II. Causes of War

“The prohibition of war! The prohibition of war!” wrote Yokota Kisaburo, a professor of international law at Tokyo Imperial University in 1931, who dared to speak out against Imperial Japanese conquests in Manchuria and beyond at a time when doing so could destroy his career or get him assassinated.Footnote ¹⁷ “That is the main subject of the twentieth century.”Footnote ¹⁸

This review essay is a response, although inadequate, to Yokota’s imperative. Why are there wars? Drawing on two rich books by the leading historians Margaret MacMillan and Richard Overy, the essay aims to connect the theoretical concerns of international law with the sweep of centuries of international relations. MacMillan is a distinguished diplomatic historian whose work has illuminated not just the outbreak of World War I, but also the processes of peacebuilding, from her magnificent study of the Paris Peace Conference after World War I to her incisive chronicle of Richard Nixon’s opening to Mao Zedong’s China.Footnote ¹⁹ Overy is a creative and wide-ranging historian who focuses on World War II, with outstanding work spanning from strategic bombardment to the reasons for Allied victory to a recent major reconceptualization of World War II as an imperial contest.Footnote ²⁰ Both of them have taken a step back from the specifics of twentieth-century political-military history to write ambitious, insightful books that consider war as a general phenomenon.

One of the particular strengths of international law as a discipline in the study of war is its insistence on considering the prevention of war, rather than just taming its conduct.Footnote ²¹ Like just war theorists, international law treats both the jus ad bellum and the jus in bello.Footnote ²² There was a failed Allied effort to prosecute Kaiser Wilhelm II for his aggressive role in starting World War I.Footnote ²³ At the Allied international military tribunals at Nuremberg and Tokyo after World War II, the main accusation was a criminal conspiracy to wage aggressive war.Footnote ²⁴ That legacy was invoked by the International Criminal Court in its revival of the crime of aggression, despite objections that such prosecutions might interfere with peace processes, antagonize the great powers, or prove fruitless.Footnote ²⁵ Like Yokota, who kept up a lifelong engagement with international law even when became chief justice of the Supreme Court of Japan, international lawyers have long taken seriously this charge to outlaw war. But to prohibit war, it would help mightily to know what really causes it.

The origins of war have confounded such thinkers as Albert Einstein and Sigmund Freud, who had a memorable correspondence on the matter in 1932—which gives Overy his title, Why War? Footnote ²⁶ To that question Homer barely bothered with an answer: The Iliad begins in the middle of the Trojan War with Achilles sulking and refusing to fight, with the narrative less interested in why the Greek armies attacked Troy than in the greatest Greek warrior’s coming one-on-one confrontation with Hector, the Trojan prince and champion. William Shakespeare often treated war as a remedy for regicide,Footnote ²⁷ but he too could breeze past its causes. At the start of Troilus and Cressida, his own chronicle of the Trojan War, he wrote that “our play leaps o’er the vaunt and firstlings of these broils, Beginning in the middle.”Footnote ²⁸

MacMillan and Overy come up with several explanations for those firstlings, which have considerable overlap between them. Surveying the centuries, MacMillan finds that “certain motives appear again and again: greed, self-defense, and emotions and ideas” (MacMillan, p. 35). These make a rough parallel to Overy’s main reasons for war (shuffling the order in which he listed them): resources, security, and belief. To that Overy adds a fourth: power (Overy, p. 3). Those categories will structure this essay.

III. The Human Condition

It should be noted at the outset that neither of these books have much use for international law. When Overy mentions law at all it is usually as a justification for war, mordantly citing Sultan Mehmet II’s sack of Constantinople in 1453: “Under Islamic law, three days of looting were permitted, but Mehmet, hoping to make the city his capital, permitted only one” (Overy, p. 121). MacMillan devotes a powerful chapter to international law, although she approaches the Sisyphean problem of legal constraints on war with a tragic sensibility: “Like ants with their nests, we laboriously build up a more or less agreed structure only to see it kicked apart by the heavy foot of war. We soldier on to reconstruct what have come to call the laws of war” (MacMillan, p. 206).

She points to two ideal-type traditions of jus ad bellum and jus in bello typified by advocates: for the former, Bertha von Suttner’s advocacy against war; for the latter, Francis Lieber’s efforts to codify modern laws of war-fighting (MacMillan, pp. 203–05).Footnote ²⁹ Under the jus ad bellum, MacMillan argues that “even the powerful and ruthless have looked for reasons or excuses of some sort to justify making war” (MacMillan, p. 206).Footnote ³⁰ She condemns the “wars for gain or dominance” that she documents throughout her book, writing respectfully of the principle of waging war only in self-defense (MacMillan, p. 211). She rightly connects the purposes of war with its conduct, since soldiers’ lives should not be squandered for objectives beyond the limits of a just war (MacMillan, p. 209). And she notes accurately that Nuremberg and Tokyo charged defendants not just with conventional war crimes but crimes against peace, but cannot forget that “the Soviet Union, now on the side of the virtuous, had connived with Hitler to carve up the center of Europe” (MacMillan, pp. 226–27).Footnote ³¹

