While Ernst B. Haas is recognized among European integration scholars as a founding figure of neo-functionalism, his later writing on nationalism is less known to European integration scholars. I contribute to this special issue by discussing Haas' later work on liberal nationalism and by reflecting on its insights for the study of collective identities in the European Union today. In a nutshell, Haas expected that (1) national and supranational identities are inherently rational and the expression of a deliberate choice, (2) citizens shift their identities from the national to the European level due to utilitarian considerations, (3) nationalism is not per se destructive, but it comes in many moulds and can be the basis for the international community. I argue that Haas' emphasis on rational and deliberate identity choice clashes with our current understanding of identity as implicit and subliminal. Moreover, his optimism is challenged by the rise of radical-right-wing parties who have successfully mobilized exclusive nationalist identities against European integration. On the other hand, Haas' emphasis on the double-edged nature of collective identity is an important reminder that also European identity, while striving to overcome exclusive nationalism, can reify its exclusiveness at the supranational level.