Russia's illegal aggression against Ukraine has been a watershed moment for the European Union (EU). The return of large-scale conventional warfare to the European continent, unseen since the Second World War, shattered the illusion of perpetual peace and forced the EU to confront the reality of hard power. Originally created to maintain internal peace, the EU was never conceived to handle the challenges of war. Yet, the war in Ukraine required the EU to repurpose its machinery of government to do just that.

Embracing a comparative analytical framework, Federico Fabbrini examines how the EU constitution has functioned in response to Russia's aggression. It scrutinizes the EU's legal reactions across five key policy areas: foreign, security, and defence policy; economic and fiscal policy; justice and home affairs; energy and industrial policy; and enlargement and reform. In doing so, it investigates whether the EU constitution has enabled the EU to respond effectively to the war, how EU treaties have been interpreted to authorize war-related actions, and whether these responses have adhered to constitutional limits.