Mongols, Manchus, Aztecs and Arab conquest dynasties all behaved like the Romans and went to war whenever the opportunity arose.

The twin Song Dynasties (960–1279) demonstrated a variety of attitudes to war. The first Song emperor, Taizu, was a model realist, fighting offensive wars after conducting cautious initial probes to test whether victory was likely, and carefully building up adequate forces. Yet his successors initiated six offensive wars resulting in only one success, one costly draw and four defeats. Several factors muddied their calculations: emotional revisionism demanding the return of ‘lost territories’, domestic political conflicts, an emperor’s overweening ambition or the wrong choice of allies. Other Song rulers favoured conciliation over aggression, opting for diplomacy, cultural cooperation and trade, not because of weakness, but because they wanted to pursue peaceful economic and social development. By contrast, the last Song emperors—and the last Ming emperors too—hastened their own civilizational collapse by striking out impulsively, in denial of their weakness, rather than settling for accommodation. The Song experience is not favourable to realism.

Nor is European history. Between 1400 and 1940, most European rulers who started wars were defeated.footnote7 It may be an exaggeration to say that there was no careful calculation of means by monarchs during the medieval period, but war was mainly what these rulers did when they felt slighted or ambitious, or when they needed to divert the turbulent energy of their younger sons or to bolster their domestic power. Together with the desire to maintain status, honour and glory, these motives then dictated calling out the barons, borrowing or levying taxes, and setting off for battle with whatever baronial forces showed up, which the king could not predict. Waging war was less a choice than what medieval monarchs were constrained to do. Early modern European rulers fielded professional armies and navies, but they still went to war when feeling slighted or ambitious. It was not always a ‘choice’, since belligerent stances often escalated into unintended war.

Realism in Waltz’s sense fares better in postcolonial Latin America. Here, 19th-century initiators lost six wars and won just two; there were also five mutual provocations and five costly stalemates. All eight of the leaders who initiated wars, whatever the outcome, were thrown out of office for doing so. Decision-making then became more rational, as rulers learned from the experience of bad wars to try to avoid them in future. In contrast to other continents, there were no serial aggressors—in fact the number of wars in the region declined over time.

In World War One, the aggressors demanded geopolitical status and defended the honour of client states, rather than pursuing economic goals—though German rulers hoped to acquire more profitable colonies. Many calculations were made, but war resulted from a cascade of diplomatic misunderstandings and incoherent policy formation.footnote8 A plethora of political and diplomatic initiatives produced unpredictability, resulting in a brinkmanship which meant, perversely, that no one would back down. Most rulers were confident of victory, but they also mistakenly believed that the war would be short since their economies could not support it for long. How wrong they were. The rulers of Germany, Austria-Hungary, Russia and the Ottoman Empire secured not only their own defeat but the fall of their monarchies as well. Some courtiers had warned of this, but they had lost the domestic power struggles—again: power, not reason. All the players lost heavily, except for the two outsiders who picked up the pieces, the Americans and the Bolsheviks. The war was irrational for everyone else.

In World War Two, rationality was disrupted by ideology. Had France and Britain allied with the Soviet Union to deter Hitler, the conflict might have been delayed or even prevented. But the rulers of these countries feared communism more than fascism and so, in 1939, an isolated Stalin signed the Non-Aggression Pact. In the Far East, Japanese rulers underestimated the strength of Chinese nationalism, while in the Pacific both Japanese and American rulers miscalculated each other’s reactions. War, in each case, was initiated by German and Italian fascists or by Japanese semi-fascists; in each case this was suicidal. What drove them to it? The Axis rulers were outnumbered militarily by technologically superior rivals, but nevertheless believed that martial spirit would overcome daunting odds of success. Economic motives were subordinate to the goal of imperial conquest in these regimes, each believing themselves superior to decadent liberal Europeans, communist powers and China. For Axis rulers, this war embodied Weber’s ‘value rationality’, where commitment to ultimate values overrides instrumental rationality.

In the Korean War, too, American, Chinese and North Korean rulers all underestimated their enemies, blinkered by ideology. The only possible result was a bloody stalemate, in which none of their objectives was achieved and which created a bitter divide across Korea that still poisons East Asia today. In Vietnam, us defeat was a result of underestimating the opponent’s ideological solidarity. The recent spate of wars against Muslim countries has seen battlefield victories for the us and its allies, yet neglect of political-power relations has predictably thwarted the achievement of American goals—as it will for the Israelis in Palestine. American intervention inflicted enormous damage on Afghanistan and Iraq, and contributed to the chaos rending Libya, Syria and Yemen. The Cold War aside, the United States has failed to fulfil its objectives in any significant war since 1945—an unimpressive record for a solo superpower.

