Military cooperation is ubiquitous in international politics. States have a long tradition of signing treaties of alliance for collective defense, like the North Atlantic Treaty Organization, as well as actively fighting wars together, like the coalitions that twice faced Napoleonic France in hopes of preserving the European balance of power. They have colluded to conquer and partition their neighbors, as provided for in the Molotov-Ribbentrop Pact signed between Nazi Germany and the Soviet Union, and they have built coalitions to threaten war lest targets change their policies, from the Boxer Rebellion in 1900 to the Iraqi annexation of Kuwait in 1990 to ethnic cleansing in Kosovo in 1999. In fact, fully 40% of interstate wars in the past two centuries have been multilateral (Sarkees and Wayman 2010), and while only about one-quarter of international crises since the Second World War have seen coalitions form on at least one side (Wolford 2014a), the United States has built coalitions of varying sizes to support nearly half of its own uses of force since 1948 (Tago 2007), and almost all since the end of the Cold War (Kreps 2011). Yet despite their pervasiveness, to say nothing of their popularity with contemporary great powers, we still know little about how military coalitions and the efforts taken to hold them together affect patterns of international war and peace.

The occurrence and expansion of armed conflict are of enduring interest to students of international relations, all the more so when states cooperate militarily: coalitions have been integral to some of the most destructive events in international history, and multilateral wars are among the longest, bloodiest, and widest-ranging in their implications. In the twentieth century alone, two world wars redrew the global political map as rival coalitions fought on a nearly apocalyptic scale for the domination of continents and oceans, eliminating and creating both individual states and entire international orders and fundamentally altering the power relationships in the international system.