AT A COST OF 405,399 American lives, the nation united with its allies to crush the Axis aggressors into unconditional surrender. Almost all Americans believed they had won a “good war” against totalitarian evil. The Allies saved Asia and Europe from enslavement and finally halted the Nazis’ genocidal campaign against Jews and many others whom the Nazis considered inferior. To secure human rights and protect the world against future wars, the Roosevelt administration took the lead in creating the United Nations.

Wartime production lifted the nation out of the Great Depression. The gross national product soared to four times what it had been when Roosevelt became president in 1933. Jobs in defense industries eliminated chronic unemployment, provided wages for millions of women workers and African American migrants from southern farms, and boosted Americans’ prosperity. Ahead stretched the challenge of maintaining that prosperity while reintegrating millions of uniformed men and women, with help from the benefits of the GI Bill.

By the end of the war, the United States had emerged as a global superpower. Wartime mobilization made the American economy the strongest in the world, buttressed by the military clout of the nation’s nuclear monopoly. Although the war left much of the world a rubble-strewn wasteland, the American mainland had enjoyed immunity from attack. The Japanese occupation of China had left 50 million people without homes and millions more dead, maimed, and orphaned. The German offensive against the Soviet Union had killed more than 20 million Russian soldiers and civilians. Germany and Japan lay in ruins, their economies and societies as shattered as their military forces. But in the gruesome balance sheet of war, the Axis powers had inflicted far more grief, misery, and destruction on the global victims of their aggression than they had suffered in return.

As the dominant Western nation in the postwar world, the United States asserted its leadership in the reconstruction of Europe while occupying Japan and overseeing its economic and political recovery. America soon confronted new challenges in the tense aftermath of the war, as the Soviets seized political control of eastern Europe, a Communist revolution swept China, and national liberation movements emerged in the colonial empires of Britain and France. The forces unleashed by World War II would shape the United States and the rest of the world for decades to come. Before the ashes of World War II had cooled, America’s wartime alliance with the Soviet Union fractured, igniting a Cold War between the superpowers. To resist global communism, the United States became, in effect, the policeman of the free world, repudiating the pre–World War II legacy of isolationism.