The return of traditional high-intensity warfare to Europe has led to important changes in the European security apparatus. Russia’s full-scale invasion of Ukraine has certainly re-emphasised the importance of the North Atlantic Treaty Organization (NATO) and collective defence. When NATO was founded in 1949, territorial defence was its only raison d'être, but, from 1991 onwards, deterrence and defence came to be included into a broader range of tasks and progressively put on the back burner. Since the Russian Federation illegally seized and annexed Crimea in 2014, however, the Alliance’s focus has progressively shifted from crisis management and cooperative security back to its original deterrence and collective defence mission, enshrined in Article 5 of NATO’s founding document, the North Atlantic Treaty. The article pledges its signatories to the common defence, which means that an attack on one will be regarded as an attack on all. This simple proposition has been the cornerstone of Europe’s security, peace and stability for the last seventy-five years. Yet, this norm has seemingly attracted little to no attention from legal scholars. With war raging in Ukraine and the possibility of conventional attacks on European allies becoming a reality again, understanding how credible this mutual defence commitment is and how it would play out in practice is of the utmost importance. Therefore, this chapter seeks to examine NATO’s mutual defence clause through a lawin-context approach, giving special attention to the strategic context in which Article 5 was adopted and has come to operate. The chapter is structured as follows. Section 1 introduces the topic and discusses the methodology. Section 2 analyses collective defence in NATO from a legal perspective, while Section 3 from a strategic standpoint. Section 4 concludes.