The attacks on Pearl Harbor on December 7, 1941 would change the role of the USA . Eventually, China joined the Allies, as did most of the rest of the world. By the beginning of 1942, the major combatants were aligned as follows: the British Commonwealth, the United States, and the Soviet Union were fighting Germany and Italy; and the British Commonwealth, China, and the United States were fighting Japan. From then through August 1945, battles raged across all of Europe, in the North Atlantic Ocean, across North Africa, throughout Southeast Asia, throughout China, across the Pacific Ocean and in the air over Japan.

On the morning ofAugust 6, 1945, the United States Army Air Force dropped the nuclear weapon, "Little Boy" on the city of Hiroshima.

Three days later the detonation of the "Fat Man" bomb over Nagasaki, Japan occurred.

In his 1999 book Downfall, historian Richard Frank analyzed the many widely varying estimates of casualties caused by the bombings. He concluded "The best approximation is that the number is huge and falls between 100,000 and 200,000.

Most of the casualties were civilians. The role of the bombings in Japan's surrender, as well as the effects and justification of them, have been subject to much debate. In the U.S., the prevailing view is that the bombings ended the war months sooner than would otherwise have been the case, saving many lives that would have been lost on both sides if the planned invasion of Japan had taken place. In Japan, the general public tends to think that the bombings were needless as the preparation for the surrender was in progress in Tokyo

The survivors of the bombings are called hibakusha. (被爆者), a Japanese word that literally translates to "people exposed to the bomb". The suffering of the bombing is the root of Japan's postwar pacifism, and the nation has sought the abolition of nuclear weapons from the world ever since. As of 2006, there are about 266,000 hibakusha still living in Japan.