In this chapter, I distinguish military coalitions from other, more commonly studied forms of cooperation and show how they can provide new answers to enduring questions about (a) the conditions under which states cooperate, (b) the probability of war, and (c) the expansion of conflicts. A military coalition exists when two or more states threaten another with collective military action in an international crisis; that is, they promise to cooperate in the imposition of military costs on a target state unless their demands are satisfied. Coalitions formed in about one-quarter of interstate crises from 1946 to 2001 (Wilkenfeld and Brecher 2010) as well as 40% of interstate wars since 1815 (Sarkees and Wayman 2010), and that fraction contains some of the longest, bloodiest, and most consequential conflicts of the modern era.When coalitions make demands backed up by the threat of war, they sometimes win concessions peacefully; other times, they must carry out their threats and wage war together. Further, while most coalitions manage to keep their targets isolated, some provoke balancing from third parties, expanding conflicts beyond their original participants. What accounts for these varied patterns of coalition-building, escalation to war, and conflict expansion? And to what extent can a single theoretical approach help shed light on them all? In this chapter, I contend that extant work on military cooperation – chiefly focused on treaties of alliance and diplomatic multilateralism – is ill-equipped to answer these questions. I then outline my alternative, as well as a new dataset, based on coalitions built in response to specific crises, highlighting the role of decisions over military cooperation and its maintenance in the course of individual conflicts, as opposed to prior commitments and the sanction of international organizations, as is the case in the extant literature.

Formal alliances, in which states sign and ratify treaties that commit them to certain actions in the event of war (Morrow 2000), are the most commonly studied form of military cooperation. However, once general deterrence fails and crises arise, allied coalitions are actually less ubiquitous than those formed around no prior commitments. In the sample of late twentieth and early twenty-first century crises analyzed here, roughly three-quarters of coalitions involve no allied states.