Some of the most brutal and long-lasting civil wars of our time those in Afghanistan, Bosnia, Lebanon, and Iraq, among others involve the rapid formation and disintegration of alliances among warring groups, as well as fractionalization within them. It would be natural to suppose that warring groups form alliances based on shared identity considerations such as Christian groups allying with Christian groups, or Muslim groups with their fellow co-religionists but this is not what we see. Two groups that identify themselves as bitter foes one day, on the basis of some identity narrative, might...