Abstract
For the past two decades, international relations research has been dominated by what Lake and Powell (1999) refer to as a partial-equilibrium approach that focuses on explaining one aspect of international behavior while holding other aspects of the international system constant. This approach has generated a number of progressive research programs on topics such as the democratic peace, rationalist explanations for war, and open economy politics. At the same time, however, we know that central assumptions of the partial equilibrium approach—that, in short, it is feasible to hold all else constant—frequently are violated, especially over the longer term. In this book, Levy and Thompson advance a useful framework for thinking about a more systemic or, in their terms, coevolutionary approach to studying international politics, as well as a number of specific arguments within this perspective about the international history of war over the period from prehistory to the present.
The central argument in this book, which provides the theoretical framework for all subsequent arguments, is that throughout history war has coevolved with five other variables—political organization, military organization, political economy, threat environment, and weaponry—with significant changes in one realm typically producing significant changes elsewhere. Using this framework, they then develop specific arguments about the origins of war, major historical “accelerations” in warfare, cycles of political centralization and decentralization, and the differing trajectories of war in the contemporary developed and developing world. This agenda is extraordinarily ambitious, spanning the breadth of human existence both historically and geographically.
After an introductory chapter, the authors turn in chapter 2 to mining the anthropological literature on the origins of war to argue that the capacity for organized violence emerged from armed hunting bands but that it took the development of more complex political organizations, motivated by the development of agriculture, for warfare to become common. Chapter 3 turns to their general theoretical argument, identifying and defining their six master variables, dismissing possible alternatives, and developing the argument that significant changes in one variable tend to produce significant changes elsewhere, with war occupying a critical juncture in this nexus given its role as a selection mechanism. This chapter also introduces more specific subsidiary arguments on which subsequent chapters expand.
Chapters 4 and 5 examine three major historical revolutions in warfare, which Levy and Thompson refer to as “accelerations” given the gradual, if extremely significant, nature of the changes involved. Here they are particularly interested in two arguments: that changes in the frequency and intensity of warfare both arose from and produced changes in their five other coevolving variables and that these changes were driven in particular by major shifts in political economy, most notably the development of agriculture and the industrial revolution. Chapter 4 presents plausibility probes of ancient Egypt, Greece and Rome, ancient China, and Mesoamerica prior to the Spanish arrival. While inevitably cursory—they devote four pages to Mesoamerica, which “is the shortest [trajectory] reviewed in this chapter, lasting only about 2,650 years” (p. 117)—these studies nonetheless do a good job of bringing in relevant historical and anthropological literature to illustrate their arguments. Chapter 5 turns to the developments after 1500 in the West, where warfare became markedly more severe but ultimately, precisely because of its severity, significantly less frequent. After again tracing the connections among the coevolutionary variables, they spend the second half of the chapter developing an explanation for the reduction in the frequency of major power war, which they contrast to competing arguments by Mueller (1989) and Kaysen (1990).
Chapter 6 examines the coevolution of war with societal variables over the entire period of what Levy and Thompson refer to as the Western trajectory, beginning at the dawn of civilization in Mesopotamia and transitioning through Greece and Rome to modern Europe and the United States. A central argument in this chapter is that military innovation produces paradigmatic armies and navies that experience unusual military success, resulting in a period of political centralization that ultimately declines, in the process accelerating the intensity of regional conflict, until eventually war becomes so severe that states become deterred from resorting to it. Chapter 7 turns to the non-Western world, where they argue that the emergence of a territorial integrity norm has produced an unusually benign external threat environment that facilitates the survival of weak states, which in turn are beset by civil (rather than interstate) wars. Chapter 8 concludes by recapitulating their argument and findings and drawing some tentative implications for future research.
