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?

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.