This doctoral dissertation focuses on the European Union (EU)’s foreign and security policy and the ad hoc initiatives by informal groupings of the European “Big Three” (France, Germany, the United Kingdom (UK)) outside of the formal EU treaties’ policy framework, in response to major international crises. Despite the setup of a so-called Common Foreign and Security Policy (CFSP), the “EU response” to major crises has indeed often taken the form of initiatives by informal groupings of a limited number of EU member states, acting on behalf of the Union but not necessarily based on a mandate. With the most prominent and active role in the face of major crises, informal groupings of the European ‘Big Three’ are also often referred to as “directoires” of the big member states, pointing to an apparent trade-off between the effectiveness and legitimacy of these initiatives at both the internal or intra-EU level (that is, in the formulation of a position or policy) and external or extra-EU level (in addressing the crisis or conflict). Seeking to address this dual effectiveness-legitimacy issue and elucidate the articulation between informal groupings’ ad hoc initiatives and EU foreign policy, this thesis proposes a new theoretical and conceptual framework expanding on the regime-based intergovernmentalist approach to policy coordination and taking it one step further by supplementing the input-based, issue-specific preferences and power dynamics with an innovative process-based model of issue-specific mainstreaming and compartmentalisation which allows mapping the various features of informal groupings’ ad hoc initiatives at the internal and external levels. While drawing insights from the precedent case of the Balkan Contact Group initiative on Bosnia and Kosovo (1994-1999), this thesis offers an in-depth tracing and analysis of the Franco-German-British E3 initiative on the Iran nuclear crisis (2003-) and of the Franco-German Normandy initiative on the Russia-Ukraine conflict in Eastern Ukraine (2014-2022), based on qualitative semi-structured elite interviews, oral and written archival testimonies, document analysis and secondary sources. Providing elements of response to the dual effectiveness-legitimacy issue while elucidating the roles of informal groupings’ ad hoc initiatives in EU foreign policy, this thesis counterbalances dominant assumptions of the literature on the phenomenon as a “paradox” or “differentiation”. It brings fresh new light on the study of foreign policy cooperation in Europe, of EU foreign and security policy and of the EU as a foreign policy actor.