World War II put an end to America's historical isolation from international power politics and the realist ideology that shaped Old World affairs. With the development of transoceanic military technologies and the global threats presented by Nazi Germany and the Soviet Union, the United States was suddenly vulnerable to major war and conquest. In response to this new state of affairs, three American thinkers - Reinhold Niebuhr, Hans Morgenthau and Kenneth Waltz - developed a modern strategic framework that sought to introduce American leaders and the educated public to the harsher realities of international politics. They did so by emphasizing a clear-eyed and coldly rational approach to the play of interests, egotism and the drive for power in the conduct of foreign affairs and by demonstrating a willingness to view major war as a viable option. Yet even as the United States began to embrace this new realism, atomic weaponry threatened to make it absurd. How could a realist approach to war be sustained in the face of global annihilation? This story of how the three chief architects of a powerful ideology struggled with the implications of their own creation offers a context for contemporary debates about the resort to war and weapons of mass destruction.