The starting point for assessing the evidence about the origins of war is understanding the positions regarding the biological component of war making.
The two poles of this argument are, on one side, that aggression and war are human instincts, with the implication that war is and always has been an inevitable part of human existence, and, on the other, that war is a purely cultural construction with no biological component, implying that war can be eliminated from human affairs. It is the politics of the implications on both sides that matter and that cause this argument to be so heated at times. On purely biological grounds, the issue is unanswerable, based as it is on a false dichotomy between nature and culture. Humans are such complex biological mechanisms, genetically programmed to form complex, linguistically based cultures, that separating the influence of biology from culture in the makeup of different societies is as impossible as separating the influence of nature from nurture in the makeup of individuals. It is, inevitably, both, which gets us no closer to an answer as to the potential biological origins of war. Nor does looking at our closest relatives, the great apes, help, for the social biology of primate groups varies widely and includes cultural components. Just among chimpanzees, our closest relatives among the apes, levels of interpersonal and intergroup conflict vary widely, though tending to the nonviolent, and so offer no clear lessons about human nature. We must turn to the evidence from early human history.
Such evidence is, of course, quite sparse, and written evidence does not show us a time when war did not exist. Therefore, many historians and anthropologists have turned to studies of those hunter-gatherer groups that survived into the twentieth century to try to gain insight into our primitive past, on the assumption that such peoples live as our ancestors did millennia ago. One much cited case involves the Yanamamo of the Amazon basin, who exhibit high levels of interpersonal (especially intergender) and intertribal violence. This has often been cited as evidence in support of the proposition that the inclination to warfare is part of human nature. But the assumption that such peoples live as our early ancestors did is questionable. Anthropologists have reinterpreted the original field studies of the Yanamamo, taking into account the broader colonial history of South America. They point out that, far from being isolated, the Yanamamo have been in direct and indirect contact with neighboring (and often more complex) societies for centuries—societies that themselves practiced organized warfare, exported weapons, and in general tainted the supposedly pristine evidence provided by the Yanamamo or, indeed, by any other hunter-gatherer society that managed to survive into the twentieth century. Furthermore, every example of a violent group is matched in the anthropological literature by a counterexample of a group living peacefully. The anthropological record, at least as it pertains to studies of living peoples, is therefore problematic for answering questions about the origins of war.
That brings us to the archaeological record of early human existence. The archaeological record is also problematic because it is far from complete. It does, however, provide the tentative outlines of an answer, although there is intense debate among specialists about this issue. Archaeologists and paleo- anthropologists have studied skeletal remains of Homo erectus, the widespread ancestor of modern humans, as well as tools from the limited tool kit Homo erectus used (basically, stone hand axes) associated with the skeletal remains; these come from sites across Eurasia, dating to between 2 million and 100,000 years ago. Added to this evidence are the prehistoric skeletons they have found of Homo sapiens, the modern human species, dating to between 150,000 and 10,000 years ago and scattered across a far wider range than Homo erectus achieved, along with modern humans’ much more varied toolkit. Of these several thousand skeletons of both species, few bear any unambiguous signs of human-inflicted violence, and those that do are isolated cases. None of the exceptions are Homo erectus; it seems that, at the least, organized violence is the product of the modern human species among hominids, dating to no earlier than about 150,000 years ago. But even among the modern human remains, evidence of organized, weapons-assisted aggression between groups (as opposed to individual murder) is nonexistent prior to perhaps 10,000 years ago.
Therefore, if we define war as organized human violence against other humans, as opposed to the odd murder, there is almost no evidence for it prior to about 8000 bce. Aside from one isolated case in Egypt, a mass burial of several hundred skeletons showing clear signs of the impact of human weaponry at a site in northern Iraq from around that date is the earliest of a type that becomes increasingly common from that point on. At roughly the same time, fortifications began to appear in the same part of the world—as, for instance, at Jericho in northern Palestine (though the earliest forms of these walls were probably for flood control rather than defense against human enemies). Evidence of this sort spreads rapidly from this point of origin through Europe and the Near East and also appears independently later in places such as northern China and Central America.
This suggests that warfare was an invention, a cultural phenomenon that, though it inevitably contains some biological component simply because humans are biological creatures, was not a product of biological determinism. (Whether the dynamics of the interaction of human societies since then makes war inevitable as a social and cultural phenomenon is a separate question.) What we must then ask about are the conditions that characterized the places where and times when warfare sprang into existence after millennia of peace.
The places and times involved suggest that the conditions are associated with (though not dependent on) the emergence of agriculture. Put another way, the same conditions that gave rise in various places to agriculture also contributed to the invention of warfare. Rising population is the crucial variable. In especially fertile areas and rich hunting grounds, greater numbers of people began to put pressure on even those food resources. At the same time, because resources were at least initially plentiful, populations in such areas, even before agriculture, became less nomadic. They would thus have increasingly staked claims to territorially defined settlements. This trend was simply reinforced by agriculture where and when it arose. Fixed settlements could accumulate resources that provided both a tempting target for plunder and a possession worth defending.
