The days of the simple explanation are over. People fight wars because they are greedy. Or because their genes predispose them to violence. Or because nations naturally require expansion. Or because governments want to inspire loyalty among their citizens. Jack Levy (Rutgers) and William Thompson (Indiana Univ.) will have none of it. Or rather, they will have all of it. Instead of adopting the typical academic habit of emphasizing a single determinative factor, this capacious and engaging volume identifies five elements that have structured the nature of warfare from prehistory to the present: political and military organization, threat environment, political economy, and weaponry. These are admittedly broad categories, but Levy and Thompson (hereafter, L&T) argue persuasively that we must analyze the problem of war from a catholic perspective or risk oversimplification. They are not concerned with individual conflicts or issues such as motivation or ideology. Even the narrowest specialists among us will learn much from the panoptic view presented here, in which warfare is subsumed within a constellation of six co-evolving elements that shape human experience.

Within their set of determinative factors, L&T highlight political economy as central, showing how major shifts in warfare and its consequences reflect broad transformations in human systems of production. Thus, one style of warfare predominated until the agricultural revolution, a second until the rise of the West and the advent of industrialization, and a third since then. This bald summary may suggest a schematic if not static style of argument, but the authors stress throughout the dynamic nature of warfare as it evolves within their featured institutional parameters.

The first style of warfare derived from competition for resources among hunter-gatherers and, most importantly, the development of segmented societies where violence against one member was perceived as violence against the whole group. The escalation of warfare in turn encouraged greater group cohesion. Thus, war, weapons, and political organization progressed in tandem.

The second "period of accelerated evolution"[1] of warfare came in the last half of the first millennium BCE with the rise of the Roman and Han empires (92). Both civilizations flourished because of better organized militaries, deadlier weapons, and stronger political consolidation. L&T are, however, alert to differences in the cases they compare. The experiences of Egypt, Greece, Rome, China, and Meso-America are not precisely analogous, but each example reveals the co-evolutionary nature of the factors the authors single out initially.

The third transformation came in the industrial age. Europeans devised much more lethal weapons and the social consequences of industrialization spurred more democratic governance, which, in turn, necessitated new ways of raising and using armies.

Despite the book's theoretical nature, for each of the epochs they mark out, L&T demonstrate how specific political and economic conditions opened different possibilities for military dominance by those actors who organized most effectively. The wars in early modern Europe occurred within a threat environment shaped by robust competition, fluid boundaries, and high national autonomy. The victors gained new territories and resources while consolidating their own power and national identities. In contrast, Latin American, sub-Saharan African, and Middle Eastern conflicts in the post-1945 era have occurred within reasonably fixed boundaries, with frequent intervention by external actors and national fission rather than fusion. Such observations, scattered through the book, attest to the authors' mastery of the secondary literature and keen eyes for identifying patterns and variations.

Changing international relations in the industrial age, particularly since 1945, have profoundly altered the locales and modes of war. Aware that much of the world is not fully industrialized, L&T wisely distinguish wars in the developed world from those in less developed regions. Strikingly, wars between industrialized powers have increased in severity but declined in frequency over the last century, as the costs and consequences of waging war have risen dramatically. Consequently, inhabitants of the developed world are now less likely to die in a war than at almost any time in history. But those living in less developed nations face a much greater risk: in their often intra-state wars, violence affects civilians as much as or more than it does soldiers. While the authors do summarize prevailing explanations for this, their own wide-angle perspective does not disclose just how and when such conditions might change.

Most welcome in The Arc of War is the authors' willingness to challenge conventional wisdom and easy answers. They reject any teleological theory of twentieth-century total wars. Nothing inevitable, they maintain, drives humans either to make war or to do so more brutally than did their predecessors. The book's co-evolutionary model demonstrates that warfare changes along with the societies and governments that propagate it. Just as the first generation of English textile manufacturers could not foresee General Motors, so early adopters of gunpowder could not anticipate nuclear weapons.

Another methodological innovation of the book is its attention to the feedback cycle: the authors' evolutionary model requires scholars to recognize that the factors they want to isolate create new contexts, which, in turn, change the factors that once seemed stable. Specialists who focus on weapon systems, for instance, must appreciate how the development of nuclear weapons made war so horrifically deadly that it lost its appeal as a strategy of international affairs.

In so astutely sketching and explaining grand changes in the history of warfare, L&T provide a useful context for discussing particular wars in specific places and eras. The theory of co-evolution also needs, as they admit, more testing and evaluation by researchers in disparate fields. But the methodological shift they propose will help scholars grasp the perpetually contingent relations among the factors they study. While this complicates efforts to predict the future, L&T do foresee broad continuity in the years ahead, as developed nations avoid wars and underdeveloped ones suffer from them.

The rise of global terrorism and consequent asymmetric wars as well as the persistence of North-South conflicts all fit uneasily into the authors' typology, which is better suited to explain changes occurring over decades. We might instead consider how warfare and its relation to the other factors in their analysis would change, if, say, the digital revolution were to produce a political-economic realignment on the scale of the one set in motion by the industrial revolution. Might smaller nations or autonomous groups develop the capacity to disrupt the communications networks of major powers in a way tantamount to war's destructiveness? Such capability would "flatten" the threat environment in an unprecedented way, since real-world distribution of resources has always made some nations impossibly stronger than others. The Arc of War is a compelling demonstration that we cannot properly anticipate such changes—the modes and effects of warfare are woven too deeply into the fabric of human society. Change itself is all we can anticipate with certainty.