Russia’s invasion of Ukraine in February 2022 brought about diplomatic condemnation across Europe and military support for Kyiv amid an uptick in European collective identity. Yet national responses differed significantly in terms of their contribution, their consistency, the degree of underlying contestation, their willingness to lead, and their choice of institutional frameworks. Explaining this variation requires an understanding of the interaction between systemic forces and a host of unit-level attributes, including military capabilities and geopolitical position, ideas and identities, and domestic politics. This introductory article examines each of these factors with a view to explaining the underlying variation in European responses, engaging with theoretical insights from Neoclassical Realism and Foreign Policy Analysis. Drawing on examples from the Special Section, we show how the divergent responses of several European countries – France, the United Kingdom, Poland, Germany, Estonia, and Sweden – can be explained by reference to these three categories.