As for the jus in bello, she warily writes that it is “not a legal code as we would recognize it in domestic society” (MacMillan, p. 213).Footnote ³² Although her account of the origins of both Geneva and Hague law is cautious, she does find much in the evolving jus in bello that is useful: conventions about prisoners of war, attempts to control battlefield tactics or newfangled weaponry such as the crossbow or explosive bullets (MacMillan, pp. 213–15), the principle of discrimination in order to spare civilians (MacMillan, p. 218). Yet she notes gloomily, “The actions of the British in Iraq in the early 1920s, where the new tool of air power was used to bomb rebellious districts into submission, of the Italian and German air forces in the 1930s in Spain during the Civil War or the Japanese in China after 1937 demonstrated that, as much as the international community might deplore attacks on civilians in general and on their resistance in particular, the powers ignored international agreements when it suited them” (MacMillan, p. 222).

MacMillan is both respectful and skeptical about the enterprise of international law. She warns about the “maze of contradictions” (MacMillan, p. 210) in building a workable legal system, wonders who will judge the rights and wrongs, and notes that the outbreak of war often arises from a long and tangled history of reciprocal harms. Alert to colonialism as a force in the making of world order, she is right to point out that “[t]he West does have a long and shameful history of observing one set of rules for itself and another for those it considers less ‘civilized’” (MacMillan, pp. 212–13). She singles out George W. Bush’s administration for torture at Abu Ghraib, which she notes was licensed at the time by the U.S. Department of JusticeFootnote ³³ (MacMillan, p. 223). Above all, she worries that the increasing lethality of modern warfare will rip through all legal constraints. The limits on justifying war are strained by the Clausewitzian inherent escalation of war, with limited war intensifying into absolute war: “Rules are easily broken or ignored, however, in the passions raised by war” (MacMillan, p. 179).

To be sure, some international lawyers would see international law as having such deep roots in human interactions since antiquity that it has a claim to be part of the human condition,Footnote ³⁴ or at least to stand as an important belief system that has long served as a source of authority or legitimacy.Footnote ³⁵ Yet both of these books look elsewhere than law for the fundamental truths about war.

In his incisive book, Overy tries to understand, if not what causes war, then at least how some influential disciplines have tackled that question. He finds a sharp disconnect between the human sciences on the one hand and social scientists and historians on the other. The human sciences, including biology, psychology, and anthropology, have explained that war is an evolutionary adaptation or a cultural determination (Overy, p. 2). Humans, on this account, are subjected to natural forces or to cultural pressures (Overy, p. 2). In contrast, for social scientists and historians, it is human beings who cause the wars for four main kinds of reasons: for resources, belief, power, and security. Whatever the cultures that entrench warfare, they are made by humans. In short, the human sciences deprive humans of agency, while the social sciences restore it (Overy, p. 3). That distinction is crucial for international criminal law. If the human sciences are right, that would badly undermine the prosecution or punishment of individual leaders for launching wars: how can they be charged as criminals for doing what comes naturally?

Overy and MacMillan, following Freud, agree that more “advanced” civilization has not gotten rid of war. There is, depressingly, a growing body of archeological evidence of human warfare going back the Neolithic era or even the Pleistocene past. The historian Kathleen DuVal shows that Indigenous nations in North America formed alliances and fought wars long before European colonizers, aiming to expel or destroy them, became dominant over the continent. Native Americans, she argues, were not primitive but neither were they peaceful.Footnote ³⁶ Overy, like MacMillan, argues that as states became more powerful and bureaucratized, they could kill their enemies more effectively; indeed, warfighting was a major reason why they got more effective (MacMillan, pp. 3–4).

The explanations from evolutionary biology are in many ways unanswerable. They derive their force from the undeniable fact of warfare across the centuries, or even from observations of primate behavior.Footnote ³⁷ (As Gilbert and Sullivan put it in 1884, “Darwinian Man, though well-behaved, At best is only a monkey shaved!”Footnote ³⁸) Charles Darwin’s theory of evolution is mostly about how a plant or animal adapts to its environment and its main mechanism is sexual selection rather than violence. Still, Darwin did briefly mention that some ancient tribes wiped out others, although not necessarily as a matter of fitness. In Germany, Darwin’s theory was reimagined as being about competition between so-called races (Overy, p. 13). That phantasmic nexus between purported racial fitness and war became an ugly feature of the prewar era.Footnote ³⁹ Overy notes that Sir Arthur Keith, a British post-Darwinian writer, contended that war was a biological adaptation because it promoted the strong and killed off the weak (Overy, pp. 11–14).