Donbas to Gaza

Putin also seems far from attaining his ambitious goals. The normal blend of fear and over-confidence fuelled the build-up to his invasion of Ukraine in 2022. The understandable part of Russian fears derived from the eastward expansion of nato, begun in 1999 as Poland, Hungary and the Czech Republic joined, with seven more countries acceding in the early 2000s—nearly all wanting to join because they feared Russia, and all but Slovenia ex-members of the Soviet bloc. During this period, the us and nato took full advantage of Russia’s inability to mount more than verbal protests. By late 2021 there were nato missile sites in Romania and Poland, nato exercises in the Baltic states and Black Sea, and us military aid going to ex-Soviet states in Central Asia. In November 2021, Washington signed a Charter on Strategic Partnership with Kiev that called for Ukraine to join nato, and the us promised ‘unwavering support’ for Ukraine’s reincorporation of Crimea. Blowback came in February 2022 when Russian troops, massed along Ukraine’s border, invaded across three fronts.

If Russian fears of nato partly explain Putin’s warmongering, four further, less rational factors played important roles. The first was ideological: a sense of ‘grandeur’, emotionally supercharged by the humiliation of the post-Soviet collapse, combined with belief in the irreversible decline of Western hegemony. The second was military: Russia had become over-confident due to its successes in Chechnya, Georgia and Syria against much weaker forces. The third was political: support for his regime was beginning to flag and playing the nationalist card was popular. Fourth was Putin’s contempt for Ukrainians, intensified by growing divergences between the two regimes; like many aggressors before him, he despised his enemies and disparaged their powers. But Ukrainans, equipped with modern weapons and fuelled by the emotional power of defending their homeland, fought with skill, courage and tenacity. Initial Russian setbacks lasted long enough for anger to grow abroad. Putin had unintentionally strengthened the solidarity of his foes and the West’s response was stronger and more united than he had expected. But it should not have surprised him, for the us could now seize the opportunity to cut Russia down to size without committing its own troops. Biden was able to fight a proxy war, Putin was not. Caught in the middle of their irrational struggle were mangled Ukrainian bodies and devastated cities—the normal horrors of war, but affecting Westerners in a way that the wartime sufferings of non-white people in Africa and Asia had not.footnote9

The current Israeli onslaught against Gaza is the latest in a unique series of wars and militarized disputes since the foundation of the state of Israel in 1948. Almost all ended in Israeli victories, forcing the Arab states into lopsided peace deals with Tel Aviv at the expense of the Palestinians, while the periods of peace allowed Israelis to establish more and more settlements across the occupied territories. Religious differences are one of the central drivers, not because the combatants try to impose their faiths on each other, but because both believe they have a divine right to the same land. The Hebrew Bible claims that God promised it to the children of Israel, while Arabs say the Land of Canaan was promised to Ishmael, from whom they claim descent. Muslims and Jews revere the same holy sites—Al-Aqsa and the Dome of the Rock; Cave of the Patriarch and Temple Mount—making Jerusalem the epicentre of conflagration. Neither the Palestinians nor the initial political elite of Israel were renowned for their religiosity, but in an age of nationalisms their ethnic identity as Jews and Arabs has greatly reinforced this struggle.

The situation is also unique in involving the imposition of a settler-colonial state upon an indigenous people by another people, fleeing a genocide. Liberal assumptions might suggest that the terrifying experience of the Shoah would make Israeli Jews more sensitive to others’ suffering. To the contrary, many seem to believe that to survive as a people, they must use to the full whatever coercive power they have. Since Israeli Jews have the military and political power to seize Arab lands, most believe they have the right to do so in the name of ethnic survival. Their ambition is boosted by access to international capital, which has enabled them to build a modern state, a high-tech military and a successful economy. Palestinians meanwhile are predominantly poor, dependent on the Israelis for the provision of essential services in their two enclaves, abandoned by foreign powers and subject to ongoing ethnic cleansing. This is the context for Hamas’s claim that armed struggle can bring satisfaction, if not actual gains. The Israeli response is state terrorism, with a twenty-fold disparity in fatalities—rising since October 7 to over a hundred-fold, with some 30,000 Palestinians killed compared to fewer than 2,000 Israelis. Thanks to pro-Israel American Jews’ ability to organize the defeat of us politicians critical of Israel, the decline of American antisemitism and growing pro-Zionist sentiment among Evangelicals, Israel has long been America’s most favoured ally, rewarded with massive economic and military patronage. Of all the American policy failures in the Middle East, this is the one case where the us had the power to put pressure on both sides. Instead it is firmly backing Israel. Peace and genuine settlement have long been little more than a glimmer on the far horizon. Now, for both sides, even that has been extinguished.footnote10

