This study aims to explain the pattern of contributions to NATO’s military campaign in Libya. It combines collective action theory with hypotheses on balance of threat, alliance politics, and domestic constraints in a multicausal framework, which is tested with qualitative comparative analysis. The results suggest novel inferences on the interactions between partisan politics, and the benefits states wish to secure by contributing to a multilateral operation. Contrary to conventional wisdom, parties situated at the left of the ideological spectrum were more inclined to support Operation Unified Protector than parties situated at the right. Whereas left-wing governments participated if they had the resources to contribute significantly to the fulfillment of the protection mandate and either highly valued their alliance with the United States or were not facing imminent elections, right-wing governments only contributed if their countries’ interests were threatened by the crisis in Libya or their participation was critical for the operation’s success.