To give a sense of how deeply this racialism ran not so long ago, it is worth noting that even after World War II, Keith promoted a kind of scientific racism, casually claiming that “the Welsh and the Scottish are less resistant” to assimilation “than the Irish or the Italians,” and that “the Armenians and Parsis … share the isolating racial mentality of the Jew.”Footnote ⁴⁰ In two ugly chapters about Jews in a book published in 1948, he wrote about what he considered clannishness and “inbreeding communities” of Jews, concluding: “My deliberate opinion is that racial characters are more strongly developed in the Jews than in any other Caucasian people.”Footnote ⁴¹ Just three years after Auschwitz, he criticized “the Jew” for not accepting blame for antisemitism: “Very rarely does he ask the question: ‘Why are my people objects of antipathy to so many Gentiles?’”Footnote ⁴²

After surveying evolutionary biology, Overy reckons it “an unavoidable conclusion that for most of human existence the pursuit of inclusive fitness included not only sociality and cooperation but also conflict when the circumstances made it seem necessary” (Overy, p. 31). Yet the argument from evolutionary biology only takes us so far. It sometimes blurs the lines between small-scale violence and modern warfare. It skirts the logical error of the naturalistic fallacy: even if war is natural, that does not make it good. Its claims about our bestial ancestors may be overblown: Frans de Waal, a specialist on chimpanzee behavior, observed that these primates negotiate, accommodate, and engage in what he called “peacemaking among primates.”Footnote ⁴³ Yet most importantly, while evolutionary biology may establish that humans have an obvious propensity toward warfare, it also shows that we have an inclination toward peaceful cooperation as well.Footnote ⁴⁴ As an explanation, it cannot answer the more fine-grained question that really concerns international lawyers and social scientists: under what circumstances is warfare more or less likely?

If not in evolutionary biology, perhaps the answer lies in individual psychology. The psychoanalytical literature here is rather inadequate, replete with unverifiable claims about repressed aggression, suppressed Freudian sexual fears, or anxieties developed by infants. As Overy frostily writes about one such assay, this was “a view for which there was not a shred of clinical evidence” (Overy, p. 35). The most famous of these arguments came after World War I, when, in that correspondence with Einstein, Freud provoked the peace-minded physicist with his own updating of the argument from biology. Freud remembered ruefully how exhilarated he had been when the Great War began, “giving all my libido … to Austria-Hungary.”Footnote ⁴⁵ He contended that humans are hard-wired with a “death instinct,” which seeks destruction—the opposite of his claims of the power of an erotic drive, which wants life to thrive. Seeing that all societies are warlike, he concluded that this belligerence must be a biological phenomenon. “Here is then the biological justification for all those vile, pernicious propensities which we are now combating. We can but own that they are really more akin to nature than this our stand against them”—that is, more natural than pacifism.Footnote ⁴⁶ War, he wrote, was “a natural thing enough, biologically sound and practically unavoidable.”Footnote ⁴⁷

More recently, psychological studies of human judgment and decision making have newly explained some recurrent pathways to war. Our brains are not very good at complicated decision making, as Daniel Kahneman, the Nobel-winning psychologist taught—pioneering, along with Amos Tversky, a fascinating literature. Kahneman showed that our brains operate on two systems, thinking fast and slow: what he calls System 1, which is fast, automatic, impressionistic, biased, often wildly wrong; and System 2, which is slow, deliberate, more thoughtful, but gets easily tired and overwhelmed. Almost all of the biases in System 1 lean toward hawkish foreign policy decisions: we tend to exaggerate how evil our enemies are; we fail to understand how threatening our own actions look to them; and because we weigh losses more heavily than gains, we have an irrational aversion to cutting our losses even when a war is going badly. These pathologies in decision making make us belligerent, although we can try to correct against them by an awareness of these biases.Footnote ⁴⁸ These insights have led to a rich, emerging literature in political science about how psychology can drive international conflict.Footnote ⁴⁹

Tellingly, one recurrent psychological bias is that people imagine themselves more in control of their destiny than they actually are, which helps to explain why so many leaders have expected to win quickly in an opening salvo—as the German General Staff did with the Schlieffen Plan in 1914—overestimating their chances of winning a conflict that all too often turns out to be escalatory and protracted.Footnote ⁵⁰ As MacMillan writes, “Too often nations start hostilities without thinking ahead to what they hope to achieve and what sort of peace they would like” (MacMillan, p. 45). She aptly singles out Tojo Hideki and the Imperial Japanese army leadership for plunging into a simultaneous attack on the United States, the British Empire, and the Dutch Empire without a clear sense of how to get out of the war (MacMillan, pp. 45–46).Footnote ⁵¹ From the Soviet and American wars in Afghanistan to the contemporary wars in Ukraine and Gaza, governments have planned for the opening engagements, only to find themselves mired in an interminable morass.Footnote ⁵²

IV. War as Plunder

Both MacMillan and Overy agree on the importance of material motivations for war. Overy couches that in more neutral language, while MacMillan treats this motive as sinful, calling it greed: “Greed for what others have, whether it is food for survival, women for servitude or procreation, precious minerals, trade or land, has always motivated war” (MacMillan, p. 35). Both MacMillan and Overy point to Imperial Japan’s drive to conquer new colonies in search of raw materials, markets, and land for a growing population on the home islands—which would spur Japan first into Manchuria and then into the rest of China (MacMillan, p. 34; Overy, p. 97).Footnote ⁵³

