How do ethnic power-sharing coalitions affect the risk of violent conflict? We argue that government leaders anticipate costly conflict and form larger ruling coalitions asuncertainty over threats increases. Our theoretical perspective contrasts with work on spoils politics and the security dilemma that predicts small and exclusive coalitions. We develop a formal model of coalition formation in ethnically divided societies that considers both rational group leaders’ desire to maximize their own power and the anticipation of future costly conflict. The model locates the key source of violent conflict in uncertainty over the size of radical sub-groups. Where ruling elites manage to satisfy violent sub-group leaders by sharing power, the risk of rebellion decreases although the risk of coups persists. Using the Ethnic Power Relations data to model ethnic coalitions in 137 states between 1946 and 2013, we find that including more dangerous internally fragmented groups into ruling coalitions decreases the risk of ethnic armed conflict while having a negligible effect on coups. A novel selection estimator reveals that the conflict-reducing effect of power sharing becomes stronger once we consider the endogeneity of coalition formation and conflict.