Sebastian Harnisch, Florian Böller

• How can a liberal perspective on military alliances and defence cooperation be applied to the case of NATO’s establishment, institutionalization, persistence, and evolution? To answer these questions, this chapter first establishes a coherent liberal argument drawing on three variants of liberalism: republican, commercial, and ideational. It then demonstrates with short case vignettes how each of these three strands helps one understand the formation and institutionalization of NATO, the politics of intra-alliance conflict management, and how divergent ideational preferences have impacted alliance cohesion and thus the development of NATO. In conclusion, the chapter considers how liberal approaches are relevant to ‘new’ security and political challenges, such as the rise of populism, cybersecurity, and Russia’s war in Ukraine.