The idea of war from greed is an ancient one, going back at least to Socrates. In Plato’s Republic, Socrates contends that, in a city that has some material indulgences beyond what is strictly necessary, the inhabitants will encroach on their neighbors’ land: he expects that “we cut off a piece of our neighbors’ land, if we are going to have sufficient for pasture and tillage, and they in turn from ours, if they let themselves go to the unlimited acquisition of money, overstepping the boundary of the necessary?”Footnote ⁵⁴ Then they will “go to war as a consequence.”Footnote ⁵⁵

In The Federalist Papers, Alexander Hamilton argued that commerce would be no bar to aggression: “Is not the love of wealth as domineering and enterprising a passion as that of power or glory?”Footnote ⁵⁶ He points out that ancient Athens and Carthage, though commercial republics, were distinctly aggressive; he notes the same contemporary pattern in Venice, the Netherlands battling England for control of the seas and fighting Louis XIV, and Britain’s commercial interests driving it into war against Spain.Footnote ⁵⁷ And for Vladimir Lenin, imperialism emerged from the most advanced stage of finance capitalism, as predatory empires hunted for resources, markets, and land.Footnote ⁵⁸

Overy writes that Marxist analysis has difficulty with the age of monopoly capitalism, since “the burden of proof that specific wars were engineered by big business has proved persistently difficult to find, despite the presence of powerful industrial and financial organizations dedicated to military production and research. It was not the armaments barons who pushed Europe into war in 1914” (Overy, pp. 116–17). Yet he does believe that control of natural resources—especially oil—was a reason for the Suez War in 1956, the Biafra war in Nigeria in 1967–70, the Sudanese civil war in 1983–2005, the Gulf War in 1991, and the United States’ invasion of Iraq in 2003, among others (Overy, pp. 131–32, 136–38). During World War II, he notes, the Allies controlled some 90 percent of natural oil output (Overy, p. 136). When the United States slapped Japan with an oil and gasoline embargo in retaliation for the takeover of southern Indochina, it became a major part of the rationale for launching the Pacific War in 1941: Japan decided to conquer oil supplies in Southeast Asia, particularly Dutch-controlled Indonesia, in order to fuel its navy.

In a fascinating chapter, Overy brings this literature up to date with a study of ecology as a reason for conflict—anticipating the ways that the unfolding climate disaster might, among its myriad dire consequences, drive states into war. He carefully distinguishes ecological warfare, which is driven by a shortage of vital natural resources, from simple material conquest, which is meant to boost imperial or national power by weakening others (Overy, p. 112). There have been previous crises from glacial and interglacial periods, which brought changes in weather that threatened the survival of humans and other animals (Overy, p. 99). Overy argues that competition for scarce resources could have caused warfare over the centuries; so could migrations in search of better land which ran up against other inhabitants, as in prehistoric Chile and Germany (Overy, p. 88). More recently, Overy writes that “environmental stress and food scarcity” are important in explaining widespread killing in the American southwest between 900 and 1300, due both to climate change and fights between hunter-gatherers and landed farmers (Overy, pp. 90–91). He argues that food scarcity drove ecological warfare among the Maori of New Zealand (Overy, p. 91).

Overy finds it harder to adduce cases of ecology wars after the creation of kingdoms and empires, when other confounding factors make it harder to pinpoint the environmentally driven motives (Overy, p. 95). Trade and better farming were cheaper solutions than warfare. Still, he contends that Adolf Hitler’s drive for Lebensraum was drawn in part—Hitler’s thinking is always derivative—from German geographers and other writers who thought that the population of the master race would need new territory (Overy, pp. 95–96). Despite the limited evidence for wars driven by scarce resources, Overy expects more of them as the climate crisis worsens (Overy, p. 99). As he points out, it was during moments of climate crisis that the Mongols and Manchus invaded China; he even suggests that warfare driven by climate change may have helped, at least to some extent, to destroy the Song, Ming, and Qing dynasties (Overy, p. 101). Overy does not overhype his conclusions, admitting that much of the evidence is tenuous, but he does make a reasonable case that, other things being equal, ecological crisis increases the likelihood of interstate violence (Overy, p. 108).

V. In the Posture of Gladiators

If war is back, realists would say that is because it never really left. For those who see the international system as a dangerous brawl in anarchy, state-controlled violence is an inevitable feature of a disordered world. If so, we should learn to accept war, at most seek to manage or mitigate it, but not to dream of abolishing it.Footnote ⁵⁹ Realists scorn the judgments of international criminal law, since state leaders must have recourse to violence in anarchy.Footnote ⁶⁰ Even the liberal political theorist Judith Shklar approved of Nuremberg’s charges for crimes against humanity but not for crimes against peace, believing that the road to war was usually too tangled for legal judgment.Footnote ⁶¹

