This paper examines the potential strategic and legal consequences for the European Union’s defence cooperation following a hypothetical defeat of Ukraine. With the EU deeply involved in supporting Ukraine against Russian aggression, the implications of a possible defeat go beyond immediate military losses and encompasses broader ramifications for the EU and NATO, especially in their military defence, cooperation, and credibility. This paper focuses on how defeat could affect European military strategy, its legal position on defence commitments and the Union’s overall political cohesion.