Sharks versus minnows

From the ancient world to the present day, initiating a major war has more often resulted in failure—and massive devastation—than success. There are exceptions: some wars were rational in the sense that they were initiated for profit and achieved that end. These were mainly wars of imperial conquest against much weaker adversaries—‘sharks versus minnows’—or low-cost raiding parties. Defensive wars with a good chance of success may also be considered rational. In all of them, the benefit is zero-sum: for some to gain, others must lose. War brought benefits to conquerors but massacres and dispossession to the defeated. Conquest produced the social forms that are revealingly called both ‘empires’ and ‘civilizations’: Egyptian, Akkadian, Assyrian, Roman, Hellenic, Persian, Turkic, Muslim Arab, Mughal, Mongol, Chinese, Spanish, British, Aztec, Inca, Maya, American and so on. These civilizations grew by slaughtering and subjugating numerous other peoples, tribes and city-states, while claiming to bring order, freedom, civilization and, sometimes, the true faith. But the figure of the great conqueror is now almost obsolete. Putin’s might be the last (failed) attempt to embody it. States, legitimized by nationalism, now inhabit a sanctified world order. Today there is one great global civilization containing rival imperial cores, which exploit their peripheries; war between these centres would be irrational as it would have the capacity to end all human civilization.

Other wars might be considered consequentially rational, in that we can see with hindsight that they sparked unintended benefits, such as economic development. Conquest can spur creativity by blending distinct social practices, as is sometimes argued for the Mongol Empire. It may occasionally produce social order, as imperialists have always claimed. Ibn Khaldun noted that in the early Arab wars, conquerors seized great wealth for themselves and their followers, always at the expense of the conquered. Imperial rule did boost economic growth and tax returns for the first two generations, but then came decline and the dynasty’s collapse.footnote11 Claims for the unintended benefits of war in the modern period have been made by some,footnote12 but the evidence is weak and the benefits pale compared to the devastation.

The counterfactual of whether civilization might have been better served by peace may be insoluble. But in Song China, peace favoured major technological innovation, proto-industrialization and economic development—and wars put an end to this growth. Chroniclers of pre-modern wars saw them as zero-sum and stressed the devastation of the regions where they occurred. Since 1945, statistical data drawn from national income accounts show that war has reduced gdp per capita, even though the destruction of human life and fixed capital isn’t measured.footnote13 Generally, across the wars I have surveyed, far more people lose than win. Given the certainty that war kills millions, most wars seem pointless and irrational in terms of both means and ends. Why are there nonetheless so many of them?

2. who makes the decision?

In most realist accounts of war, the actors are states—or ‘units’, as international-relations theorists say. Yet historical sociology suggests that it is human beings—rulers and their entourages—that are the decision-makers, whether in a monarchy, an oligarchy, a representative democracy or a dictatorship. In the cases I surveyed, the calls were made by a small coterie of rulers, advisers and a few powerful figures—and sometimes by a single person. The extreme case is the authority vested in recent presidents to release nuclear missiles that could destroy the world. We cannot blame whole nations for wars, or the entire capitalist class—although there are some colonial bankers, media barons and arms merchants who might be held responsible. Most capitalists prefer to do business in peace, though they quickly adapt to exploiting profits from war. Contrary to received opinion, representative democracies have been no less likely to go to war, whether against authoritarian regimes or other democracies—provided we include their numerous colonial wars against the direct democracies of indigenous peoples.

Peoples themselves are rarely responsible for wars, not because they are virtuous but because they are not greatly interested, in either sense of that word: their personal interests are not at stake, and they don’t have much appetite for foreign affairs. In parliamentary democracies, elected representatives depend on their constituents for re-election, and so mirror their lack of interest in foreign policy. In the American Congress, most representatives and senators leave foreign policy to the relevant committees. If senior committee members agree with the Administration, foreign policy is simply rubber-stamped, unless powerful interest groups intervene or a gross violation of human rights provokes moralizing rhetoric. This is why congressional votes for war in the us have been so lopsided.

