Insufficiency of informal alignment: why did Finland choose formal NATO membership?

Abstract In May 2022, less than three months after Russia started its war of aggression against Ukraine, both Finland and Sweden applied for membership in NATO, thereby revoking their longstanding policies of non-alliance. What led to this shift? And what made formal NATO membership more attractive to these countries than their previous relationship with the alliance, which already amounted to informal alignment between them and NATO? To answer these questions, this article reviews the process and debates preceding the Finnish NATO application, which also proved decisive for Sweden's alliance bid. In doing so, the article contributes to the existing literature on Finland's NATO process as well as the broader scholarly debate about alliance formation. It argues that two factors prompted the Finnish NATO application: first, a dramatic shift in Finnish public opinion on NATO membership and, second, the emergence of a broadly shared view among Finland's elites of the insufficiency of Finland's existing security arrangements in view of Russia's increased threat potential, with NATO membership seen as the only viable remedy. The article duly highlights the role of public opinion in shaping a state's alignment preferences. Moreover, it finds strong evidence for realist (threat) and rationalist (efficiency) explanations for alliance formation.