This essay introduces Max Weber’s sociology of modern culture to International Relations. Previous treatments of Weber in the discipline have focused on Morgenthau’s use of Weberian ideas rather than on the differences between their positions. In appropriating Weber’s ‘ethic of responsibility’ for his theory of power politics, Morgenthau neglected Weber’s sociology of ‘rationalization’ and analysis of the displacement of cultural values in modern policy-making. In Morgenthau’s theory foreign policy is judged in terms of consequences for state power, while for Weber policy is judged in terms of consequences for cultural values. This crucial difference in their understanding of the political ethics of realism is anatomized. Using Weber’s sociology of modern culture and often misunderstood view of the relation between science and values, the article then traces the repercussions of Morgenthau’s influential understanding of realism in strategic policy science.