America was wounded but roused to national unity by Pearl Harbor. Roosevelt settled on a fundamental strategy of dealing with Hitler first, while doing just enough in the Pacific to block the Japanese advance.

With the ugly exception of the Japanese-American concentration camps, World War II proceeded in the United States without the fanaticism and violations of civil liberties that occurred in World War I. The economy was effectively mobilized, using new sources of labor such as women and Mexican braceros. Numerous African Americans and Indians also left their traditional rural homelands and migrated to war-industry jobs in the cities of the North and West. The war brought full employment and prosperity, as well as enduring social changes, as millions of Americans were uprooted and thrown together in the military and in new communities across the country. Unlike European and Asian nations, however, the United States experienced relatively little economic and social devastation from the war.

The tide of Japanese conquest was stemmed at the Battles of Midway and the Coral Sea, and American forces then began a slow strategy of island hopping toward Tokyo. Allied troops first invaded North Africa and Italy in 1942 - 1943, providing a small, compromise second front that attempted to appease the badly weakened Soviet Union as well as the anxious British. The real second front came in June 1944 with the D-Day invasion of France. The Allies moved rapidly across France, but faced a setback in the Battle of the Bulge in the Low Countries.

Meanwhile, American capture of the Marianas Islands established the basis for extensive bombing of the Japanese home islands. Roosevelt won a fourth term as Allied troops entered Germany and finally met the Russians, bringing an end to Hitlers rule in May 1945. After a last round of brutal warfare on Okinawa and Iwo Jima, the dropping of two atomic bombs ended the war against Japan in August 1945.