This article examines the EU's decision-making on military operations to assess the contested nature of the EU as an international security actor. It investigates both the decision-making process on the launch and subsequent development of EUNAVFOR Atalanta, from 2008 to 2014. First, empirically, the article, based on policy documents and semi-structured interviews, unpacks the contestation concerning the use of military force in the EU by distinguishing between the justification of military force and the policy embeddedness of military force at the more operational level. Second, it shows how the strategic interaction among different advocacy coalitions, which is characterized by a process of cooperative bargaining, explains the particular development of EUNAVFOR Atalanta in terms of its justification and policy embeddedness.