Public opinion does play a greater role in modern societies than it did in the past, but amid popular ignorance it is often manipulated by political leaders, interest groups and media barons. When geopolitics are fraught, foreign threats can become ‘nationalized’ if the public is persuaded that its way of life is under attack. As war starts, a ‘rally round the flag’ atmosphere usually lasts long enough to support the rulers. Volunteers sign up in droves, boosted by propaganda about the enemy’s atrocities. But after the first rush of enthusiasm, conscription may be needed, even if soldiers continue to obey the order to fight under military discipline. Varying degrees of commitment among troops—high when defending the homeland, and within highly ideological armies; lower in most wars with professional or conscripted soldiers—need to be reinforced by repetitive drilling, harsh discipline and battlefield terrain from which escape is difficult. A secret ballot held the day before action would probably see most soldiers vote against it, except perhaps in elite regiments.

Democracy is absent from decisions about war and peace. The people know little about the enemy, beyond what their rulers tell them. In the past, people saw war as a defence of their lord or monarch; obedience was their duty, reinforced by ritual and coercion. Today, many people will identify with the media’s image of the nation and its enemies. Like Russians and Israelis now, Americans have supported a war claimed to be in self-defence, for good against evil, as leaders invariably assert both. True, there have been societies—such as the mounted tribespeople of Eurasia and the Middle East—whose men have seemed addicted to war, while the women accepted such attitudes as normal. Decisions for war were made by the khan or emir and his intimates, but there was popular enthusiasm. Patriarchal ideology has tended to smother pacific tendencies among men, who fear being tarred with cowardice. Women are often complicit in this ethos, which plays an important role in making men endure the horrors of battle.

True also that in a few societies, quasi-representative decisions for war have involved larger groups than rulers and their coteries, though these could rarely be claimed as expressions of a popular will. In some Greek city-states, decisions were made by the citizen body, amounting to 20 to 40 per cent of adult males. Many were involved in decision-making in the early Sumerian city-states, as also in Tlaxcala, Mexico, and among native American peoples. Nevertheless, in the case of Rome, the Senate usually manipulated the popular assemblies into war. The English Parliament generally left such matters to monarchs and their ministers—except during the mercantilist 18th century, when bankers and merchants joined in. Nineteenth-century colonial-policy debates reliably emptied the House of Commons; popular interest was roused only when atrocities committed against British people were publicized and then paid back ten-fold. Rulers also lead populations into war under false pretences: Hitler’s lies about the murders of Germans in Danzig in 1939, Roosevelt’s distortion of the uss Greer’s 1941 brush with a German submarine and Johnson’s distortion of the Gulf of Tonkin incident in Vietnam in 1964 were pretexts for war, believed by most citizens. Bush the Younger and Blair fed false information to gullible publics about Saddam Hussein’s supposed links to terrorists and development of weapons of mass destruction. Putin’s lies about his war in Ukraine are many.

The us Congress is vested by the Constitution with the power to declare war, but in the 20th and 21st centuries it has usually ratified decisions already made by presidents (World War Two was a partial exception). When Israel invaded Gaza in October 2023, Biden offered unqualified support before consulting Congress. In 2001, during the panic induced by the 9/11 terrorist attack, Congress passed—with only one dissenting vote—the Authorization to Use Military Force Act, allowing the President to take military action against ‘terrorists’, or those who harbour them, without congressional approval. By 2018 this act had been used 41 times to attack nineteen countries. In January 2024, Biden unilaterally redesignated Yemen’s Houthis as terrorists. Popular demonstrations in favour of war or peace do occur, but they involve a tiny proportion of the population. War becomes unpopular if it goes badly, or if it requires conscription or extra taxes or debts. There may be war or peace factions within the governing class, lobbying by special interest groups, or student and intellectual mobilizations. But that is as popular as war and peace decisions generally get. So the problem shifts away from why states make war, to why rulers do.

3. rulers’ motives

Since rulers make wars, their goals and personalities matter. A few focus on stability, the economy, social welfare or justice, and oppose conscription and higher taxes. Others favour war as profitable or heroic, and willingly raise taxes and initiate conscription. Personal war records are an important indicator, since sequential victories enhance prestige and loyalty and make future wars more likely. Rulers may be capable or incompetent, calm or impulsive, suspicious or trusting, quick or slow to take offense. Contrast three successive Ming emperors: Yongle, the warrior; Xuande, the administrator; and Zhentong, the incompetent. Contrast the cruel warrior Henry V with the mentally challenged Henry VI, or peace-loving Chamberlain with bellicose Churchill, or micro-managing Obama with erratic, ignorant Trump. In Latin America, four of its fifteen wars could be attributed to reckless presidents initiating or provoking wars they would probably lose. Since personality differences are contingent, realists dismiss them as ‘noise’ in their models. But we must not confuse models with explanation.

