Hans Joachim Morgenthau (February 17, 1904 – July 19, 1980) was one of the major twentieth-century figures in the study of international relations. Morgenthau's works belong to the tradition of realism in international relations theory, and he is usually considered, along with George F. Kennan and Reinhold Niebuhr, one of the three leading American realists of the post-World War II period. Morgenthau made landmark contributions to international relations theory and the study of international law. His Politics Among Nations, first published in 1948, went through five editions during his lifetime.
Morgenthau also wrote widely about international politics and U.S. foreign policy for general-circulation publications such as The New Leader, Commentary, Worldview, The New York Review of Books, and The New Republic. He knew and corresponded with many of the leading intellectuals and writers of his era, such as Reinhold Niebuhr, George F. Kennan, Carl Schmitt and Hannah Arendt. At one point in the early Cold War, Morgenthau was a consultant to the U.S. Department of State when Kennan headed its Policy Planning Staff, and a second time during the Kennedy and Johnson administrations until he was dismissed by Johnson when he began to publicly criticize American policy in Vietnam. For most of his career, however, Morgenthau was esteemed as an academic interpreter of U.S. foreign policy.