It is commonly argued that political elites in Europe are increasingly acting in accordance with shared norms, identities and practices, thus shaping the character of international cooperation in Europe, not least in the field of security. However, in contrast to such expectations, European security cooperation often displays highly irregular and unpredictable patterns. This article offers a conceptual framework that seeks to make sense of these irregular patterns without refuting the assumption that social institutions in the sphere of international security shape cooperation in fundamental ways. Our point of departure is the observation that European states are embedded in international orders that produce norms and practices that sometimes complement and sometimes compete with each other. We contend that a general situational mechanism traceable through a number of domestic-level factors conditions the propensity of European states to coordinate national security policy. The framework, designed to make sense of the often-irregular patterns of European security cooperation, is illustrated by examples from European states’ response to the 2011 crisis in Libya.