Monarchs, dictators and presidents rarely decide policy alone. They often listen to opinions at court, in councils or assemblies. Yet rulers try to appoint like-minded advisers and debates within national politics will influence their perception of external realities. War and peace decisions can often depend on which faction dominates domestic issues. Debates over Japanese imperialism in the early 20th century were settled when the balance of power in Tokyo tilted to the right due to the Great Depression, the repression of the working class, collapsing political parties and the assassination of moderates. The Emperor and his circle duly shifted towards aggressive imperialism. Bush the Younger came to power on domestic issues, ignorant of the outside world. He let Vice President Cheney make most appointments to foreign and defence posts. Cheney chose hawks who were keen to launch wars.

Rulers also use wars to shore up their political power. Some Marxists have argued that war is used to divert class struggle. The major example of this was the run-up to World War One, where working-class power did figure in monarchs’ calculations. But revolution was the consequence of war for them, as sceptics at court had warned beforehand. War is prone to increase class conflict rather than reduce it, especially in defeat. More commonly, rulers beset by rivals have launched wars to divert intra-elite conflict, or to counter allegations of weakness—like Taizong or England’s Henry V. Weak rulers who launch wars are often reluctant to back down for fear of compounding their negative image. These ‘audience costs’footnote14 loomed large for ancient Chinese dukes and emperors, medieval monarchs, leaders plunging into World War One, General Galtieri, Saddam Hussein. Monarchs may wish to prove that they really are the Son of Heaven or anointed by God. Putin wants to prove he is Peter the Great. Rulers who fear their own generals may purposefully weaken their armed forces to lower the threat of coups. They become less likely to go to war as a result, but more vulnerable to attacks by emboldened rival states. For fear of his generals, Muhammad II of Persia separated his massive army into smaller detachments stationed in different cities, meaning Chinggis Khan could pick them off one by one and destroy the Shah’s empire. Roman emperors used praetorian guards for protection against the army. Inca rulers sought to coup-proof themselves by reducing army power, as have various Middle Eastern regimes—Saddam Hussein self-destructed this way. Stalin almost did as well, after purging the Red Army officer corps in the 1930s.

The financial cost of war has often been a rational restraint on rulers since tax increases are unpopular. Rulers were often reluctant to squeeze extra yield from peasants for fear of rebellion or longer-term damage to the economy—which, in turn, would reduce the taxes and men available for future wars. Easy targets or short wars were not ruinous, nor were rule-governed wars with few casualties. But if economic profit were the sole motive of rulers, there would have been far fewer wars, since so few of them pay.

Emotions and ideologies

Historians have pointed to ‘greed and glory’ as rulers’ main motives for war. Political scientists suggest an overlapping pair: ‘greed and grievance’. Those launching aggressive wars usually promised economic benefits to their soldiers and subjects; but acquiring more territory, tribute or submissive clients also brought rulers the gratification of honour and high-standing, for their states and for themselves, which they saw as one and the same—they were ‘statesmen’, after all. Glory was the highest achievement, for it was seen as eternal, whereas economic profit was only for the present. Motives of honour and glory combined in an ideological-emotional package, alongside motives of material gain. I would identify a third motive: rulers’ intrinsic enjoyment of domination over others, as stressed by Nietzsche. It can be seen in the attitudes of the great conquerors, often shared by their soldiers’ glee in looting and raping, as also in raiding wars. American leaders today revel in being ‘the leader of the free world’ or representing ‘the greatest power on earth’. These three motives—greed, status-honour-glory and domination—have repeatedly combined in ways that bend and distort rational calculation.

Realist rationality requires balancing the economic costs and benefits of war against casualties and the likelihood of victory—yet this is not easy. Rulers would need to assess four separate metrics at once, and there is no single equation for this. The cost in lives, however, was often irrelevant for rulers, as few risked their own in battle. By the 20th century they were desk killers, sending young men to distant deaths. Few campaigns have been abandoned because rulers feared heavy losses. Setbacks were more likely to intensify calls for ‘sacrifice’, which they were not making themselves. In the past, many rulers saw their soldiers as ‘scum’, drawn from the uncivilized lower classes. Modern soldiers have been wary of being used as cannon fodder: French troops rebelling in World War One demanded their sacrifice be ‘proportional’. Afghan troops fled in 2021 when their sense of proportionality was shattered by sudden American withdrawal.

