Nationalism---that is, felt attachment to one's nation---plays an important role in various political behaviors. But national identity, like other identities, is not exogenously fixed. Instead, the literature increasingly views identity as malleable with respect to major social, political and economic events. We explore how one such event, Brexit---one of the most significant policies affecting international integration in the United Kingdom---affected individual perceptions of national identity. Our theoretical framework considers Brexit a polarizing event in which expressive nationalism changed in ways consistent with policy preferences on the referendum, and political identities more broadly. To empirically understand the implications of Brexit for identity, we exploit a decade of individual-level panel data in the UK spanning both the Brexit referendum and the actual exit from the EU to evaluate how felt attachment to the British nation and its constituent nations---England, Wales and Scotland---changed. Our results demonstrate the polarizing nature of the referendum, but also the way expressions of national attachment moderated over time, even as subsets shifted their identities toward the prototypical nationalism that corresponded to their political identities. Our results have implications for short and medium run identity formation, the interpretation of public opinion, and the causes of attachment individuals have to global and national identities when those identities are made salient by integration and isolation policies.