This PhD thesis provides the first systematic and comprehensive analysis of the collective defence mechanisms in Article 5 of the North Atlantic Treaty (NAT) and Article 42(7) of the Treaty on European Union (TEU). Russia’s ongoing invasion of Ukraine has re-emphasised the importance of collective defence, of which the North Atlantic Treaty Organisation (NATO) and its mutual defence clause have been the cornerstone for over seventy-six years. However, a similar mutual assistance/defence clause also binds the Member States of the European Union (EU). In light of rising fears of military escalation and potential conventional attacks on NATO and/or EU territory, and amid uncertainty surrounding the United States’ commitment to European security under the current Trump administration, understanding how these mutual defence clauses operate, interact, and apply in practice is of critical importance. Using a law-in-context approach, this thesis analyses and compares these overlapping mutual defence clauses. Specifically, the thesis explores the historical origins of these clauses, teases out their legal meaning, and, drawing on international and European security scholarship, examines the geopolitical and strategic contexts in which they operate. This thesis considers as case studies the two circumstances in which these mutual defence clauses were triggered for the first—and so far only—time. First, the invocation of Article 5 NAT following the 11 September 2001 terrorist attacks in the United States of America (USA). Second, the activation of Article 42(7) TEU in response to the 13 November 2015 terrorist attacks in Paris. The thesis argues that the obligations under Article 42(7) TEU are legally more stringent than those stemming from Article 5 NAT: although both entail obligations of result, the former obliges EU Member States to provide assistance ‘by all the means in their power’, whereas the latter requires NATO Allies to take ‘such action including the use of armed force’, as deemed necessary. Yet, in practice, Article 5 NAT has proved more credible: while both organisations lack enforcement mechanisms to address instances of inadequate action or inaction by their signatories, the US nuclear umbrella has thus far lent credible military strength to Article 5 NAT, something Article 42(7) TEU still lacks. By analysing the EU and NATO’s collective defence mechanisms, this research seeks to equip legal professionals and policymakers with insights that can foster a better understanding of the legal and institutional challenges that must be overcome to advance defence integration and cooperation in Europe.