Near the end of his career, Raymond Aron explained that he had never ceased to ‘think or dream or hope – in the light of the idea of Reason – for a humanized society’, a hope that extended to the conduct of international affairs as well. The purpose of this essay is to examine how Aron’s liberalism manifested itself in his theorising about international relations and in some of his less abstract diplomatic recommendations. In tracing the effects of Aron’s fundamental theoretical commitments on his more concrete policy analyses, we examine how Aron’s liberalism affected his approach to two of the most contentious issues in French foreign policy, namely, the decolonisation of Algeria and the creation of the force de frappe, France’s nuclear weapons programme. We argue that the tensions and contradictions in Aron’s foreign policy prescriptions have their origins in his ambivalence over the source and character of human reason.