Five factors are shown to be strongly related to civil war duration. Civil wars emerging from coups or revolutions tend to be short. Civil wars in Eastern Europe and the former Soviet Union have also tended to be relatively brief, as have anti-colonial wars. By contrast, 'sons of the soil' wars that typically involve land conflict between a peripheral ethnic minority and state-supported migrants of a dominant ethnic group are on average quite long-lived. So are conflicts in which a rebel group derives major funding from contraband such as opium, diamonds, or coca. The article seeks to explain these regularities, developing a game model focused on the puzzle of what prevents negotiated settlements to long-running, destructive civil wars for which conflicting military expectations are an implausible explanation. In the model, regional autonomy deals may be unreachable when fluctuations in state strength undermine the government's ability to commit. The commitment problem binds harder when the center has an enduring political or economic interest in expansion into the periphery, as in "sons of the soil" wars, and when either government or rebels are able to earn some income during a conflict despite the costs of fighting, as in the case of contraband funding.