Which factors contribute to the cohesion of alliances among rebel groups during civil wars? Through the case of the Syrian civil war, which experienced a high level of fragmentation and cohesion among rebel groups, we emphasise how the establishment and implementation of power-sharing mechanisms in horizontal wartime institutions ensure lasting cohesion in alliances. Moreover, we posit how the mere establishment of these mechanisms is insufficient, and they must be effectively implemented. The implementation of power-sharing mechanisms in horizontal wartime institutions increases the costs of defection, and it reduces the incentives for unilateral defection from an alliance by coalition members.