As realists correctly point out, states confront each other in conditions of international anarchy, where no sovereign stands above them. States disagree about who is allowed to use violence and are jealous of their own right to use force. As Thomas Hobbes wrote in Leviathan, “in all times, Kings, and Persons of Soveraigne authority, because of their Independency, are in continuall jealousies, and in the state and posture of Gladiators; have their weapons pointing, and their eyes fixed on one another; that is, their Forts, Garrisons, and Guns upon the Frontiers of their Kingdomes; and continuall Spyes upon their neighbours; which is a posture of War.”Footnote ⁶²

Citing that famous passage from Hobbes, it is not until the end of his book that Overy gets to what most political scientists studying international relations would see as the starting point for explaining war: the search for security under these conditions of anarchy. He concurs with realists that states exist in “a Hobbesian state of nature,” a war of all against all. With no higher power to protect them, states are forced to rely on self-help. Overy thinks that alliances, treaties, and international institutions may help to avoid conflict, but warns that self-interest and the fundamental lack of trust between rival sovereigns will always impede cooperation (Overy, p. 198).

As Overy notes, realists divide into two main camps (Overy, p. 219), which the political scientist Jack Snyder has labeled as offensive realism and defensive realism.Footnote ⁶³ According to offensive realism, mistrustful states expand in pursuit of hegemony at the expense of rival powers, since dominance will ensure their survival. All states are in a race for supremacy and therefore all states are revisionists. Powerful states attract allies.Footnote ⁶⁴ In contrast, according to defensive realism, states can achieve security through defensive policies or a balance of power. Defensive realists warn that bellicose policies—the ones advocated by offensive realists—will be self-defeating, provoking rival states to adopt their own belligerent policies. Powerful states spur the formation of opposing alliances.Footnote ⁶⁵ Offensive realism counsels rapid aggression; defensive realism counsels prudent restraint. Overy is dissatisfied with both, warning that a military buildup undertaken in the name of offensive realism may simply spur an arms race, while unilateral disarmament in the name of defensive realism might tempt aggressors (Overy, p. 220).

Both forms of realism pose a problem for international criminal prosecutions for crimes against peace. In offensive realism, even the revisionist states are propelled toward their conquests not by a criminal motive, but by the fear that other states would otherwise do likewise unto them. And if defensive realism is correct, then leaders charged with aggression would reply that they were simply reacting to the international system. Defensive realism looms large in both books: Overy discusses the search for security as a cause of war, while MacMillan is especially attuned to self-defense as a cause of war. Political scientists have long warned about the dangers of the security dilemma: in conditions of anarchy, unilateral steps by a state to ensure its own security wind up provoking counter-steps by another state, resulting in less security for either. Robert Jervis termed this the spiral model of international relations.Footnote ⁶⁶

The problem is, again, an ancient one. When the Athenians are about to launch a massive invasion of Sicily, Thucydides quotes a leader of a Sicilian city: “the fact that they are coming with so large a force is far from being a disadvantage to us. Indeed, it is much better that way, when one considers the rest of the Sicilians, who will be terrified, and therefore all the more willing to become our allies.”Footnote ⁶⁷ Thus MacMillan writes with disapproval, “Sometimes, it is argued, a preventive war, against a threat which is more anticipated than actual, is the best form of self-defense” (MacMillan, p. 36). According to Thucydides, the Spartans chose war out of fear that Athens, already strong and threatening, would only grow more so. Yet MacMillan cautions, “Suspicions and fears of others, from rival gangs to countries, can create perceptions of threats even where they might not exist, just as they do for our cousins the chimpanzees” (MacMillan, p. 37). She points out how the United States and China have been spiraling toward conflict well before Trump’s latest trade war, suggesting the need to think more creatively to avoid the trap of a ruinous war (MacMillan, p. 38).

For realists, the best guarantee of peace is not international organizations, war crimes tribunals, or a global legal order, but a balance of power.Footnote ⁶⁸ In the classic Sanskrit kingly handbook from the Mauryan Empire, The Arthashastra, Kautilya writes that one makes peace when one is evenly matched with one’s enemy—and even then only to build up one’s own strength.Footnote ⁶⁹ In some realist theories of international relations, war is a result of an imbalance of power. Either a state gets too weak and tries to knock its rival down to size, or a state gets too strong and tries to crush a rival. That is why Henry Kissinger, in his study of the peace built at the Congress of Vienna in 1814–1815 after the Napoleonic Wars, lauds statesmen such as Viscount Castlereagh and Prince Klemens von Metternich for seeking equilibrium among the five great powers, rather than retribution against France.Footnote ⁷⁰ Some realists even advocate for the proliferation of nuclear weapons because they believe that their terrible destructive power will ensure conventional peace based on a balance of terror.Footnote ⁷¹

Overy is wary about these claims about the balance of power. The international balance of power, he notes, is always in flux (Overy, p. 198). Like some historians, he is underwhelmed by the systematic aspirations of security studies: “History is made to fit the theory, rather than the other way round” (Overy, p. 221). Unlike some of the nuclear proliferation optimists in the realist school, he takes seriously the prospect that rogue states might actually use nuclear weapons, and points to Putin’s nuclear threats in Ukraine with concern (Overy, p. 223). He does not trust some of the standard social-scientific measures of military and economic power, and is unconvinced that power transitions will result in war (Overy, pp. 191–92). As for the present-day discussion about China as a rising power challenging the hegemonic dominance of the United States, he writes that there is still such a power gap between these countries that “to talk of power transition in this case is at best premature, at worst an ill-considered provocation” (Overy, p. 194).

