This paper critically analyses the escalating security challenges facing the European Union, primarily defined by the Russian Federation's military aggression against Ukraine after 2022 and the return of great power tensions. This transformation of the security environment has definitively ended the era of post-conflict peace on the European continent and exposed deep structural vulnerabilities in assuring sovereign European defence. The analysis identifies three key structural challenges: the critical fragmentation of Member States' defence capabilities and capacities, the limited ability to respond to high-intensity conflict, and the industrial unpreparedness of the European Defence Technological and Industrial Base. It is asserted that the EU’s persistent reliance on external guarantees, particularly amid rising transatlantic uncertainty, jeopardises its political sovereignty. To overcome these structural limitations, the paper advocates for a deeper integration trajectory towards the concept of a Total Defence Union. It posits that strengthening European defence is a strategic imperative for maintaining Europe's stability and international standing in an era of increasing great power rivalry.