Military coalitions—officer, troop, and materiel amalgamations provided by multiple sovereign actors brought together to wage joint warfare—are more often than not ad hoc relationships. In other words, they are not drawn from preexisting institutions with prewar agreements on strategic aims, strong checks on cheating and free-riding, or well-integrated systems for sharing command responsibilities and resource management. They must be formed, but how? Marina Henke, in her groundbreaking book Constructing Allied Cooperation: Diplomacy, Payments, and Power in Multilateral Military Coalitions, illuminates this critical process. She asks these important questions: How are multilateral military coalitions built? Which states are able to build them? And how do they do so? Given the increasing prevalence of military coalitions since the end of the Cold War, these questions are crucial to understanding contemporary warfare.

To understand the process of military coalition formation, Henke first provides a framework for potential coalition members. She starts with the concept of pivotal states. Pivotal states are those with the strongest preference with regard to the formation of a coalition, and thus, they organize the coalition-building process. The pivotal state, which is both willing and able to pay the disproportionate costs of organizing the coalition, looks to create its ideal coalition. It assesses potential members according to three criteria: what the state can contribute to the coalition, how much joining the coalition intrinsically motivates the state, and what external incentives pivotal states can offer to secure their participation. Given these criteria, Henke then places potential coalition members into three categories: bandwagoners (those that eagerly join the call to action), self-starters (those that initiate coalition negotiations with pivotal states), and coalition recruits (those that are drafted by pivotal states).

Once pivotal states identify potential coalition members, Henke argues, they use their preexisting bilateral and multilateral connections to structure the coalition recruitment process. Diplomatic embeddedness, as Henke terms it, is a broad construct incorporating a range of interactions among elected officials, bureaucrats, diplomats, and military officers. Critically, diplomatic embeddedness provides information that pivotal states can use to structure potential coalition participation. It allows the pivotal state to learn about the capabilities and preferences of potential coalition members vis-à-vis the conflict in question, as well as other interests held by potential coalition members that the pivotal state can use for leverage, including side payments. It also provides mechanisms for negotiating coalition membership via negotiation venues and cooperation brokers. International organizations typically play the role of cooperation brokers. Moreover, pivotal states can also use diplomatic embeddedness to channel side payments, because direct payments may be politically unpopular for leaders of some coalition member states. Finally, diplomatic embeddedness increases the likelihood that coalition members will fulfill agreements via punishment, information, and trust.

To explore the proposed framework and mechanisms—the ways pivotal states use diplomatic embeddedness to form military coalitions—Henke combines large-N regression analysis, in-depth elite interviews, and archival research. Of particular interest to a variety of scholars are Henke’s four case studies: the US-led creation of the Korean War coalition, the United Nations-African Union Mission in Darfur (UNAMID) coalition, the Australian-led International Force East Timor (INTERFT) coalition, and the French-led European Union Military Operation in Chad and the Central African Republic (EUFOR Chad/CAR) coalition. Indeed, the cases provide a fascinating window into pivotal state processes for instructing diplomats and military attachés to collect information on deployment preferences, instructing military officers involved in training foreign military contingents to report on the deployment preferences of their trainees, approaching foreign diplomats and other officials at bilateral or multilateral gatherings and asking for their views on a particular deployment, requesting information from foreign policy alums of military training academies, leveraging joint training and operations, approaching international organization officials for recruitment information, and asking friendly states for information and advice.

There is much to like about this book, which has the potential to enrich both scholars from various disciplines and practitioners. Although there is a vast existing literature on collective warfighting, it is dominated by research on alliances, formalized agreements between states to militarily assist one another in times of need. Constructing Allied Cooperation joins the great work by Patricia Weitsman, Sarah Kreps, and others exploring ad hoc coalition formation. Moreover, although we can be sure formal alliances will continue to play a critical role in the contemporary world, established, institutionalized coalitions may not always be appropriate for confronting emergent challenges, as threats arise in new geographic areas and conflict domains. If new arrangements must be made, thinking carefully about what partners to bring into the coalition will be essential.

This book does have some limitations, although just a few, which provide opportunities for future scholars. Henke convincingly demonstrates the deliberate use of diplomatic embeddedness by pivotal states in her cases. Yet theoretically, the process by which pivotal states use it is not clear. First, it is unclear how pivotal states use the various aspects of diplomatic embeddedness sequentially, both for recruiting any one state to join the coalition or for recruiting multiple states. Understanding the sequence by which the various aspects of diplomatic embeddedness are exploited would illuminate unintended consequences of the process of coalition creation. Relatedly, how does the pivotal state use various aspects of the diplomatic network? Henke notes that inducements are more likely to work than coercion, but surely in the negotiation process the pivotal state sequences the types of inducements offered. Second, Henke’s emphasis, and rightly so for her first book on the subject, is on states. But coalitions increasingly involve nonstate actors, by default outside of Henke’s diplomatic embeddedness framework. Nonstate actors have been an enduring feature in interstate warfare and are an increasingly present coalition member. Third, what are the reputational, political, economic, or even strategic costs to the pivotal state for capitalizing on its diplomatic embeddedness? Finally, how does the construct of diplomatic embeddedness fare in an increasingly economically interdependent world? Does it provide more costly, or perhaps fewer, opportunities for pivotal states to create their desired wartime coalition?

Overall, Constructing Allied Cooperation: Diplomacy, Payments, and Power in Multilateral Military Coalitions is an important, informative, and interesting read. It debunks the notion that one can just buy allies; process and, more importantly, depth of diplomatic ties matter. To construct a military coalition is a difficult process that requires extensive diplomatic effort via multiple diplomatic channels, both military and civilian. Thus, only states with such diplomatic depth will be successful at creating a multinational military coalition. It is therefore a necessary book for those interested in diplomatic networks, collective action, alliances, and military coalitions.