Why do most civil wars occur in a relatively small number of countries? We answer this question by analyzing how civil wars diffuse in multiethnic states. Our theory outlines two motivation and two opportunity mechanisms that trigger additional ethnic rebellions in the same state. First, ongoing civil wars motivate members of other ethnic groups to mobilize in reaction to the negative externalities of nearby conflict. Second, ethnic groups emulate nearby rebel groups as a means of addressing preexisting grievances. Third, fighting multiple civil wars drains state capacity, opening the door for additional challengers to rebel against the government. Finally, long-lasting civil wars signal that the state is unable to defeat active rebels, thus creating incentives for new challengers to take up arms. We test our mechanisms in all multiethnic states with a history of armed conflict between 1946 and 2006. Using Geographic Information Systems, we construct overlap and minimum distance measures between ethnic groups’ settlement patterns and conflict zones. Our statistical analysis indicates that new ethnic civil war onsets are more likely in the vicinity of ongoing armed conflicts. Ethnic civil wars also diffuse as governments face an increasing number of rebels and longer rebellions.