The worldwide depression of the 1930s helped produce a war that eventually raged across vast stretches of Europe, Asia, North Africa, and the Pacific. In Japan, Italy, and Germany, economic collapse and rising unemployment nurtured political movements that emphasized military aggression. Elsewhere, including the United States, governments turned inward to concentrate on domestic recovery and to avoid expensive foreign entanglements. After war was declared officially in Europe in 1939, President Roosevelt announced his intention for the United States to remain neutral--but privately, he mobilized public opinion in support of the Allies. Roosevelt’s efforts to eliminate isolationist sentiment in the United States became increasingly important as German forces swept through Europe. By 1941, Britain fought almost alone. In June of that year, Germany turned against the Soviet Union, with which it had earlier signed a nonaggression pact, thus bringing the Soviets into the war on the side of the Allies. Meanwhile, tensions escalated in the Pacific. Unwilling to abandon plans for territorial conquest, Japan launched a surprise attack on American military bases at Pearl Harbor, Hawaii, and in the Philippines in an attempt to weaken the United States. The United States thereupon declared war on Japan, and . Japan’s Axis allies, Germany and Italy, declared war on the United States.

The most important issue in the early months of the war involved the strategy against Germany, especially the possibility of a second front in occupied France that would serve as a springboard for an assault against Germany itself. The Soviet Union was suffering horrible losses to German attacks and pressed vigorously for a second front in the west. But Britain and the United States launched their first offensive attack in North Africa, followed by an invasion of Italy. Finally, the largest invasion force in history was assembled, and the Allies invaded France in June 1944. In the spring of 1945, Germany surrendered.

At first, Japan continued to expand in the Pacific. But, by late 1944, Japanese naval and air forces had been virtually destroyed, and American planes began continuous aerial bombings of Japan. Still, Japan seemed determined to fight (even in the face of certain defeat). In August 1945, the United States decided to use the atomic bomb in an effort to end the war without invading the Japanese home islands. At the time, most Americans sighed with relief that the devastation of much of the world was over. The United States established itself as the richest, most powerful country on the globe.

Ultimately, the success of the United States in the war depended on mobilization at home. From 1941 to 1945, American life was transformed. The federal bureaucracy nearly quadrupled in size, and the concept of greater government regulation was accepted and survived the war. The economy expanded rapidly with a dramatic increase in government spending. Science and technology forged the links of mutual interest between business and government. The sale of war bonds, rationing of essential items, and progressive taxation gave Americans a sense of shared sacrifice, easing the class tensions so prevalent in the 1930s. Dramatic social changes accompanied the wartime mobilization. War altered the composition of the workforce. As military service drained the supply of white male workers, women and blacks became more attractive as production workers. Jobs were plentiful and savings piled up. Great population movements from rural to urban areas of the country eroded social and geographical distinctions. The United States and its people lived through many socioeconomic changes during these few years.