Given the significant and complex effects that migration has on issues of interest to international relations scholars, it is surprising (and lamentable) how little attention it has received in the subfield. Fortunately, that lacuna has begun to be addressed over the past decade, and Kelly Greenhill's book makes a solid contribution to the emerging IR literature on the subject. In addition to broadening the scope of international relations scholarship, it justifies the unique contribution IR frameworks can have on migration and refugee studies.

Greenhill sets out to explain a phenomenon that many, including migration experts, did not believe existed: the use of state-engineered migration as a tool of coercive statecraft in the post–World War II era. Indeed, as she rightly points out, this rather insidious means of political suasion has been used numerous times over the relatively short period examined, and with a striking degree of success. Weapons of Mass Migration recounts 64 instances in which, as she puts it, “displaced people have been used as pawns” in international statecraft, involving as many as 10 million people in a single occurrence. Moreover, Greenhill's relatively conservative calculus includes only those instances with documentation that clearly supports both intent and execution of this tactic, though there is considerable anecdotal evidence to suggest that its use is even more widespread. Those states that engage in such acts of coercive diplomacy (often weaker than their target) have succeeded in achieving at least some of their political aims 71% of the time, she notes, but it is the displaced who suffer the human costs involved. This is an issue with clearly significant implications for interstate relations, human rights, and human security.

The author addresses two central questions, the first largely descriptive, the second more theoretically grounded: 1) How often is forced displacement used for coercive diplomacy? 2) How, why, and under what conditions does it succeed or fail? Providing a thorough descriptive account of a largely overlooked phenomenon would itself make a contribution to the literature, but the book provides both an extensive accounting of the cases where migration was used as a tool of coercive diplomacy (in the appendix) and in-depth case studies of several prominent examples (Chapters 2–5). These cases include migration from Cuba, the Balkans, Haiti, and North Korea. These historical chapters offer a rich and detailed account of the events, the context in which they took place, and wider implications. Theorists might prefer the large-n dimension of the book to be dedicated to a broader testing of the hypotheses generated, rather than the more descriptive accounting of the instances of coercive, engineered migration that were identified by the author. Instead, the theoretical arguments are tested against a rather small sample of cases using the structured, comparative case method. Although the scope of testing is thus limited, the book offers an exquisitely detailed account of events.

Greenhill responds to her second set of questions by arguing that “coercive engineered migration can be usefully conceived of as a two-level game, generally asymmetric, coercion by punishment strategy, in which challengers on the international level seek to influence the behavior of their targets by exploiting the existence of competing domestic interests within the target state(s) and by manipulating the costs or risks imposed on their civilian population(s)” (p. 3). Coercion is exercised either by threatening to overwhelm the capacity of the target country to logistically accommodate a large influx of migration and/or by using migration to create domestic political instability by exacerbating existing tensions between domestic interest groups. To these causal mechanisms, Greenhill adds a dimension of “hypocrisy costs,” which are created when target states' treatment of migrants and refugees contradicts their professed commitment to liberal norms and values. Not surprisingly, this should make liberal democracies more vulnerable to coercive engineered migration—a hypothesis supported by the empirical evidence drawn from the post–World War II era presented in the book.

Although it is not suggested in the book, one might also consider how succumbing to coercive engineered migration would affect domestic perceptions of the target state's capacity to regulate migration, and, hence, its very sovereignty. In National Security and Immigration (2006), I showed how public perceptions of a weakening capacity of state control heightened insecurities regarding immigration/refugee flows in Europe and the United States over the past half century and contributed to a growing political mobilization of groups favoring more restrictive policy. Indeed, concerns regarding an erosion of what I referred to as “societal sovereignty” could produce a strong shift in domestic political coalitions since its appeal is not limited to the right wing. Mikhail Alexseev (2005) documented a similar effect on domestic politics and explained the effect using the framework of the security dilemma to account for the spiral logic of threat perception.

There are also other likely candidates for factors that strongly affect a state's vulnerability (in terms of political sensitivity). For example, kinship ties may factor prominently. A large population of ethnic kin present in the target could significantly impact its willingness to accept the migrants/refugees. Conversely, we would expect the opposite to be the case if there were historical grievances between a subpopulation of the target state and the incoming migrants. Receptivity/vulnerability to engineered migration may also be strongly affected by the skill composition of the migrants and/or the economic conditions and economic needs of the target country. In short, there are numerous likely factors that shape the politics of migration policy domestically that can affect the type of statecraft outlined in the book.

One might also wonder why geography did not figure more prominent as a variable. Although decreasing transportation costs partly explain the increasing size and scope of migration flows (see Massey et al., Worlds in Motion, 1998), geography still plays a major role in any migration equation. In the sense used here, we might think of geography as having a strongly constraining effect on the “force projection” of threats regarding coercive engineered migration.

Greenhill categorizes the actors that engage in coercive engineered migration into three groups: generators, agents provocateurs, and opportunists. If we are to speak of coercive engineered migration sui generis, the inclusion of the latter category seems inherently problematic. The author defines “opportunists” as states that “play no direct role in the creation of the migration crisis, but simply exploit for their own gain the existence of outflows generated or catalyzed by others” (p. 30). While these actors may seek to exploit a situation that has presented itself, they do not, in fact, engineer either the migration or the crisis. Given that more than a third of the cases (26/64) fit this category, it might reduce our perception of the significance of coercive engineered migration since it magnifies the degree of frequency of the phenomenon. In terms of both international relations and human rights, it would seem that generators and agents provocateurs are the more insidious actors since they are in fact producing the migration flow.

In addition to adding to the IR literature on statecraft more generally, Greenfield offers a glimpse at the international politics of smaller states, and in particular, their relations with larger, more powerful countries. As she notes, “Crisis generation represents one of the few areas in which weak actors may possess relative strength vis-à-vis their targets—and, in the case of migration crises, also vis-à-vis their even weaker victims” (p. 28).

Weapons of Mass Migrations is innovative, well written, rigorously researched, and timely. It is both theoretically innovative and policy relevant, and will likely spur several new paths for IR research and migration studies.