Isolationism was the byword of American foreign policy in the 1920s and 1930s. During the buildup to U.S. entry into World War I, a major shift in the two major parties' posture toward foreign affairs had taken place. Before 1916 Democrats had generally followed Bryan's lead in opposing a more assertive, interventionist role in world affairs. Under Wilson's leadership, however, the party gradually embraced a more active role for the United States in world affairs in which it identified its economic and strategic well-being with that of other democracies and in which it would be willing to use force in behalf of world peace. A similar change was taking place in the Republican Party. Its support of the Spanish-American War and acquisition of the Philippines, together with the activism of Roosevelt, Lodge, and other prominent Republicans, had earned the party a reputation for favoring a larger role for the United States in world affairs. But in 1916 the Republicans refused to seriously consider nominating Roosevelt for the presidency. In the debate over the Versailles treaty, the party identified itself with nationalism and isolationism against Wilsonian internationalism. In truth, the rank and file of the Republican Party, especially outside the East, identified more with Borah and his followers than with Lodge and his. In the 1930s, influenced by the Great Depression and the gathering war clouds in Europe, the Republican Party, as well as a majority of Americans, would invoke the concept of Fortress America and insist that the rise of the fascist powers in Europe and Asia posed no threat to the United States.
During the height of the Great Depression, the Democratic administration of Franklin Roosevelt chose not to challenge the Republican consensus. But as the 1930s progressed with Hitler gobbling up Austria and Czechoslovakia and Japan's invasion of China, Roosevelt began inching the country toward nonbelligerent alliance with Great Britain, China, and the other nations standing against the Axis. With the outbreak of World War II in 1939 and the fall of France in 1940, the debate over America's proper role in world affairs escalated, with the Democrats generally opting for interventionist policies and the Republicans clinging to isolation. In 1940 isolationists formed the America First Committee. The organization was Midwest-centered and made up largely of business-oriented opponents of the New Deal, although it included former Progressives and elements of the extreme left who espoused the "merchants of death" thesis. Opposing them were the Committee to Defend America by Aiding the Allies. Ideologically, interventionists tended to be liberal New Dealers and politically they were generally Democratic, although members of the eastern, liberal wing of the Republican Party supported all-out aid to the Allies. In the spring of 1941, the Roosevelt administration went head-to-head with the isolationists in Congress and secured passage of the Lend-Lease Act. With that measure the United States became a non-fighting ally of Great Britain and the Soviet Union. With the attack on Pearl Harbor on 7 December 1941, the United States became a full-fledged belligerent, and partisan opposition to intervention effectively ended.