From the Patriotic Front struggle against the minority rule in Rhodesia to the seven-party mujaheddin alliance in Afghanistan, inter-rebel alliances make the armed opposition more resilient and successful in the face of government repression. Why then do some rebel groups cooperate with each other while others do not? Drawing on the principal-agent theory, I argue that the presence of foreign ... [Show full abstract] sponsors is likely to encourage alliance formation in civil wars especially when two rebel outfits share a state sponsor. Shared sponsors may demand cooperation between their agents and credibly threaten to punish them for non-compliance. They may also insist on the establishment of umbrella institutions to improve their monitoring and sanctioning capacity, and to increase the legitimacy of their agents. I test this argument using the UCDP Actor dataset with new data on alliances between rebel groups. I find strong evidence that shared sponsors increase the probability of inter-rebel alliance.