"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 Second World War 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 distils Niebuhr's disparate and now difficult to access work 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 U.N, 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 the Responsibility to Protect, and a pragmatic hawk on China as it rises today. It also highlights his many contributions to international relations theory, from Realism to Liberalism to existentialism to the English School to constructivism. This is the first book that focuses exclusively on the international relations thought of Reinhold Niebuhr, one of Classical Realism's most important figures, one whom diplomat and Realist George Kennan called, "the father of us all.""--