In another part of his book, Overy makes a powerful analysis of some of the worst revisionist states that could perhaps be lumped under offensive realism.Footnote ⁷² Yet he refuses to treat their behavior as reactive, saying instead that they chose war out of a simple drive for power. He does not use the legal language of Nuremberg or Tokyo nor brand these leaders as suspected criminals, but he certainly assesses them harshly as aggressors. He firmly rejects the colonialist pretense that the territories of the Roman Empire, or later the British Empire, were acquired defensively. He recoils at the conceit that Roman expansion across much of Europe, North Africa, and Asia Minor could be described as anything but aggression (Overy, p. 173). Rome, he writes bluntly, fought its endless wars in “the pursuit of power at the expense of other polities” (Overy, p. 171).

This kind of power-seeking expansionism finds its rawest expression in what he calls “hubristic power,” the ambitions of an individual leader using warfare to swiftly build a vast new empire: Alexander, Napoléon Bonaparte, Hitler (Overy, p. 176). Here his argument about hubris dovetails with MacMillan in her discussion of honor and glory as irrational but powerful reasons for war, where her prime examples include Alexander and Napoléon (MacMillan p. 38). As she writes, “Honor and glory are abstract concepts yet they can matter more than life itself” (MacMillan, p. 38). (Today, she notes, governments are more apt to talk of credibility than of honor, although she sees them as similar.)Footnote ⁷³ Napoléon was intoxicated with his own superiority and dominance: “My power depends on my glory and my glory on my victories” (Overy, p. 181).

For Overy, hubristic warfare is not about systems or cultures, but about individual personalities—a viewpoint that could readily align with Nuremberg and Tokyo’s principles of individual criminal responsibility.Footnote ⁷⁴ In both Overy and MacMillan’s major examples, the hubristic leaders did not need to answer directly to their subjects; indeed, they readily sacrificed the lives of their followers to further their drive for conquest. Only a sixth of Napoléon’s troops in the Grande Armée ever came back from Russia (Overy, p. 184). “Here I am also ice-cold,” Hitler once said. “If the German people is not ready to support its self-preservation, so be it: then it should disappear” (Overy, p. 189).

This points to dictatorship itself as a possible cause of war. The reduction of government to the whims of a particular individual turns political science into psychology: a powerful person acts out a personal psychodrama on an international stage, perhaps willing to take big risks—or, as international lawyers would say, to behave as criminals. There are other reasons to worry about autocracies as they make foreign policy: dictatorships concentrate power in the hands of a single tyrant or a small leadership clique, who might have particular strategic goals that propel them to conflict;Footnote ⁷⁵ the habitual use of repression at home in a dictatorship may foster violent norms that spill over into the conduct of foreign relations; the political insecurity of a narrowly-based regime of shaky legitimacy might drive it toward conflict; the ideology that propelled a revolutionary dictatorship to power might drive its decision-making; or dictators might have difficulty getting good policy advice or accurate information, because they rely on those who are loyal rather than those who are competent.Footnote ⁷⁶ The political scientist Jessica Weeks has shown that particular kinds of dictatorships—personalistic strongmen and military juntas—are substantially more likely to start wars than civilian non-personalist machines, which are about as likely to start wars as democracies.Footnote ⁷⁷

If hubristic leaders or military dictatorships are prone to war, it is somewhat surprising that neither MacMillan nor Overy spend much time on domestic politics as a cause of war.Footnote ⁷⁸ Jack Snyder argues that military and imperialistic factions within a country can logroll together to drive their state into war, while Thomas Christensen contends that leaders will sometimes inflate foreign threats in order to mobilize domestic resources.Footnote ⁷⁹ The classic warning about authoritarian governance as a cause of war comes from Immanuel Kant.Footnote ⁸⁰ In his 1795 essay “To Perpetual Peace,” the Prussian philosopher feared a mismatch between the costs and the benefits of war under a monarchy, where a leader has much to gain and the people have much to lose:

He did not contend that the citizens would be peaceful because of any inherent goodness or pacifism, nor that the king was inherently wicked or belligerent. Instead, self-interest does the work in his argument, contrasting the people’s self-interest with that of a king or authoritarian ruler.

