The rise of fascism in Germany, Italy, and Japan led to the outbreak of World War II. Initially, the American public opposed U.S. intervention. But by 1940, President Roosevelt was mobilizing support for the military and preparing the country for war. The Japanese attack on Pearl Harbor in December 1941 brought the nation fully into the conflict. War mobilization dramatically expanded the federal government and led to substantial economic growth. It also boosted geographical and social mobility as women, rural whites, and southern blacks found employment in new defense plants across the country. Government rules assisted both the labor movement and the African American campaign for civil rights. However, religious and racial animosity caused the exclusion of Jewish refugees and the internment of 112,000 Japanese Americans.

By 1942, Germany and Japan seemed to be winning the war. But in 1943, the Allies took the offensive — with advances by the Soviet army in Europe and the American navy in the Pacific — and by the end of 1944, Allied victory was all but certain. Germany finally surrendered in May 1945, and Japan surrendered in August, after the atomic bombing of the Japanese cities Hiroshima and Nagasaki. The United States emerged from the war with an undamaged homeland, sole possession of the atomic bomb, and a set of unresolved diplomatic disputes with the Soviet Union that would soon lead to the four-decade-long Cold War. Federal laws and practices established during the war — the universal income tax, a huge military establishment, and multibillion-dollar budgets, to name but a few — became part of American life. So, too, did the active participation of the United States in international politics and alliances, an engagement intensified by the unresolved issues of the wartime alliance with the Soviet Union and the postwar fate of colonized nations.