Why do people who have lived together peacefully for years suddenly start killing each other? Tilly, a leading historical sociologist, thinks there are patterns to collective violence that run through its various forms -- barroom brawls, peasant rebellions, labor strikes, ethnic struggles, civil wars, and even interstate wars. Although Tilly relies on jargon and abstractions in his quest for a unifying framework to make sense of these diverse types of violence, a dedicated reader will pick up some interesting insights. Tilly argues that the activation of latent political identities that separate people into "us" and "them" often triggers violence. But the violence emerges less from preexisting hatred than from sudden uncertainties and shifting social conditions, particularly the declining capacity of authorities to enforce agreements or police existing boundaries. Tilly supports this claim with the useful finding that the character and intensity of collective violence depend mightily on the type of government and its capacities. Democratic regimes tend to experience less group violence than authoritarian ones because of broader participation and a more extensive array of rights and institutions, and thus, he concludes, they are the best cure for collective violence.