Collective defense is back at center stage for NATO. This is primarily due to Russia’s 2014 invasion of Ukraine, its illegal annexation of Crimea, and the subsequent and more brutal second invasion of 2022, all of which prompted NATO to fundamentally reassess how it provided security to its members states. Since then, the alliance has had to relearn lost skills, refill the ranks, and reinvest in materiel necessary for territorial defense in a way that it has not since the late 1980s. This chapter puts NATO’s return to collective defense in historical context, examining how the alliance pursued security during the Cold War and the post-Cold War period through the mid-2010s. The chapter also briefly outlines some of the most significant challenges NATO has confronted since 2014 in adapting to the ‘cold peace’. The key question facing NATO today is whether the allies have the wherewithal to fully reembrace collective defense while simultaneously remaining committed to crisis management and cooperative security.