The extraordinarily broad theoretical and empirical scope of this book inevitably implies that specific arguments cannot be fully substantiated. Instead, this book is best understood as providing a template for conducting work in a systemic or coevolutionary vein. Thus, although this review could raise questions or concerns about specific arguments—how, for example, should we square the argument that changes in political economy have primacy in explaining the three historical accelerations in war with the observation that the industrial revolution postdated the onset of the third acceleration by several centuries?—the more important question is what this book reveals about the promises and perils of a coevolutionary method.
At a time when most international relations scholars are focused on narrower and more time-restricted questions, there is obviously room for more expansive and ambitious, if necessarily less conclusive, approaches to studying international relations. Moreover, given the ubiquity throughout history of arguments that fundamental change is afoot—as in arguments today about the obsolescence of war, the renewed significance of religion, or the revolution in military affairs—it is important that we be able to speak in a theoretically informed way about the significance and limitations of such developments. This approach will be particularly appropriate for a number of important questions that mainstream approaches have difficulty addressing, such as how to explain specific major historical changes such as the emergence of the state system or the accelerations in war on which Levy and Thompson focus, or why Europe remained a collection of separate states through post-Roman history whereas other regions, such as ancient Egypt and China, alternated between periods of political centralization and decentralization.
Recent work focused on these sorts of questions has been predominantly constructivist, and it is thus useful to have an alternative viewpoint that assigns a more limited role to ideational variables. Given this contrast, however, it is somewhat surprising that the authors limit their engagement with constructivist or ideational perspectives to a relatively brief justification for omitting ideational variables from their six-variable framework (p. 58) and to a response to Mueller’s ideational explanation for the decline of interstate war (pp. 145-146). More direct engagement would have helped to better delineate the areas of overlap and difference between these perspectives.
Along similar lines, a core contribution of this study is the identification of the six key variables that coevolve over time. This framework is as important for what it leaves out or downplays—such as climate or population (p. 57)—as for what it includes. Future work will however have to grapple with conceptual and definitional challenges in the use of these variables. Thus, for example, Levy and Thompson use “political organization” to refer both to the nature of political regimes—democracy, monarchy, etc.—and to the degree of political centralization, as with the cycle of unification and dissolution into competing chiefdoms in ancient Egypt; for many questions, analysts will want to separate the system of government from the level of regional consolidation.
That said, the biggest challenges for coevolutionary work will relate to inference. The most obvious difficulty is that standard hypothesis-testing strategies are inappropriate in contexts in which all variables are theorized to be endogenous to all others (Jervis, 1998), while advanced statistical techniques for dealing with endogeneity are inapplicable in the typically limited-N analysis of macro-historical change. Even when endogeneity is less problematic, relationships will frequently be nonlinear, as with the observation that both mild (Mesoamerica) and extreme (areas threatened by horse-mounted raiders) threat environments hindered military reform, whereas an intermediate environment in Europe produced both incentive and opportunity to innovate (p. 210).
These challenges imply that conclusions will necessarily be less definitive for coevolutionary work than for narrower studies. Indeed, if coevolutionary work reminds us that generalizations reached through conventional midrange theorizing may be context bound in unanticipated ways, the question for coevolutionary approaches is whether it will be possible to identify widely accepted generalizations at all. In this context, explicit standards for empirical analysis will increase confidence in findings, as will efforts where it is possible to identify narrower but more easily testable hypotheses. Levy and Thompson do allude at times to standards for empirical inquiry (e.g., pp. 169, 184) and also provide an example of how one might use quantitative data to test coevolutionary hypotheses in chapter 2 (esp. pp. 47-51), but given the goals of this project, they do not explicitly address these methodological challenges. Subsequent studies will not have this luxury, however.
In sum, Levy and Thompson have written an extremely interesting book that provides a template for examining a wide range of broad historical changes related to war. These big questions undoubtedly deserve greater attention than they have been receiving, even given the inevitable difficulty of developing definitive answers. This book thus should help provide the impetus for further research, which however will need to more explicitly address some of the conceptual and methodological challenges that this study elides.