Kant sought political institutions that would take that self-interest and transform it into peaceful political behavior. To bring perpetual peace, he proposed a republican constitution. The free and equal citizens of a republic would, if given the choice between war and peace, have an interest in avoiding conflict. As he wrote:

Building on Kant, there is a long tradition of hoping that democratic politics may help to build peace, either because it empowers publics who fear wartime conscription and taxation, or because there are non-violent norms inculcated in democratic polities.Footnote ⁸³ Today the finding of a separate peace among democracies is one of the most robust in the subfield of international relations; as the political scientists Kosuke Imai and James Lo recently wrote, “the positive association between democracy and peace is much more robust than that between smoking and lung cancer.”Footnote ⁸⁴ The decline of democracy over the past nineteen years may be linked to the rise in war.Footnote ⁸⁵

Some scholars have argued that international law ought to play a role in encouraging the development of democracy, such as Thomas Franck’s much-debated claim soon after the collapse of the Soviet Union that democracy was becoming “a global entitlement, one that increasingly will be promoted and protected by collective international processes.”Footnote ⁸⁶ Drawing on Kant and the subsequent literature on the democratic peace, he argued that “the right to democracy can readily be shown to be an important subsidiary of the community’s most important norm: the right to peace.”Footnote ⁸⁷ Still, as he noted, even though the UN Charter in Article 4(1) limits UN membership to “peace-loving” states, it is more important that the Charter’s Article 2(7) mandated that the world organization not meddle in matters “essentially within the domestic jurisdiction of states.”Footnote ⁸⁸ More recently, Kim Lane Scheppele, with Hungary under Viktor Orbán’s hybrid authoritarianism in mind, has suggested that regional organizations ought to police democratic backsliding among their members.Footnote ⁸⁹ Whatever one thinks of the merits of these debates, they are worth discussing. Yet Overy never mentions the democratic peace.

VI. Ideas and Ideologies

If war is a crime, it is also often a crime justified in the name of some higher ideal. Max Weber linked ideas with interests, but with the emphasis still on material interest: “Not ideas, but material and ideal interests, directly govern men’s conduct. Yet very frequently the ‘world images’ that have been created by ‘ideas’ have, like switchmen, determined the tracks along which action has been pushed by the dynamic of interest.”Footnote ⁹⁰ In contrast, in recent years, there has been a renewed interest among political scientists in revolutionary ideology as a factor in political life.Footnote ⁹¹ Both MacMillan and Overy insist on the importance of ideology, with both of them emphasizing religion and nationalism, and MacMillan also focusing on revolutionary socialism.

Overy warns that secular historians too easily presume that faith is just a façade for some earthly motivation, such as class interest or a resource grab (Overy, p. 143). In contrast, both he and MacMillan single out the Crusades as major wars waged in the name of religion. Both authors also emphasize the Crusaders’ systematic massacres of Muslims and Jews in Jerusalem in 1099 (MacMillan, pp. 40–41; Overy, p. 146)—a depressingly apt point at a time when Trump’s secretary of defense has a tattoo with the Crusader slogan of “Deus vult.”Footnote ⁹² When war is waged to achieve an earthly paradise, then all manner of horrors are permitted to reach that paramount goal. As Overy disapprovingly puts it, “belief can mobilize popular engagement in warfare and justify its necessary excesses (Overy, p. 142).

He means political ideology as well as religion, ranging from Wilsonianism to Wahhabism (Overy, pp. 162–65), as well as Cold War anti-Communism in the West. Following the Italian historian Emilio Gentile, he says that Communism, fascism, and Nazism are “‘political religions,’ which are as demanding of their members as any puritanical church,” promising an earthly paradise of racial utopia or class utopia (Overy, p. 160). Still, it might be worthwhile to point out that ideology can be mobilized as a force for non-violence as well, such as M. K. Gandhi’s insistence on satyagraha in India’s freedom struggle against British colonialism,Footnote ⁹³ or Japan’s embrace of pacifism after its defeat in World War II.Footnote ⁹⁴ There are strategic advantages to peaceful action as well: as the political scientists Erica Chenoweth and Maria Stephan have shown in an important study of hundreds of conflicts by nonstate actors against states from 1900 to 2006, non-violent resistance campaigns are considerably more successful than violent ones.Footnote ⁹⁵

The most extreme and consequential example of an ideological war is Nazi Germany in World War II, with genocidal antisemitism inexorably mixed up with eastward expansion for Lebensraum.Footnote ⁹⁶ Overy properly treats the Holocaust as a war for the annihilation of the Jewish people: “The ideological drive to eliminate Jews from Germany and Germany’s new-won empire in Europe ended with mass extermination, carried out with guns, machine guns, and poison gas, as if the Jews really were a militarized enemy rather than the fearful, disoriented, and powerless victims they actually were” (Overy, p. 161). The German historian Karl Dietrich Bracher wrote that World War II was “the logical outgrowth of National Socialist ideology and rule.”Footnote ⁹⁷ Hitler repeatedly made a fantastical claim that the 1917 Russian Revolution had really brought the Jews to power in the Soviet Union. For Nazis, the vast country ought to be ripe for conquest: full of racially inferior Slavs, chaotically misgoverned by the lowest of all the races in the Nazi imagination, the Jews. When the “Judeo-Bolshevik” Soviet Union collapsed under German attack, the Nazis thought, it would be a triumph of Nazi racial thinking. As Lucy Dawidowicz put it, “War and the annihilation of the Jews were interdependent…. He had set into motion a twofold war—one that was traditional in its striving for resources and empire and that would be fought in traditional military style, and one that was unconventional inasmuch as its primary objective was to attain National Socialist ideology and that would be conducted in an innovative style of mass murder.”Footnote ⁹⁸

