EU institutions have expanded their involvement in defence since 2016, prompted by Brexit and Donald Trump’s first presidential victory. This trend has become even more pronounced since the 2022 Russian invasion of Ukraine. NATO remains central to Europe’s territorial defence, as evidenced by Finland and Sweden’s accession, yet there also exists a legal basis for the EU to play a role. Article 42(7) of the Treaty on European Union (TEU) obliges Member States to assist each other in the event of armed aggression, echoing Article 5 of the North Atlantic Treaty (NAT). This provision has received limited attention from legal scholars. With the war in Ukraine intensifying, concerns over Trump undermining Article 5 NAT and rising risks of military and hybrid attacks on Europe, it is crucial to understand how the EU’s mutual defence clause would be implemented. This includes its interplay with the EU’s solidarity clause in Article 222 of the Treaty on the Functioning of the EU (TFEU) and NATO’s own mutual defence clause. To this end, this article undertakes a legal and strategic analysis of Article 42(7) TEU. Following an introduction that outlines the topic and methodology, Section 1 offers a legal perspective, while Section 2 a strategic perspective. Section 3 concludes that, while significant progress has been made, the EU is not yet in a position to credibly threaten the use of force in the exercise of collective defence.