During and after the Cold War, two disparate research traditions developed within international relations. One—the more dominant—examined patterns of international conflict, largely through a state-centric lens; a very different tradition explored possibilities for peaceful change by focusing on nonstate actors and transnationalism. Idean Salehyan's signal contribution, in this carefully argued book, is to bridge these literatures. His focus is civil wars, which some might be tempted to view as a form of (violent) domestic politics. However, as Salehyan shows, such wars have important transnational dimensions that are anything but peace inducing. Rebels without Borders thus makes important contributions both to studies of civil war and to IR theory more generally.

For students of civil war, Salehyan provides definitive evidence that open polity models are most appropriate for theorizing them. Such models recognize that the object of study—civil war, in this case—has causal elements that transgress state borders; Salehyan operationalizes this transnationalism by examining border-spanning rebel groups. While the casual reader of newspapers may not be surprised to learn about such groups—think of the Contra rebels in the Nicaraguan conflict of the 1980s or, more recently, the rebel groups criss-crossing the Congolese and Rwandan borders in Central Africa—their role had been largely neglected in previous theory on civil conflict.

If Kristian Skrede Gleditsch can be credited for first alerting us to the role of transnationalism in civil war (“Transnational Dimensions of Civil War,” Journal of Peace Research 44 [May 2007]: 293–309), then Salehyan goes a step further, developing a theoretical argument to explain it. Specifically, he draws upon work on political opportunity structures and conflict bargaining to advance a theory of transnational rebellion (Chapter 1). Some theorists may have qualms with his framework, as it generates few novel or counterintuitive insights—for example, one “overarching” claim is that “hosting foreign rebels will bring the host and target countries into conflict with one another” (p. 60). Still, for a civil-war research program where more data at times appears to be the prescription for unanswered questions, Salehyan's theoretical moves are welcome and scientifically progressive.

The core of the book (Chapters 2, 3) is a superb example of how to develop a rigorous theoretical argument, operationalize it, and then test it quantitatively on a rich data set. In these pages, the author anticipates and effectively rebuts criticisms of the data collection or the methods employed. More important, he demonstrates that transnational rebel groups play a central role in accentuating and prolonging civil war: They circumvent a state's monopoly of violence within its own borders, and provide access to a range of resources unavailable to domestic groups.

These findings have implications for both theory and policy. On the former, Salehyan's analysis should serve as a wake-up call to students of transnational politics, who for too long have theorized only its nonviolent aspects—nongovernmental organizations spreading human rights norms, say. Regarding policy, the author correctly argues that responses to civil war stressing domestic remedies alone—building democratic institutions or fostering economic growth—“will only go so far in preventing political violence” (p. 172).

All this said, the book is not without limitations, in terms of method, theory, and social theory. Methodologically, the rigor of the first half vanishes in the case studies (Chapters 4–5). There is no method in these chapters, which is unfortunate given the availability of an increasingly sophisticated case-methods literature (e.g., see John Gerring, Case Study Research: Principles and Practices, 2007). As a result, the case studies—despite the author's claims (pp. 108, 110, 122–23, 164)—fail to provide evidence for the causal mechanisms he posits to explain his correlational findings. For example, bargaining dynamics based on cost/benefit calculations figure prominently in Salehyan's theory, yet Chapters 4 and 5 provide no process-level evidence that such dynamics were at work. The process tracing that might provide the necessary evidence is notable only by its absence (e.g., see Andrew Bennett and Alexander George, Case Studies and Theory Development in the Social Sciences, 2005).

As for theory, the bridge that Salehyan builds between conflict research and transnationalism needs to be strengthened. Contrary to the author's claims (p. 8), research on transnational politics is rich, progressive, and diverse (e.g., see Thomas Risse, “Transnational Actors and World Politics,” in Handbook of International Relations, ed. Walter Carlsnaes, Thomas Risse, and Beth Simmons, 2002). It could thus help students of civil war better theorize the multiple transnational actors—diasporas, rebel groups, social networks—and the factors—financial flows, technology, socialization—that may be at work in cross-border civil conflicts. Given this diversity in the broader literature, it is not clear why Salehyan's study is limited to one particular kind of transnational actor (rebel groups) motivated to act only in pursuit of material gain.

These last comments point to a third limitation of the book, at the level of social theory. Like much of the contemporary literature on civil war, Salehyan's argument has its roots in political economy and rational choice (e.g., see Jeremy Weinstein, Inside Rebellion: The Politics of Insurgent Violence, 2007). Yet, it is surely good practice to put one's preferred social theory at some risk of being disconfirmed; it is thus surprising that the book considers no alternative explanations. Would a nonrationalist/constructivist account of transnational rebellion produce observable implications identical to those advanced by Salehyan? If so, how would we adjudicate between them?

Perhaps, though, this line of argumentation is unfair: I am asking the author to do what no one else has done. Yet, even within Salehyan's own research program on civil war, scholars such as Severine Autesserre, Lee Ann Fujii, Roger Petersen, and Elisabeth Wood have developed and tested sociological theories. In particular, Wood has demonstrated that emotions can play a key role in overcoming the collective action problems that insurgents face when mobilizing in the face of government repression (Insurgent Collective Action and Civil War in El Salvador, 2003). Given that such an argument seemingly contradicts a core claim by Salehyan—that motivations alone cannot explain insurgent violence (pp. 21, 36)—it is odd that it is not addressed.

The point here is not to berate the author for his choice of a particular social theory. Rather, my concern is a lost opportunity to explore synthetic theoretical arguments that better explain the complex social world around us, resulting in scholarship more attuned to the real-world concerns of policymakers. Such a style of argumentation has been endorsed by a growing number of scholars, including those working on civil war (e.g., see James Fearon and Alexander Wendt, “Rationalism v. Constructivism: A Skeptical View,” in Handbook of International Relations, ed. Walter Carlsnaes, Thomas Risse, and Beth Simmons, 2002).

These criticisms need to be kept in context. It is in fact Salehyan's parsimonious argumentation, transparent methods, and clear prose that make it easy for a sympathetic reviewer to find gaps in the analysis. While Rebels without Borders may not be the last word on transnationalism and civil war, it is nonetheless a good start, one on which those with more ambitious theoretical or social-theoretic agendas should build.