VII. The Past Is Prologue

Throughout both of these books, the authors never lose sight of the suffering that war brings to soldiers and civilians. MacMillan emphasizes how elites have driven soldiers into war, from Augustus Caesar to King Louis XIV to Napoléon (MacMillan, pp. 68, 86). This lines up with a careful recent study of the civil war in Sierra Leone by the political scientists Macartan Humphreys and Jeremy Weinstein, who find that combatants were often coerced or pressured into fighting, with few of them fighting for political motives.Footnote ⁹⁹ In parallel to her discussion of the causes of war, MacMillan writes that soldiers, too, will sometimes be motivated to fight “for gain, to defend themselves or because of ideas and emotions” (MacMillan, p. 124). Yet she also emphasizes the ways in which soldiers are coerced into serving, either directly by their states or indirectly by their societies. She quotes Tim O’Brien’s powerful, harrowing book drawing on his experiences fighting in Vietnam, The Things They Carried: “Men killed, and died, because they were embarrassed not to. It was what had brought them to war in the first place, nothing positive, no dreams of glory or honor, just to avoid the blush of dishonor” (MacMillan, p. 156).

MacMillan has an admirable commitment to conveying the misery and horror that war inflicts on soldiers.Footnote ¹⁰⁰ She quotes an Imperial Japanese soldier fighting in China in the late 1930s: “I wonder if the people at home, overjoyed with the thought of ever-victorious Japanese soldiers, understand the unending pain that comes with this victory” (MacMillan, p. 165). She treats war as a profound immorality, where it suddenly becomes socially acceptable to kill and wound fellow human beings, to steal (MacMillan, p. 162), to blow up the buildings and infrastructure that make possible a decent life: “What is grotesque or appalling in peacetime—the smell of death or unburied corpses, dirt, rats, lice, foul water or rotten food—is simply part of the fabric of war” (MacMillan, p. 161). In addition to the terrors of combat, soldiers must also endure “waiting around, boredom and grumbling about food, lice, the rats, weather or the senior officers” (MacMillan, p. 159). Worse, the century of total war has in many ways erased the dividing line between soldiers and civilians. MacMillan argues that too often we focus on the troops and not enough on civilians caught up in the war, facing starvation, killing, deportation, or rape (MacMillan, p. 176).Footnote ¹⁰¹

Overy ends his book with a chilling augury: “If war has a very long human history, it also has a future” (Overy, p. 230). MacMillan, too, writes, “War and the threat of war are still very much with us” (MacMillan, p. 234). All of the primary causes of war discussed in these books—greed, security, ideas, and power—are still main features of international politics today.

It is no longer fanciful to imagine worse wars to come, including the prospect of conflicts among great powers. With Trump back in power, Putin may see a historic opportunity to rupture NATO once and for all, by pressuring a member state or clawing off some territory in the Baltics and then gambling that the United States under Trump will abandon the alliance. In early 2025, Denmark’s defense intelligence service warned that Russia could be ready for a major European war within five years, especially if the Russian government concluded that the United States will not back up its European allies.Footnote ¹⁰² Putin has recently escalated his challenge to NATO with Russian strikes on an American factory in Ukraine and European diplomatic compounds in Kyiv, and then a barrage of Russian drones against Poland.Footnote ¹⁰³ When those drones in Poland were shot down by NATO warplanes, Poland’s Prime Minister Donald Tusk told his parliament, “This situation brings us the closest we have been to open conflict since World War II.”Footnote ¹⁰⁴ In East Asia, Trump has alienated American regional allies and cast doubt that the United States would defend Taiwan, weakening a conventional deterrence that already had credibility problems. Of course, these would be terribly dangerous gambles for Putin or Xi, but Putin in particular has shown an appetite for risk in in waging five recent wars, and Russia under him has become an increasingly militarized society and economy.Footnote ¹⁰⁵

As MacMillan notes, wars among the great powers have become more and more bloody as modern warfare becomes nationalized and industrialized. When Japan defeated Russia in 1904–05, the combined death toll was between 130,000 and 170,000 people; World War I took around nine million human lives; and World War II claimed as many as fifty million civilian lives (MacMillan, p. 90)—more of a World Massacre than a World War. It is haunting to imagine how many people could perish in a future war between powers bristling with nuclear weapons. That is some of what Stanley Hoffmann, himself a refugee from Nazi Germany’s annexation of Austria and invasion of France, meant when he wrote about “the need to understand, and to make others understand, a violent world that threatens at any moment to destroy any possibility of private happiness—to make history fall like a roof lifted by a tornado on the inhabitants of the house.”Footnote ¹⁰⁶