Read outside its immediate historical context, Languages in Contact (1953) by Uriel Weinreich (1926–1967), most particularly its Preface by André Martinet (1908–1999), contains statements that can seem contradictory and mystifying. Describing his student Weinreich’s book, Martinet characterises bilingualism as “divided linguistic allegiance”, and uses the metaphor of a battlefield to describe the feelings of language variation experienced by bilinguals – but also by monolinguals, suggesting that the mainstream doctrine of languages as self-contained and unified is nothing more than a useful abstraction. Martinet’s own allegiances were divided between loyalty to his student and to his profession, since his own best-known work tended in the direction of the abstraction. All this was taking place in a febrile atmosphere at Columbia University, as “loyalty investigations” were being implemented by the Dean of Students to root out suspected communists – people thought to have allegiances divided between the two sides of the Iron Curtain. This paper tries to make the curious statements in the Preface and the book proper comprehensible by reading them within these professional and political contexts. It considers too how Martinet and Weinreich conceive of the bilingual brain on the model of two nations within a single state.