The war in Europe, which started between Nazi Germany on one side and Poland, Great Britain, and France on the other, developed into a second World War. Although the Roosevelt administration favored the defeat of Germany and its principal ally, fascist Italy, the United States did not enter the war until after Japan attacked Pearl Harbor, Hawaii, December 7, 1941. Congress declared war on Japan the next day. Three days later, Germany and Italy declared war on the United States.

World War II, unlike World War I, was a long, two-front struggle for the United States. It required not only armed forces of more than 16,000,000 men and women, but industrial and agricultural production that strained the capacity of the nation. About 20,000,000 women took industrial jobs to relieve the labor shortage. The United States supplied munitions and food not only for its own forces, but also, through the program known as Lend-Lease, for its allies. The Lend-Lease program had been started before the United States was in the war, mainly to help Great Britain after the fall of France in 1940. The Soviet Union was given much Lend-Lease aid to withstand the attack upon it that Germany began in June, 1941.

The demands of the war resulted in more government control over civilian life than the nation had ever known before. Scarce consumer goods, such as sugar, meat, and gasoline, were rationed. Prices of many commodities were controlled and rents were "frozen." Many workers were prevented from changing jobs. Strikes at factories working on war contracts were forbidden.

President Roosevelt was reelected for a fourth term in November, 1944, defeating Governor Thomas E. Dewey of New York. In April, 1945, barely a month before Germany surrendered, Roosevelt died. Vice President Harry S. Truman became President. On him fell the decision of whether the United States should use against Japan a new and horribly destructive weapon—the atomic bomb—to hasten the end of the war in the Pacific area. One bomb was dropped on the city of Hiroshima on August 6, 1945, and another on Nagasaki on August 9. Five days later Japan agreed to unconditional surrender.