Leading twentieth-century figures in the study of international politics. He made landmark contributions to international relations theory and the study of international law, and his Politics Among Nations, first published in 1948, went through many editions and was for decades the most-used textbook in its field in U.S. universities. In addition, Morgenthau wrote widely about international politics and U.S. foreign policy for general-circulation publications such as The New Leader, Commentary, Worldview, 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, 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. For most of his career, however, Morgenthau was an academic critic of U.S. foreign policy rather than a formulator of it.