Realist analysis neglects the vitally important role of emotions and ideologies in the decision to go to war. They serve to fill the gaps in human rationality, where scientific knowledge falls short, enabling action amid uncertainty—for war is always a risky shot in the dark. Emotions like resentment, hatred and ambition play a major part in escalations towards war, in environments more conducive to fever than calm—amplified by the ‘anarchy’ of inter-state relations. Disputes may escalate through provocations, hostile rhetoric, sabre rattling, a clash of patrols, the sinking of a ship or the maltreatment of citizens abroad, all serving to fuel emotions. Publicizing the other side’s atrocities increases the odds of further escalation. Adversaries are seen as ‘terrorists’, America as the Great Satan, Iran as part of the Axis of Evil. Negotiating with evil is difficult. Pragmatism is called upon for peace-making, but war involves an appeal to emotions that intensify once combat is joined, making it harder to disengage. Aggression fuelled by self-righteousness can override contradictory information that might counsel peace. When both sides are emotional, damaging mutual brinkmanship follows.

The evidence suggests that the mixture of overconfidence and unreasonable fear has played a significant role in launching modern wars. An analysis of 26 wars in the 20th century found that failures of decision making were mainly due not to imperfect information or commitment problems, as realists would say, nor to material interests, as Marxists and economists would argue, but to sentiments of honour, status and revenge.footnote15 A study of the role of provocation in modern warfare showed that great powers had twice been overrun by unprovoked aggressors, but on a further six occasions, aggressors were provoked by the victim’s ‘fantasy-driven defensive bellicosity’, the threat heightened by ‘their own tendency to exaggerate the dangers they face, and to respond with counterproductive bellicosity.’footnote16 An earlier investigation, stressing the role of over-confidence in decisions for war, found that rulers underestimated their adversary, or the chances of others coming to its aid, due to ‘a lack of realistic empathy with either the victims or their potential allies.’footnote17 These studies did not include colonial wars, where lack of empathy was even starker. These emotional states—fear, over-confidence, lack of empathy—confound rational decision-making. We see all three in Russia and Israel today.

An important factor here is the bonding effect that societies exert on their members, as Durkheim noted. Ibn Khaldun called this asabiyya, normative solidarity, which generates a collective will to pursue further goals. He claimed this was the most fundamental bond of human society and the basic motive force of history, explaining soldiers’ commitment and bravery in war. But normative solidarity has a dark side: a lack of empathy and understanding of the enemy. For Durkheim, national society is a cage, imprisoning a people within its stereotypes of ‘the other’. In wartime, troops sing as they march into battle, confident they will be home soon, unable to imagine the enemy troops at that moment doing likewise, with the same brio. Because rulers deny justice to the enemy’s cause, they minimize its righteousness, morale and staying power, which can endure even after defeat. Again, Putin and Netanyahu are good examples. How can they think that their appalling offensives will bring eventual compliance with their rule? For that matter, did Hamas leaders really understand what fury from the skies their atrocities of October 7 would unleash? Rulers view enemy resources opaquely, guided by external signifiers of weakness—rumours of disunity or discontent, low troop morale, supposed decline, compounded by racial or religious stereotyping, or disdain for a supposed weakling leader—and so blending understandable mistakes with self-delusion.

Values and facts

Over-confidence also results from the blurring of fact and value. Rational-choice theory strives to be scientific, to keep fact and value apart; ‘what is’ governs the world, not ‘what should be’, as all social scientists are taught. Yet human beings do not operate like this. We all blur fact and value. In war, this appears as the belief that our cause is just and so we should achieve victory. The English word ‘should’ carries a double meaning: victory is a moral desiderata, but also a probable outcome. French and German cognates suggest the same. In the American Civil War, both Union and Confederate soldiers believed that the justness of their cause meant they should win quickly. At the start of World War One, British troops thought they would be home by Christmas, German troops before the autumn leaves fell. Roman senators believed all their wars were just, blessed by the gods, generating righteous aggression—and victory. Chinese Confucian and Legalist theorists discussed this idea at length, mostly concluding that a just and virtuous ruler would defeat an unjust and despotic one because the people would offer him more support. Right makes might. Whether this is true is debatable, but if one side feels especially righteous, its morale may be higher and its battle performance better, as classical Chinese theorists and Ibn Khaldun argued, and as the Vietnamese plf demonstrated against the Americans. If both sides are self-righteous, however, the result is a more murderous conflict, like the Thirty Years’ War, World War Two or Israel–Palestine.