Rising population not only put pressure on resources but also changed social structures. More people in an area, both in absolute numbers and in terms of the density of settlements, led inevitably to the rise of defined social hierarchies and mechanisms of community governance. Such mechanisms, perhaps in the form of tribal leadership functions increasingly held in one family line until a hereditary chiefdom emerged, as well as in the solidifying of a socially elite and powerful group of families around the ruling family, probably first arose to deal with questions of intracommunity dispute resolution and economic redistribution. After all, the larger a group is, the harder it is to settle quarrels informally through the mediation of family members or friends, because increasing numbers of people won’t be closely connected this way. It is also harder for people to simply trade and share their resources directly within the group for the same reason. Thus, a chief or equivalent emerged to handle what in modern terms we would recognize as a version of policing and justice on one hand, and taxation and spending “for the communal good” on the other. But a recognized chief and his supporters (those who might help enforce the decisions of the chief within the community) not only met these internal needs but potentially provided the means for a more organized, centralized, and effective communal response to outside threats. The rise of such social and political hierarchies and structures did not necessitate the invention and use of war, but they facilitated such a move—that is, made it more possible, even if not making it inevitable. They were the preconditions for war.
The rise of political hierarchies, even in limited form, was accompanied by increasing trade in prestige goods, whose display showcased and legitimized the power and status of elites. Both the trade itself and the limited sources of supply that characterized prestige items (which are by their nature rare) formed another potential nexus for conflict to arise. That political decision makers and their status were implicated in this nexus of conflict made for an even more powerful inducement to war.
Many societies, in different places and times, independently reached this stage of economic and sociopolitical development. The final factor that seems to have pushed some of them over the edge into the use of organized violence against neighboring groups was environmental. First in northern Iraq, and then in several later cases where war making arose apparently independently, there seems to have been a severe environmental crisis—widespread drought brought on by a slowly warming and drying climate, for example. Sudden and widespread shortages would have placed an especially great strain on established resource levels, triggering the move to military action. Thus, formerly peaceful neighboring peoples resorted to organized violence against each other to protect their place in the world. It should be noted that both warfare and agriculture, according to this interpretation of the evidence, arose after the end of the last Ice Age. Since that period of global warming was not always conducive to sustaining food supplies, both warfare and agriculture can be seen in part as adaptive responses to significant environmental change.
Furthermore, the consequences of warfare and agriculture often reinforced each other, contributing to the spread of each. Once warfare had been invented in an area, a number of dynamics contributed to the tendency for warfare to spread rapidly beyond its points of origin, and indeed beyond places where the initial conditions held. First and most obviously, it was a successful technique, at least from the perspective of the initially victorious societies. They were, of course, the ones best placed to exploit the new way of life, because success in one war would have generated additional resources for survival, support for further war efforts, and vital experience in how to conduct war. Faced with a society that could threaten war, neighbors had no choice but to respond in kind or submit, and their conversion to a war footing forced their neighbors, in turn, to confront the same choices. In short, once there were war- making powers on the sociopolitical map, any society in contact with them had to adopt the new mode of organization or risk conquest and extermination. The spread of the idea of war by this dynamic could have been very rapid—in fact, the archaeological evidence suggests that it was.
Perhaps even more important, then and thereafter, for the perpetuation of warfare as one of the tools a society could deploy was the interaction of warfare with social class and political leadership. War became widespread, as we noted, even in areas where the initial conditions that gave rise to it did not apply—areas, therefore, presumably, lacking the necessity of war for survival, at least in some cases. The basic dynamic at work was that in the sorts of hierarchical societies that could have resorted to war successfully, the interests of social elites naturally diverged from the interests of the masses. Since elites were more likely to use their social and economic positions of privilege to specialize in the bearing of arms, thereby becoming warrior elites, they also thus stood to garner the most in terms of glory and riches from waging war. In short, warfare was potentially more beneficial to the elites than to the farmers. In fact, in many circumstances, war must have been a burden for farmers, increasing the demands of their own elites for resources and exposing them to destructive raids by the enemy. Elite support for war may have worked against the economic interests of their own farmers, contributed to political tensions within communities, and encouraged elites to tighten their control of their people. At its extreme, this process could have reduced free farmers to peasant or even slave status. Since the meager surplus wealth produced by an agricultural economy encouraged the political domination of farmers by elites, so that the elites could guarantee their (disproportionate) share of the surplus, war simply reinforced the tendency of agricultural economies to create elite-dominated, hierarchical social structures.
Furthermore, as the most intense form of crisis that societies now faced, warfare made strong leadership at the very top all the more crucial. Tribal leaders, chiefs, or kings recognized this and so favored war as policy more than their society as a whole would have. Thus, leaders made war because war reinforced their authority and status, not because it necessarily benefited their societies. In this way, too, warfare reinforced and intensified the pressures for unified leadership that all agricultural societies, operating in conditions of scarcity and limited means of communication, faced. Thus, warfare contributed to the prevalence of monarchy as the predominant form of political organization for much of human history since the end of the last Ice Age.
Between the external threat posed by warrior societies and the internal dynamics of hierarchical political organization, avoiding warfare was no longer an option. War gradually became the global constant in human history—complete with fortifications, arms races, and wide-ranging social and cultural effects—that the rest of this book will explore.