I honestly never intended to write this book – or any book, for that matter. My plan had always been a career based on journal articles, which is pretty much the norm for someone like me who publishes applied formal theory and an occasional empirical paper. However, armed with a fellowship semester in the fall of 2012, some data I'd started collecting but didn't know what to do with, and a couple of related journal articles in the pipeline, the idea to tie it all together into a book started to take shape. What, after all, did we really know about military coalitions? Was the conventional wisdom, basically a horror story of free-riding, intramural rancor, and coercive ineffectiveness, correct? If it was, why did we see so many states using coalitions in so many conflicts? Did coalition partners increase the chances of war? Did they presage expanded conflicts? I soon realized that, to do this thing right – that is, to introduce my own definition of military coalitions, to build a series of theoretical models, and to test their predictions against the data – would be very much a book-sized endeavor.

After lamenting each of the (many, many) times I'd tuned out when people talked about book writing and publishing, I took a leap into the dark and got to work. The result, I hope, can start a scholarly dialogue about the dominant mode of military cooperation in international politics: the crisis-specific, purpose-built coalition. In the chapters that follow, I introduce the data, identify some puzzles to be explained, and then analyze the choice of partner, the escalation of crises to war, and the expansion of conflicts, all with an eye to the fundamental politics of buying and selling military cooperation. Hopefully, my claims that coalitions are worthy of study, that when it comes to shaping patterns of international war and peace they are more than the sum of their parts, are convincing. At the same time, I'm also hopeful that the exercise invites scrutiny and improvement; to the extent I can be productively wrong about something in this book, I'll consider the effort a success.

The effort, though, wasn't all my own; I racked up intellectual debts over the last several years like a compulsive gambler.