Ideologies that demonize the enemy also breed over-confidence. Putin demonizes Ukrainians as fascists; American administrations demonized the ayatollahs, Saddam and Gaddafi. In consequence, Putin believed he would achieve a swift military and political victory; the Americans and Israelis knew their military power would bring victory in the field but were deluded about political aftermaths. They believed in the global justice of their cause. Americans ‘should’ be welcomed by Iraqis, they ‘should’ be able to establish democracies; Israelis ‘should’ be able to find security for their state. The salience of ideological warfare against an ‘evil’ enemy has risen in recent times, contradicting Weber’s assertion of the increasing rationalization of modernity. Modern ideologies have spawned aggressors who want to transform or destroy the society of those they attack. The most extreme case was Nazi Germany, for death or slavery awaited Jews, communists and Slavs if the Nazis won. For these groups, self-defense involved a desperately rational attempt to survive.

Rulers have also been tempted to follow the paths that had brought past success. Victories begat confidence, which made war a more likely outcome of a dispute. Rome, the last few Chinese Warring States, the remaining Japanese daimyo and the surviving major rulers in early modern Europe all grew accustomed to victory. Most finally got their comeuppance, but sequences of triumphs had ‘baked in’ martial culture and the centrality of military institutions. Rulers perceived war, not trade, as the way to wealth, career success, social status and glory. The Roman Republic was an extreme case of martial culture’s entrenchment, and Roman militarism was unusually long-lived. But warfare has become baked into successive societies, down to Prussia-Germany and modern-imperial Japan. Marx said that the tradition of all dead generations weighs like a nightmare on the brains of the living; but for rulers accustomed to victory, war is often a pleasurable daydream.


Two ways to lessen the pain of war were developed. One involved rules of engagement that kept the death rate low for dominant classes and officers. An extreme example was the Aztec ritual ‘flower wars’, but milder forms existed in China during the Spring and Autumn period, in Europe in the Middle Ages and again in the century following the 1648 Peace of Westphalia. War was not absent, but it was regulated, with rules about the treatment of prisoners and of captured cities, making it less costly—for some.Wars of deflection were another way to lessen the pain. In ancient China and in Europe, conflicts between the major core powers could be partially deflected onto weaker peoples on the periphery, or onto the enemy’s lesser allies. Empires were built through expansion into the peripheries—Rome around the Mediterranean, Zhou dynasty rulers among the ‘people of the field’, and Britain and France across the world in the 18th century, when their peace treaties typically conferred territorial gains on both, at the expense of colonized natives. Then followed a division of the spoils, as in the European ‘Scramble for Africa’ and for late-imperial China, where all the major foreign powers provided troops for an allied force to suppress Chinese resistance. The Cold War deflected usussr conflict into proxy wars; a rational strategy for the superpowers, if not the client states and movements they instrumentalized.

Conversely, repeated defeats or costly draws lowered ambition, undermining militarism—a delayed ‘realist’ reaction, as in imperial Rome after repeated inconclusive wars with the Parthians and northern barbarians. Japan’s terrible civil wars in the 16th century produced widespread yearning for peace, which Tokugawa policies were able to provide for the next 300 years. Realist pragmatism was more often a short-term effect. Four of Europe’s worst conflagrations—the Thirty Years’ War, the Napoleonic Wars, the two World Wars—led to periods of diplomatic peace. In the first three, this was temporary. The fourth now seems fragile. America’s recent spree of unsuccessful wars may not result in long-term caution in Washington, since us rulers have deflected the risk of death onto enemy soldiers, civilians and hired contractors, all dying far from the public gaze. For some Americans, the war in Ukraine is a perfect storm, weakening Russia by expending Ukrainian lives and us dollars, but not American lives. Trump does not agree. He is meaner about dollars.

4. survival?

There is one further set of realist arguments for rational wars that we should consider. Those known as defensive realists, such as Waltz, say states prioritize survival and calculate rationally the means of ensuring this. Aggressive Realists say that states calculate the economic or strategic profit from war set against its cost in treasure and lives and the likelihood of military victory. If the odds seem favorable, states will go to war. States will initiate war when militarily strong and choose defense or diplomacy when weak. I will cast doubt on these propositions.

Rulers themselves believe their decisions for war are rational and they surely try to avoid a war they believe they are likely to lose. But we can pose a simple test: Do those who initiate aggressive wars win them? Obviously, some do not, but that may only indicate understandable mistakes. What if initiators systematically lost wars or fought costly wars with no victor? Quantitative data from four studies are available.footnote18 Their figures average out at around a 50 per-cent chance of success for the aggressor, and about half the successes had pitted sharks against minnows, where the result was rather predictable. Would you take the risk of initiating a war with only 50/50 odds? Clausewitz noted that war is a gamble – but not perhaps a reasonable one.

