Reinhold Niebuhr (1892–1971) may have been the most influential and insightful American thinker of the twentieth century. In dealing with the intricacies of human nature, society, politics, ethics, theology, racism, and international relations, Niebuhr the teacher, preacher, philosopher, social critic, and ethicist was highly influential and difficult to ignore during the World War II and Cold War eras because of his intellectual heft and the novel manner in which he addressed the economic, spiritual, social, and political problems of his time. This book distills Niebuhr’s disparate and now difficult-to-access work on international relations into one volume, making it more easily accessible than ever before, at the same time bringing his work into the twenty-first century. It argues that if he were alive today Niebuhr would be a champion of the United Nations, a supporter of globalization, a fierce opponent of America’s 2003 Iraq War (for all the reasons he opposed the Vietnam War), an advocate of responsibility to protect, and a pragmatic hawk on China as it rises today. This book also highlights his many contributions to international relations (IR) theory, from Realism to Liberalism to existentialism to the English School to constructivism. This is the first book that focuses exclusively on the IR thought of Reinhold Niebuhr, one of America’s most important public intellectuals and classical Realism’s most important figures, dubbed “the father of us all” by American diplomat and Realist George Kennan.