The bombs, secretly developed by the United States with assistance from the United Kingdom and Canada under the codename 'Manhattan Project', were the second and third atomic devices to be detonated. They remain the only ones ever used as weapons, rather than for the purpose of testing. The first detonation, called 'Trinity', was conducted in a desert in New Mexico on July 16, 1945.
The decision to drop the bombs was made by US President Harry S Truman, and followed over three years of direct involvement of the US in World War II.
At the time of its bombing, Hiroshima was a city of considerable significance. It was a communications centre, storage point, and assembly area. It was chosen as a target because it had not suffered damage from previous raids. The US believed this would create an ideal environment to measure the harm caused by an atomic bomb.
Photograph: MPI/Getty Images
Also See: What you should know about World War II
External link: The Progressive: Unhappy Anniversary of the Bomb