It is hard to credit defensive Realism’s belief that survival is states’ major goal, for overwhelmingly states have failed to survive. This was not true of postcolonial Latin America, where balancing against would-be hegemons was successful in six wars and failed in none. After the 1830s all its states survived. This is unusual. Only one of over seventy polities in post-Zhou China survived. Sixteenth-century Japan saw over two hundred polities reduced to just one. The more than three hundred states of Europe were whittled down to thirty by the twentieth century. An unknown number of states and tribes disappeared from pre-Columbian America and Africa. Human civilizations have expanded by eliminating most of the world’s lesser polities. Elimination came through defeat in war, submission to the threat of force, or marriage and inheritance contracts. Most vanishing states died through war or the threat of war. But vanishing rarely occurs today.

I began by asking why rulers choose war to achieve their ends, instead of softer sources of power. But choice is not quite the right word, since decisions also embody social and historical constraints of which the actors may not be wholly aware, since they constitute part of the taken-for-granted reality. Social structures are created by humans but then become institutionalized, constraining subsequent action. War and peace decisions are influenced by constraints inherited from the past. There are multiple causes of war; motives and means need to be set in their historical, ecological and geopolitical contexts, as do erratic processes of escalation. If their varied interactions subvert realist reason, they may also defeat any simple causal theory. In response, some ir realists have broadened rational choice to include all these factors—emotions, ideologies, domestic politics and so forth. But if these are regarded as rational, the theory becomes circular and we cannot identify irrationality at all.

At the heart of both realist and Marxist theories of war is the idea of combined economic and military power—seizing material resources through war. This is sometimes rational for the winners, although as we have seen it is overwhelmingly zero-sum: for some to benefit, others must suffer. But since greed, a status-honour-glory triad and love of domination are all clearly important motives for rulers, rationality cannot be said to dominate war decisions. Moreover, miscalculation has occurred too often for a rational-choice model to hold. The offensive wars that go according to plan are mostly those in which ‘sharks attack minnows’, or when wars among the sharks are deflected onto the minnows, as in the Cold War. The military superiority of sharks means they do not need much calculation of odds, for they can bank on winning. And since they write history, victory in war is seen as more likely, more profitable, more rational and more glorious than it really is.

1 See Michael Mann, The Sources of Social Power, in four volumes: Cambridge 1986, 1993, 2012, 2013.
2 Robert Jervis, ‘Cooperation under the Security Dilemma’, World Politics, vol. 30, no. 2, January 1978.
3 John Mearsheimer, The Tragedy of Great Power Politics, New York 2001, p. 31.
4 Kenneth Waltz, Theory of International Politics, Reading ma 1979.
5 This essay draws on my recent book, On Wars, New Haven and London 2023. In examining the historical record I use two types of evidence: political scientists’ quantitative research on wars since 1816, and my own analysis of long-run sequences of war during the Roman Republic, ancient and imperial China, Japan from feudalism to 1945, Europe over a millennium, pre- and post-colonial Latin America and the United States from the Civil War to today. Here I present my bare conclusions; evidence and sources are in the book.
6 See the vivid descriptions by S. L. A. Marshall in Island Victory: The Battle of Kwajalein Atoll, New York 1944; Ambush: The Battle of Dau Tieng, New York 1969; and Bird: The Christmastide Battle, New York 1969.
7 Evan Luard, War in International Society: A Study in International Sociology, London 1986.
8 Thomas Otte, July Crisis: The World’s Descent into War, Summer 1914, Cambridge 2014.
9 See On Wars, pp. 465–73.
10 See On Wars, pp. 417–20.
11 Ibn Khaldun, Muqaddimah: An Introduction to History, trans. Franz Rosenthal, Princeton 1958 [1377].
12 For example, Margaret MacMillan, War: How Conflict Shaped Us, New York 2020.
13 Clifford Thies and Christopher Baum, ‘The Effect of War on Economic Growth’, Cato Journal, Winter 2020.
14 James Fearon, ‘Domestic Political Audiences and the Escalation of International Disputes’, American Political Science Review, vol. 88, no. 3, 1994.
15 Richard Lebow, Why Nations Fight, Cambridge 2010.
16 Stephen Van Evera, Causes of War: Power and the Roots of Conflict, Ithaca 1999.
17 Ralph White, ‘Why Aggressors Lose’, Political Psychology, vol. 11, no. 2, 1990.
18Melvin Small and David Singer, ‘Patterns in International Warfare, 1816–1965’, Annals of the American Academy of Political and Social Science, vol. 391, no. 1, 1970; Dan Reiter and Allan Stam, Democracies at War, Princeton 2002; Lebow, Why Nations Fight; White, ‘Why Aggressors Lose’.