Why do rebel groups often fight each other when confronting a common, and typically stronger, enemy—namely, the government? Inter-rebel aggression is a calculated response by rebel groups to opportunities for expansion and to the threats confronting such groups. In particular, inter-rebel wars occur under the following conditions: (1) when a window of opportunity emerges allowing rebel groups to defeat coethnic rivals at low costs and thus achieve rebel hegemony, or (2) when rebel groups are confronted with a window of vulnerability, in which they experience a radical deterioration of power relative to that of coethnic rivals and then attempt to escape in a desperate military gamble. Both strict military considerations and ethnic identities shape the rebel groups' calculus. Coethnicity influences the threat perception of rebel groups and their ability to grow at the expense of rivals, thus providing both defensive and offensive motives for inter-rebel aggression. Because coethnic rebel groups want to mobilize and control the same ethnic communities, they are highly sensitive to immediate, intense conflicts of interest and see other coethnic rebel groups as serious threats. Moreover, coethnic rebel groups can generally expect to absorb the resources of defeated rivals, which in turn may improve their chances in the fight against the government. In-depth case studies of multiparty insurgencies in Ethiopia's Eritrea and Tigray provinces, based on interviews with former rebel leaders, strongly support the window of opportunity and vulnerability theory of inter-rebel war.