<p>In addition to joint activities undertaken via the EU institutional framework in response to the Russian war in Ukraine, EU countries also shaped the broader European security architecture through informal groups. These groups, which lack formal institutional structures, allow EU member states to coordinate activities to pursue common objectives. Despite the rapidly growing literature on informal groups, their occurrence and relevance in the wake of the outbreak of war in Ukraine remain understudied. This study examines informal groups' compositions, motivations, and reflects on their relevance for the European security architecture. It suggests that by resorting to informal groups, EU member states have organized the European security order functionally and geographically into several formats. By doing so, they have fostered compartmentalized multilateralism, contributing to functional relations between various layers of European security order.</p>