The Second World War (1939–45) caused 50,000,000 fatalities, including Jews in German death camps, Japanese deaths from atomic bombs, and 407,000 Americans of various racial and ethnic identities. After Japan's attack on Pearl Harbor (1941), America joined the Allied powers (Great Britain, France, and Russia) against the Axis powers (Japan, Germany, and Italy).

Blacks, like women and workers en masse, pursued equality on the home front. In the wake of the Selective Service Act (1940) some 2 million black men ages twenty to thirty-five registered for the draft, but service authorities rejected 33 percent of them (compared to 16 percent of white men). Blacks protested Jim Crow in defense industries and the Armed Forces. So, the War Department agreed to elevate a black officer to brigadier general, appoint black civilian aides to the War and Selective Service secretaries, and begin reserve officers’ training at five black colleges. But the issue of fair employment dragged until blacks threatened a mass March on Washington, which forced an executive order that banned job discrimination and established the Committee on Fair Employment Practice (FEPC). Black newspapers campaigned for a “double victory,” victory over racism at home and fascism abroad. Still, the army and War Relocation Authority relocated 120,000 Japanese Americans, 80 percent native-born, from the West Coast to the Southwest where they were held in detention camps to 1946. Xenophobia and job competition also fueled bloody race riots, more than 100 in 1943 alone. One of the worst broke out in Detroit, hub of the auto industry, taking the lives of twenty-five blacks and nine whites and destroying property worth millions. Nevertheless, black civilians continued pushing for equal citizenship.

Black servicemen and women's roles helped leverage that cause. Over 1,000,000 of them served, 701,000 in the army. Many saw duty in the navy, coast guard, marines, merchant marine, and army air corps. Half of them went overseas, there serving in infantry, coastal and field artillery, cavalry, tank, and transportation units; in signal, engineer, medical, nurses, and air corps. The army, navy, women's army and navy branches slowly began desegregating officers’ training schools. Segregation was still the official policy, but one black platoon fought bravely with white troops in Germany. A number of blacks earned military honors, notably the Navy Cross and (after a 1997 staff review) seven Medals of Honor.