What can it mean to be rational, especially in a world that seemed to be on the brink of thermonuclear destruction? During the Cold War, this fundamental question engaged the sharpest minds. Which theories of rationality could be invoked to explain human behavior, especially in the domains of international relations, war, and military strategy? And could one develop theories also for the resolution of political problems, thus providing clear normative guidance? Could the rules be given an axiomatic structure and applied to various domains of science and society in a strictly determinate fashion? Today’s fragmentation in the study of rationality undermines the Cold War hope for a unified concept of rationality. This also reveals how more traditional notions of “reason”, understood as mindful deliberation over when and how to apply a rule, have returned to the scene.