The article discusses historical enlargements of NATO in order to explore possible explanatory frameworks useful for ongoing research on the enlargement to Sweden and Finland. With the exception for Greece and Turkey 1952, the Cold War “southern and western enlargements” were characterized by a mix of idealist and liberal impulses as well as realist and military considerations. NATO was a military pact with a liberal soul, or a guardian of liberal society with a military toolbox. In the post-Cold War enlargements there were assumptions about no risks for inter-state war, relative European stability and the ability to keep Russian protests under control. In research, NATO’s “eastern enlargements” were controversial because of its contested consequences for European security, but after 24 February 2022 it is clear that the issue of the European security order must also be part of the analysis, not least to understand the drive for membership in Sweden and Finland. The fact that Sweden was a non-member of NATO has been explained both as a consequence of a “Nordic balance”, as well as internationalist norms. The striking impression after the Russian escalation in Ukraine 2022 is that most explanations of NATO’s eastern enlargements are irrelevant to the Swedish and Finnish “northern enlargement”. Further, NATOs northern enlargement has three distinct Swedish and Finnish features: the integrated relationships with NATO prior to membership, the EU memberships and the interconnectedness of the membership applications. Three explanatory frameworks are sketched in the article: a realist perspective of a “reversed Finland factor”, a liberal perspective of an interconnected and iterative application process with pluralist input from elite and general public, and a constructivist perspective of changing Western- or small state identity.