Both books under review explore connections across borders in civil conflict. They complement the recent trend in quantitative studies on civil war that has emphasized transnational dynamics but has relied to a great extent on large-n analyses that do not allow for careful examination of the mechanisms through which cross-border influences are occurring. The volume by Jeffrey Checkel provides a set of studies that show different dynamics at play in civil conflicts (from resource contributions to norms shifts). Jennifer Hazen shows the role played by international actors (from states to black markets) on the resource networks of rebel groups. Both books leave the reader thinking more critically about connections across borders: how they are made and maintained, and ultimately how they influence the outbreak and continuation of civil conflict.

Checkel’s Transnational Dimensions of Civil War has two goals. The first, captured in the title of the volume, is to explore the links across borders that influence the start or trajectory of civil conflict. To this end, five chapters address specific cases of transnational linkages and civil conflict, and a sixth uses an agent-based model to examine that relationship. The second objective is multifaceted but relates more to philosophy of science than conflict per se. All but one of the substantive chapters employs or advocates for process tracing as a method for uncovering “causal mechanisms,” and a great deal of Checkel’s introduction and Andy Bennett’s chapter center on understanding what “mechanisms” are and how we came to be focused on them in the study of political science. The position taken in the volume is that while a number of studies have demonstrated a prominent role for a transnational dimension of civil war (particularly some quantitative works), existing research has said little to nothing definitely about how transnational factors matter. Instead, theories posit a mechanism and test it by looking at observable implications. The challenge that the contributors of this volume take up, then, is to demonstrate that particular mechanisms (such as norm entrepreneurship, for example) are causing the effects in which we are interested.

Each of the substantively focused chapters follows a similar pattern. Contributors begin with a plausible causal mechanism related to some transnational dimension of civil war. After carefully laying out the logic of one or more of the mechanisms, the authors use process tracing (or agent-based modeling in one case) to test the mechanism. The actors of interest vary greatly across the studies, including transnational insurgents (Kristin Bakke), diasporas (Fiona Adamson), refugee populations (Kristian Berg Harpviken and Sarah Kenyon Lischer), the international community and rebel leaders (Hans Peter Schmitz, Stephan Hamberg), and individuals (Martin Austvoll Nome and Nils Weidmann). What the authors seek to explain also varies, including the rebel tactics, diaspora mobilization, refugee violence, demobilization of child soldiers, and shifts in the framing of rebellion.

While the individual contributions do not follow from a specific model of politics (as was the approach by Paul Collier and Nicholas Sambanis in the two volumes of Understanding Civil War, 2005), the authors appear to draw on similar themes to a high degree. The chapters deal with common themes of resources (Harpviken and Lischer, Bakke, Adamson), frames for understanding conflict (Bakke, Adamson, Schmitz, Hamberg), and learning and socialization (Bakke, Harpviken and Lischer, Nome and Weidmann). In general, there appears to be a strong foundation from the social movements literature that informs all of the studies in the volume. Elizabeth Wood’s conclusion reinforces this, orienting the findings around mobilization, the role of insurgents in shaping tactics, framing and resources, and the process of diffusion. This collective focus on the dynamics identified by the social movements literature is notable in part because it is somewhat distinct from the framework underlying major contributions identified from the quantitative literature, which often emphasizes bargaining dynamics.

The collective findings from the volume demonstrate support for a number of mechanisms through which there are links across borders. Moreover, in combination with Bennett’s and Checkel’s chapters, they provide a nice guide for how to do high-quality process tracing. What remains a challenge, however, is adjudication in some way between mechanisms. In a relatively recent change in quantitative studies, scholars are required to compare the “substantive effect” of the factors they identify as important, and some commonly used methods have emerged to do so. Bennett argues that scholars using process tracing must pursue the alternative explanations vigilantly to provide support for any one mechanism. Yet many of these contributors argue in favor of multiple mechanisms, and there remains some uncertainty about the relative weight to be placed on them. The Nome and Weidmann contribution goes some way in addressing this issue by using an agent-based model to demonstrate specific conditions under which social adaptation or norm entrepreneurship are likely to be more effective mechanisms of norms change. Still, the volume leaves largely unaddressed the question of which mechanisms are more prominent and under what circumstances.

In What Rebels Want, Hazen challenges an underlying and perhaps unintentional assumption made by a great number of conflict scholars that rebels can fight when they want to. That is, it is generally assumed that rebels (even those with low capacity) have a somewhat stable ability to engage with the state. Hazen uses process tracing in detailed case studies to show that the ability of rebels to fight can be extremely variable over even short periods of time, and that the networks (often international) through which rebels acquire arms are complex and challenging for them as they try to maintain insurgency. She argues that downturns in the resources and short-term capacity of rebels lead them to seek negations with the state.

This book includes a detailed exploration of the trajectory of rebel resource networks in Sierra Leone, Liberia, and Côte d’Ivoire, examining military, economic, and political resources. Citing the now-extensive literature showing a conflict-enhancing effect of natural resources, Hazen emphasizes the difficulty of translating natural resources into arms and highlights the degree to which rebels rely on black markets and the whims of foreign supporters. She concludes that the “most successful rebel groups will be those with the most diversified support networks” (p. 179).

A key contribution of Hazen’s book will be to influence how scholars think about rebel capacity and the role it plays in determining when rebels pursue negotiations. Yet there is, as the author notes, a disconnect between negotiations and war end. Negotiations, she demonstrates, can be used as a time for rebels to regroup and resupply. The book is framed around the question of civil war duration (see Chapter 1); however, the underlying processes that it focuses on are not explicitly linked to the war end, except to say that war is more likely to end when the “taps [are] turned off” and rebels essentially cannot “maintain any viable alternatives” to fighting (p. 23). This conceptualization, which assumes that rebels will always fight if they can, prizes the role of resource networks above many other factors in determining conflict end (such as the role of third-party guarantors, the possibility of political power sharing, territorial decentralization, or military intergeneration). Rebels are not seen as having meaningful, and thus potentially satiable, political goals. Yet it is not clear that in all successful settlements of civil war (successful being, of course, a contested concept), rebels no longer could fight. The Shanti Bahini forces fighting in Bangladesh demobilized in response to concessions from the government after years of low-level fighting. The Sudan People’s Liberation Movement in South Sudan almost certainly maintained the ability to wage insurgency at the time of settlement of the North–South Sudanese conflict in 2005. This does not in any way negate the central message of the book—that rebel supply networks are critical to their capacity and that they vary quite a bit over time and across rebel groups – but it does suggest that the role of supply networks on the outcome of conflict termination (as opposed to negotiations) is less clear and warrants further exploration.

Both books make a meaningful contribution to the study of civil war, and in the case of the Checkel volume, to the debate over what is good social science. Each also provides examples of excellent case-focused research that speaks to the quantitative study of civil war.