American participation in World War I led to a revulsion against overseas commitments, which reached its peak in the Senate rejection of the Treaty of Versailles (1919) and the new League of Nations. Rising domestic criticism in the 1920s brought about the liquidation of the military government in the Dominican Republic and moderate relaxation of American political controls elsewhere in the Caribbean. At the same time, however, the U.S. government and business community cooperated in pushing American exports and foreign loans, leading some later historians to envision an "open door imperialism" based on American economic influence abroad. An alternate view was that the United States did indeed seek such economic influence, but that most Americans then thought it possible to separate the political and economic aspects of international relations in a manner considered unrealistic by later generations.
The Great Depression of the 1930s brought an even greater emphasis on the economic side of foreign policy and a corresponding decline in interest in other aspects. The Good Neighbor policy of Franklin D. Roosevelt brought the dismantling of Caribbean military interventions and political protectorates, at the same time that Latin America was tied more closely to the American economy by means of reciprocal trade agreements. The Philippine Islands were set on the path to independence in 1934, while the Neutrality Acts of 1935–1938 were designed to minimize economic ties to belligerents in foreign wars. The Monroe Doctrine took on a new theoretical formulation as an association of hemispheric equals for collective security, and the isolationist majority in the United States eschewed any national interest in the world's affairs outside the Western Hemisphere. American imperialism was declared to be dead, never to arise again.
At the end of the 1930s there was a rapid reversal of thinking largely caused by the early victories of Nazi Germany during the new European war, and particularly by the shock created by the fall of France in 1940. Americans quickly became internationalists, the new consensus being that the world's democracies must stand together to check the crimes of "gangster nations" like Germany, Italy, and Japan. It now appeared that peace was indivisible and that the United States must be concerned with events in every corner of the globe. With the Japanese attack on Pearl Harbor late in 1941, the United States went to war in both Europe and Asia. During World War II, the United States fought as a member of a coalition that included Great Britain, the Soviet Union, Nationalist China, and many lesser members—a circumstance that drew the United States even further into global affairs. Mobilizing enormous fighting power and productivity, Americans found themselves at the close of the struggle with their armed forces deployed in Europe, Asia, North Africa, the Middle East, Australasia, and Latin America.