The UK’s official departure from the European Union in early 2020 and the acceleration of Ukraine’s EU application process following conflict with Russia in 2022, has revived debate on the development of European institutions. In the context of renewed tensions on the continent and the exposure of the vulnerabilities of European states in some key sectors (energy foremost among them), greater solidarity in stance and action seems necessary and is desired by various member states (including France and Germany). However, current institutional arrangements make this complicated, if not indeed impossible, in a very broad EU that includes some less tractable countries and some governments that no longer totally share certain values (including the democratic values) of their partners. Should we, then, expand the EU or do the opposite and deepen its mode of organization in a more federalist direction? Jean-François Drevet re-examines this dilemma, which has been to the fore since the creation of the European Community, in the light of the institutional developments the Union has undergone and the limits these have exposed over time. Confronted with the many questions raised by the Russo-Ukrainian conflict, with regard to energy independence, economic sovereignty or geopolitical leadership, the EU will not be able to sidestep this debate for much longer.