为了复苏经济，富兰克林·德拉诺·罗斯福政府实行新政，大幅增加政府对经济的干预。然而新政却没有完全生效，美国经济直到第二次世界大战后才彻底复苏。
在日本偷袭珍珠港后，美国加入反法西斯同盟以对抗轴心国。接下来的战争成为美国历史上代价最高昂的一场战争，然而前线和大后方创造的大量工作机会、以及军工产业带动的生产复苏，使美国经济完全走出了大恐慌的阴霾。美国妇女也因此大规模踏入工作职场。
As with World War I, the United States did not enter World War II until after the rest of the active Allied countries had done so. The United States's first contribution to the war was simultaneously to cut off the oil and raw material supplies needed by Japan to maintain its offensive in China, and to increase military and financial aid to China. Contribution came to the Allies in September 1940 in the form of the Lend-Lease program with Britain.
On December 7, 1941 Japan launched a surprise attack on the American naval base in Pearl Harbor, citing America's recent trade embargo as justification. The following day, Franklin D. Roosevelt successfully urged a joint session of Congress to declare war on Japan, calling December 7, 1941 "a date which will live in infamy". Four days after the attack on Pearl Harbor, on December 11, Nazi Germany declared war on the United States, drawing the country into a two-theater war.
Battle against Germany
Upon entering the war, the United States and its allies decided to concentrate the bulk of their efforts on fighting Hitler in Europe, while maintaining a defensive position in the Pacific until Hitler was defeated. The United States's first step was to set up a large airforce in Britain to concentrate on bombing raids into Germany itself. The American Air force relied on the B-17 Flying Fortress as its primary heavy bomber. Britain had ceased its daylight bombing raids, due to heavy casualties inflicted by the Luftwaffe. The USAAF suffered similar high losses until the introduction of the P-51 Mustang as a long range escort fighter for the bombers.
The American army's first ground action was fighting alongside the British, Australian and New Zealand armies in North Africa. By May 1943, the British 8th Army had expelled the Germans from North Africa and the Allies controlled this vital link until the end of the war. The American navy also played an active role in the Atlantic protecting the convoys bringing vital American war material to Britain. By midway through 1943, the Allies were fighting the war from Britain with unbroken supply lines, while at the same time Hitler's armies were very much on the back foot, with heavy bombing taking its toll on production.
By early 1944, a planned invasion of Western Europe was underway. What followed on June 6, 1944, was Operation Overlord, or D-Day. The largest war armada ever assembled landed on the beaches of Normandy and began the penetration of Western Europe that eventually overthrew Hitler and Nazi Germany. Following the landing at Normandy, the Americans contributed greatly to the outcome of the war, with dogged fighting in the Battle of the Ardennes and the Battle of the Bulge resulting in Allied victories against the Germans. The battles took a heavy toll on the Americans, who lost 19,000 men during the Battle of the Bulge alone. The allied bombing raids on Germany increased to unprecedented levels after the D-Day invasion, with over 70% of all bombs dropped on Germany occurring after this date. On April 30, 1945, with Berlin completely overrun with Russian forces and his country in tatters, Adolf Hitler committed suicide. On May 8, 1945, the war with Germany was over, following its unconditional surrender to the Allied forces.
Battle against Japan
Due to the United States commitment to defeating Hitler in Europe, the first years of the war against Japan was largely a defensive battle with the United States Navy attempting to prevent the Japanese Navy from asserting dominance of the Pacific region. Initially, Japan won the majority of its battles in a short period of time. Japan quickly defeated and created military bases in Guam, Thailand, Malaya, Hong Kong, Papua New Guinea, Indonesia and Burma. This was done virtually unopposed and with quicker speed than that of the German Blitzkrieg during the early stages of the war. This was important for Japan, as it had only 10% of the homeland industrial production capacity of the United States.
Douglas MacArthur lands at the Battle of Leyte, by U.S. Army Signal Corps
The turning point of the war was the Battle of Midway in June 1942. Following this, the Americans began fighting towards China where they could build an airbase suitable to commence bombing of mainland Japan with its B-29 Superfortress fleet. The Americans began by selecting smaller, lesser defended islands as targets as opposed to attacking the major Japanese strongholds. During this period, they inadvertently triggered what would become their most comprehensive victory in the entire war.
The Pacific war became the largest naval conflict in history. The American Navy emerged victorious, after at one point being stretched near to the breaking point, with almost complete destruction of the Japanese Navy. The American forces were then poised for an invasion of the Japanese mainland, to force the Japanese into unconditional surrender. On April 12, 1945, President Franklin Delano Roosevelt died and Vice President Harry S. Truman was sworn in as the 33rd President of the United States. The use of atomic weapons against Japan was subsequently authorized. The decision to use nuclear weapons to end the conflict has been one of the most controversial decisions of the war. Supporters of the use of the bombs argue that an invasion would have cost an enormous numbers of lives, while opponents argue that the large number of civilian casualties resulting from the bombings was unjustified. The first bomb was dropped on Hiroshima on August 6, 1945, and the second bomb was dropped on Nagasaki on August 9, 1945. On August 15, 1945, the Japanese surrendered unconditionally, ending World War 2.