The period 1991-2022 has great similarities with the period 1919-1939 as both periods are characterised by a gradual deterioration of international organisations and end with a systemic war. Why then have Sweden and Finland applied for NATO membership in the contemporary period when they remained military non-aligned in the previous period?This study develops Walt’s balance of threat theory, by operationalising Snyder’s cost-benefit analysis as intervening variables in a most different system design. This study concludes that in the first period, military non-alignment can be explained by the future costs of joining an alliance were higher than the benefits due to great power tensions. Meanwhile, in the contemporary period, the NATO application is a consequence of a rapid change in perceived threat which shifts the military balance at the same time as the negative consequences of the alliance is low. / <p>Jag vill tacka min handeledare Docent Jacob Westberg för ett gott stöd under Mastersarbetet. Jag vill också tacka Anna Lindh Bibiloteket för all hjälp med litteratur. Slutligen vill jag rikta ett särskilt tack till Sofia Lindgren på riksdagsbiblioteket som gav mig verktyg och förståelse för hur de politiska dokumenten hänger ihop, vilket har varit avgörande för hanteringen av empirin.</p>