The slimness of this book by Jack Levy and William Thompson belies its topic: a social scientific exploration of the history of warfare as an institution, especially as an institution of interstate relations. In their own (admittedly “immodest”) words, the authors seek “to explain the origins, escalation, and transformation of warfare” (p. 1). They do so by identifying six factors—political economy, political organization, military organization, threat environment, military weaponry, and war itself—and placing them within the theoretical framework of coevolution.

The notion of coevolution requires some explanation, especially as it frames the book's central argument. Without arguing for the primacy (in terms of substantive importance or sequencing) of any of the six factors, Levy and Thompson give a series of examples showing how they influence one another to produce changes in warfare. They point to the importance of political economy and in particular, the development of sufficiently dense societies, as a precondition for the origins of war; once large groups of people became attached to specific territories, they developed military organizations and adapted existing technology for military purposes in order to defend their land and people. Later in history, a relatively benign external threat environment led to fragmented political and military organization in ancient Egypt, while a more threatening environment induced rapid improvements in military organization and technology in Ancient Greece and Rome. And warfare itself—as a kind of lagged dependent variable—is identified as a critical ingredient leading to military revolutions or accelerations, and it plays a particularly interesting role in twentieth-century Europe, where the horrors of past war have made its high costs so evident that, many scholars argue, future war is all but impossible.

Given the scope of their topic, Levy and Thompson necessarily depart from the typical model of hypothesis generation and testing here. Readers hoping for a set of sharply defined predictions and accompanying large-n tests will thus be disappointed. Instead, the authors engage in an exercise more akin to process tracing, but over an expansive time line. Beginning with prehistory, they draw on scholarship from history, anthropology, and archeology to weave a story of the origins of human warfare in various parts of the world, wherein they do privilege political economy as a key explanatory factor. They then journey forward to ancient Mesopotamia and China, to early modern Europe, and through to today's world of advanced industrialized states in order to trace three military accelerations. The first acceleration saw the development of the early weapons of war. The second was characterized by the emergence of armies. In the third acceleration, shifts in political economy, such as increased population density, allowed for the expansion of these armies and the further development of the weapons they employed. Along the way, Levy and Thompson engage in a series of erudite debates with scholars from several other disciplines, highlighting their own deep knowledge of their subject matter.

This book makes several contributions to the study of war. Most importantly, it brings a social scientific lens to bear on the evolution of warfare as an institution. By linking the development of warfare to shifts in political economy and political organization—and not relying solely on military factors such as the development of armies or technological change—and by invoking current arguments about economic and political development, the authors bring their subject into the realm of political science.

One of the great strengths of this book is its synthesis of a variety of important literatures. The authors' argument speaks to recent scholarship on the future of war, engaging in a critical debate about whether war among advanced industrialized states is defunct (their answer is: not necessarily). The arc of war as described by Levy and Thompson is strikingly similar to the evolution of the institution of the state, linking their work to that of scholars of sovereignty and statehood. Indeed, this similarity could be drawn out much more: their six key factors fall broadly along military and political lines, suggesting that it is the state and war that have coevolved as much as anything. The book also stands as a welcome addition to prior scholarship on the history of warfare in its balanced focus on political and military factors. On this point, The Arc of War will be particularly useful for students of military history wanting to branch out beyond seminal, but by now standard, accounts. In particular, Chapters 4 through 6, which detail the three accelerations of warfare as well as the trajectory of warfare in the Western world, offer a compact but useful analysis of key trends in military history.

Any book of this scope will, of course, have some shortcomings. Although one of its strengths is the authors' determination to bring social science to the study of the evolution of warfare, there are several places where the book does not quite meet its own standards. The nature of the coevolutionary argument makes it very difficult to identify a distinct set of causal claims (see, for example, Figure 3.2 on p. 63); there is so much going on that it can be difficult to separate description from analysis. The reliance on coevolution also makes for a great deal of indeterminacy; the authors are not in a position to say, for example, that an increased value on one variable is likely to lead to change in a particular direction on another variable. Related, deeper explorations of salient nonevents would have been extremely helpful. Why, for example, were the dynamics of coevolution so weak in Mesoamerica (p. 117)? Why did relative isolation lead to stagnation in Tokugawa Japan but innovation in Song China (p. 210)?

One additional area the authors might explore further in future work is the role of intrastate conflict. Although Chapter 7 provides a useful review of literature on non-Western military trajectories, the authors do not attempt a full application of their argument here. Given the proliferation of recent scholarship on the political, economic, and military organization of rebel groups, it would be extremely interesting to see an analysis that marries earlier literature on development of the state and recent literature on rebel organization to the argument made here about coevolution and warfare. Is it possible, for example, that current high levels of intrastate war will lead to a strengthening by some states via a path that is distinct from the one followed by advanced industrial states but that nonetheless leads to a similar point? Exploring this type of dynamic would heighten the relevance of the study dramatically, given the typical location and nature of warfare today.

Despite any shortcomings, Levy and Thompson should be lauded for taking on such an ambitious and important project. Their mastery of military and political history is truly impressive, and the story of war is aptly told.