The atomic bomb was developed in secret by the United States during World War II in what was known as the Manhattan Project.

President Harry S. Truman made the decision to use the new weapon against Japan.

The primary motivation was to force Japan's unconditional surrender and avoid a massive, costly invasion of the Japanese home islands.

On August 6, 1945, the B-29 bomber 'Enola Gay' dropped an atomic bomb on the city of Hiroshima.

The bomb immediately killed an estimated 80,000 people, with tens of thousands more dying later from radiation sickness and injuries.

When Japan did not surrender, a second atomic bomb was dropped on the city of Nagasaki on August 9, 1945.

The bombings, combined with the Soviet Union's declaration of war on Japan on the same day, led Emperor Hirohito to support surrender.

Japan formally surrendered on September 2, 1945, officially ending World War II.

The use of the atomic bomb remains one of the most controversial decisions in military history.

The bombings ushered in the nuclear age and the new realities of the Cold War.