The author argues that the membership in a larger alliance has particular consequences for the defence cooperation between member states. For that purpose, the study examines Sweden’s application for NATO membership and its impact on German-Swedish defence cooperation. The study uses current research on differentiated integration and minilateralism as a starting point to frame the object of research. Influencing factors that either limit, enable, or promote minilateral defence cooperations are theoretically identified and empirically verified. The subject’s topicality makes expert interviews on ministry-, department-, and forces headquarters level the suitable empirical foundation. The author further argues that the complexity of the problem requires an eclectic approach to answer the research question. For this purpose, minilateral defence cooperations are defined as regimes and linked to the realistic, liberal, and constructivist interpretation of regime theory. The study finds that relative gains, absolute gains, and normative factors can explain changing preconditions under which German-Swedish defence cooperation is likely to increase in the face of Sweden’s NATO membership.