The period from 1914 to 1940 is one of the most crucial and enigmatic in modern world history, and in the history of modern U.S. foreign policy. World War I catapulted the United States into international economic and political leadership, yet in the aftermath of the war, despite grandiose Wilsonian plans, the United States quickly lapsed into relative disregard for events abroad: it did not join the League of Nations, disavowed responsibility for European reconstruction, would not participate openly in many international economic conferences, and restored high levels of tariff protection for the domestic market. Only in the late 1930s and 1940s, after twenty years of bitter battles over foreign policy, did the United States move to center stage of world politics and economics: it built the United Nations and a string of regional alliances, underwrote the rebuilding of Western Europe, almost single-handedly constructed a global monetary and financial system, and led the world in commercial liberalization.