The aim of this thesis is to answer the question of why rebel groups engage in conflict with each other despite sharing an identity. Previous research suggests that a shared identity should promote cooperation, but recent conflicts between rebel groups in Syria seem to suggest that shared identity can instead be a cause for rivalry. This thesis investigates this puzzle by using Pischedda’s theory on windows of opportunity and vulnerability and Hafez theory on proxidistant ideologies. The cases of the thesis are the conflicts between Ahrar Al-Sham and IS in 2014 and between Ahrar Al-Sham and the Al Nusra Front in 2017. The results indicate that shared identity does cause rivalry, and both theories receive support for their explanation of inter-rebel conflict. All three rebel groups are labeled as extremists, which supports Hafez theory that extremist groups are more likely to engage in conflict with rivals. The military strength and the possible costs of attack of the rebel groups seem to support Pischedda’s theory that rebel groups use specific windows in time to attack their rivals. There are however some problems that neither theory can explain, such as Ahrar Al-Shams friendly relationship to Al Nusra between 2014 and 2017. This thesis provide insight into the understudied field of inter-rebel conflict of War Studies by highlighting the importance of identity.