In this article, I build a theory of European Union (EU) expansion using Social Identity Theory. The theory proposes that the development of a national identity in relation to Europe is the most significant contributing factor to a policy to support/oppose expanding the EU to include applicant countries. According to the theory, strength of identity—whether more national or European—is the key variable in explaining the policy toward applicant countries. As a preliminary exploration of the theory, I look at why some decision-makers within EU countries support Turkey’s accession while others do not. The identities among top decision-makers within Britain, Germany, and France are used to examine the policy preferences regarding Turkey’s bid for membership into the EU during these three recent progressions: the recognition of Turkey as a candidate country in 1999, the development of a timeline for full membership in 2002, and the beginning of accession negotiations in 2005.