This article seeks to theorize the transformation of the armed forces in Northern Europe by examining its drivers during previous NATO enlargements in the region. The exploration includes the German reunification in 1990, the Polish entry in 1999, and the joining of the three Baltic States, i.e., Estonia, Latvia, and Lithuania, in 2004. Based on these experiences, the article identifies some theoretical considerations for transformation in conjunction with the current accession process of Finland and Sweden. Based on a realist logic, we conclude that the ongoing transformation should be underpinned by a new operational design, hence considering the need for flexibility and for managing potential new expeditionary shocks. To this end, we argue that the focus must initially be internally within the Alliance, i.e., to proceed with novel operational planning for northern Europe before turning the attention to the transformation of the armed forces of the new members.