This paper explores the relation between philosophy and politics in Being and Time (1927) starting from Heidegger’s suggestion that we can understand some of the linguistic and conceptual difficulties in his investigation by comparing Thucydides’ narrative prose with two texts by Plato and Aristotle. Far from simply signalling Heidegger’s proximity to Plato and Aristotle and an apolitical disdain for human affairs, carrying out and contextualizing this exercise within his interpretations of ancient philosophy shows the difficulties lie in formulating an ontology of historical, political existence. I then argue that Heidegger’s reference in Plato’s Sophist (1924-5) to a political speech in Thucydides elucidates the kind of existentiell situations underlying his understanding of authentic historicality. Situating this reference within Heidegger’s discussions of the possibilities of political speech reveals marked similarities to both phenomena thematized existentially in Being and Time and the most violent expressions of his völkisch politics in the 1930s.