NATO's post-Cold War enlargements have been carried out in rounds under American leadership, some may say, on “autopilot”. This thesis takes a theory-testing approach to understand whether Sweden's and Finland's accession marks a break with this paradigm by examining the driving forces behind the latest enlargement. By combining theories about the driving forces behind previous enlargements in a theoretical framework, these can be tested from three theoretical perspectives – realism, liberalism and constructivism. A preponderance of realistic and security-related driving forces emerges. The liberal perspective of a driving public opinion and the constructivist perspective of an obsolete non-alignment complete the picture. The result shows that few of the driving forces from previous rounds were visible in the case of Sweden/Finland, and furthermore, the driving forces differ between the countries. Thus, NATO's enlargement on "autopilot" can be considered as ended – the enlargement was driven by the countries themselves.