Although Emmanuel-Joseph Sieyès remains amongst the most studied thinkers of the French Revolution, his views on international politics remain largely unexplored, despite his significant role in shaping the foreign policy of the French republic after 1794. This article provides a new account of Sieyès as an international political thinker and actor, drawing on published and archival materials to reconstruct Sieyès' diplomatic programme and its intellectual roots. In so doing, it challenges both the notion that Sieyès was a committed practitioner of an unideological realopolitik and the common assertion that Sieyès was a follower of Immanuel Kant's famous project for perpetual peace. Instead, this article shows that Sieyès charted a distinct course, based on a plan for the ‘republicanisation’ of Europe and its reorganisation into a league of militarily and economically linked states under French hegemony, oriented towards the preservation of republicanism in a hostile world. On this basis it re-evaluates the relationship between Sieyès' and Kant's conceptions of a confederation of republican states, presenting a new account of Kant's Perpetual Peace as a curtailment of his earlier cosmopolitan and pacifist idealism in response to the international situation of the 1790s and the foreign policy pursued by Sieyès and his allies.