Much of the quantitative literature on civil wars and ethnic conflict ignores the role of the state or treats it as a mere arena for political competition among ethnic groups. Other studies analyze how the state grants or withholds minority rights and faces ethnic protest and rebellion accordingly, while largely overlooking the ethnic power configurations at the state's center. Drawing on a new data set on Ethnic Power Relations (EPR) that identifies all politically relevant ethnic groups and their access to central state power around the world from 1946 through 2005, the authors analyze outbreaks of armed conflict as the result of competing ethnonationalist claims to state power. The findings indicate that representatives of ethnic groups are more likely to initiate conflict with the government (1) the more excluded from state power they are, especially if they have recently lost power, (2) the higher their mobilizational capacity, and (3) the more they have experienced conflict in the past.