Limits of Compartmentalized Multilateralism: Informal Groupings, Differentiation and Europe’s Security Deficit

The European response to Russia’s full scale invasion of Ukraine has been marked by the proliferation of informal groupings that operate alongside, and partly in lieu of, the European Union’s formal foreign and security policy machinery. Building on scholarship on informal governance, differentiated cooperation and multilevel security orders, this article conceptualises these arrangements as constitutive elements of a broader pattern of compartmentalized multilateralism, in which issue specific and regionally bounded coalitions organise security provision across overlapping institutional sites. The central research question is whether, and under what conditions, such informality and differentiation can remedy Europe’s structural security deficit, understood as a persistent gap between threat perceptions, collective political will and deployable military capabilities. The argument advanced is that informal groupings possess distinct functional advantages – notably agility, the facilitation of leadership by like minded states, and flexible coupling to NATO and other organisations – but are subject to hard governance limits in terms of transparency, accountability, enforceability of commitments and the capacity to underpin long term capability generation. Empirically, the article draws on recent informal coalitions formed since 2022 to trace the interaction between these formats and under utilised treaty based instruments and decision rules, such as mutual assistance clauses, passerelle provisions for qualified majority voting and sector specific Council configurations. The article concludes that Europe’s security weakness lies less in an absence of institutional innovation than in the reluctance to activate, reform and embed existing formal frameworks and to invest in interoperable capabilities.