  Twentieth-century realism was born in response to the idealist perspective that dominated international relations scholarship in the aftermath of the First World War. The idealists of the 1920s and 1930s (also called liberal internationalists or utopians) had the goal of building peace in order to prevent another world conflict. They saw the solution to inter-state problems as being the creation of a respected system of international law, backed by international organizations. This interwar idealism resulted in the founding of the League of Nations in 1920 and in the Kellogg-Briand Pact of 1928 outlawing war and providing for the peaceful settlements of disputes. U.S. President Woodrow Wilson, scholars such as Norman Angell, Alfred Zimmern, and Raymond B. Fosdick, and other prominent idealists of the era, gave their intellectual support to the League of Nations. Instead of focusing on what some might see as the inevitability of conflict between states and peoples, they chose to emphasize the common interests that could unite humanity, and attempted to appeal to rationality and morality. For them, war did not originate in an egoistic human nature, but rather in imperfect social conditions and political arrangements, which could be improved. Yet their ideas were already being criticized in the early 1930s by Reinhold Niebuhr and within a few years by E. H. Carr. The League of Nations, which the United States never joined, and from which Japan and Germany withdrew, could not prevent the outbreak of the Second World War. This fact, perhaps more than any theoretical argument, produced a strong realist reaction. Although the United Nations, founded in 1945, can still be regarded as a product of idealist political thinking, the discipline of international relations was profoundly influenced in the initial years of the post-war period by the works of “classical” realists such as John H. Herz, Hans Morgenthau, George Kennan, and Raymond Aron. Then, during the 1950s and 1960s, classical realism came under challenge of scholars who tried to introduce a more scientific approach to the study of international politics. During the 1980s it gave way to another trend in international relations theory—neorealism.