Nationalism has had a complex relation with the discipline of political theory during the 20th century. Political theory has often been deeply uneasy with nationalism in relation to its role in the events leading up to and during the Second World War. Many theorists saw nationalism as an overly narrow and potentially irrationalist doctrine. In essence it embodied a closed vision of the world. This paper focuses on one key contributor to the immediate post-war debate—Karl Popper—who retained deep misgivings about nationalism until the end of his life, and indeed saw the events of the early 1990s (shortly before his death) as a confirmation of this distrust. Popper was one of a number of immediate post war writers, such as Friedrich Hayek and Ludwig von Mises, who shared this unease with nationalism. They all had a powerful effect on social and political thought in the English-speaking world. Popper particularly articulated a deeply influential perspective which fortuitously encapsulated a cold war mentality in the 1950s. In 2005 Popper’s critical views are doubly interesting, since the last decade has seen a renaissance of nationalist interests. The collapse of the Berlin wall in 1989, and the changing political landscape of international and domestic politics, has seen once again a massive growth of interest in nationalism, particularly from liberal political theorists and a growing, and, at times, immensely enthusiastic academic literature, trying to provide a distinctively benign benediction to nationalism.