This article examines the relaunch of European defense cooperation since2016 from the perspective of neoclassical realism, a theoretical approach tothe study of foreign policy which explores how domestic political andideational factors shape national foreign policy responses to internationalsystemic pressures. It argues that while Europe’s changing geostrategic andsecurity environment has created incentives for increased defensecooperation, explaining the form and content of this cooperation requiresunderstanding the preferences of key European states, especially Franceand Germany. The article focuses on two new forms of European defensecooperation: PESCO and the E21, the former inside the EU institutionalframework and the latter outside of it. The article argues that theseinitiatives are explained by the contrast between French and Germanpreferences on defense cooperation, which in turn reflect their divergentnational security priorities but also their different strategic cultures,including their differing perspectives on European integration.