In September 1939, war erupted in Europe. Roosevelt announced that the United States would be neutral, but not indifferent. In September 1940, when Britain was threatened by a German invasion, the United States gave the British 50 overage destroyers in return for naval bases in the western Atlantic. Two weeks later Congress approved the first peacetime military conscription in American history. By early 1941 Britain could no longer afford to purchase American goods, so Roosevelt persuaded Congress to enact a "lend-lease" bill. Through this program the United States eventually supplied $13.5 thousand million in war supplies to Britain and another $9 thousand million to the Soviet Union.
In the Far East, Japanese forces had invaded Manchuria (1931), China (1937) and French Indochina (July 1941). Roosevelt responded to this aggression by banning American exports of scrap iron, steel and oil to Japan and by freezing Japanese credits in the United States.
On December 7, 1941, carrier-based Japanese bombers struck at Pearl Harbor naval base in Hawaii. The surprise attack sank or damaged eight battleships and destroyed almost 200 aircraft. The United States declared war on Japan. Four days later, Japan's allies Germany and Italy, declared war on the United States.
In 1941, Japan possessed a large navy and a greater number of aircraft than could be mobilized by the United States. Prospects for a Japanese military victory depended on Japan's being able to defeat the Americans before the United States could retool its mighty industrial complex to produce military equipment. At this Japan failed, and the United States was soon producing huge numbers of ships, aircraft and weaponry.
Spurred by the fear that Germany might develop a nuclear weapon, the government spent $2 thousand million on the top-secret Manhattan Project, which produced and tested an atomic bomb in 1945.
American, British and Soviet war planners agreed to concentrate on defeating Germany first. British and American forces landed in North Africa in November 1942, then proceeded to Sicily and the Italian mainland in 1943, liberating Rome on June 4,1944, after months of bitter fighting. Two days later, June 6, "D-Day," Allied troops landed in Normandy in the largest amphibious operation in military history. Paris was liberated on August 24, and by September, American units were across the German border. In December 1944, however, the Germans launched a ferocious assault in the Ardennes region of Belgium. It took a week for the Allies to regroup and a month to counterattack and to force a German withdrawal in what became known as the "Battle of the Bulge." This proved to be the last German offensive of World War II. Finally, on April 25,1945, the western Allied forces met advancing Soviet troops at the town of Torgau, Germany. The Germans surrendered May 5, 1945.
In the Pacific, Japanese armed forces achieved a series of early victories. By May 1942, they had overrun the Philippines and forced the surrender of 11,500 Americans and Filipinos, who were treated brutally by their captors. In an atmosphere of war hysteria, 110,000 Japanese-Americans living in America's western states were forced into relocation camps. Government officials justified this action as a precaution against sabotage and espionage, but no Japanese-Americans were convicted of any act of disloyalty during the war, and many of them fought bravely in the armed forces.
By May 8,1942, the Japanese threat to Australia was checked at the Battle of the Coral Sea. In June, the main Japanese fleet, steaming toward Hawaii, was repulsed at the Battle of Midway, with the loss of four aircraft carriers.
Over the next three years, American forces advanced toward Japan by "island-hopping"-capturing some strategic islands in the Pacific and bypassing others.
American forces now prepared to invade the Japanese home islands. In the hope of bringing the war to a swift end, President Harry Truman ordered the use of the atomic bomb against Hiroshima (August 6) and Nagasaki (August 9). Japan agreed to surrender on August 14. Nearly 200,000 civilians died in the nuclear attacks, but military experts agree that the casualties, Japanese and American, would have been far greater if the Allies had been forced to invade Japan.