With the testing of the Atomic Bomb completed in the United States , the two other bombs ( Little Boy , Fat Man ) had arrived on Tinian on 26 July , being delivered by the . On 24 July , a directive was sent to General Carl A. Spaatz ordering the 509th to deliver its first atomic bomb as soon as weather would permit . The Japanese cities of Hiroshima , Kokura , Niigata and Nagasaki were potential targets . President Harry S. Truman gave his final go-ahead from the Potsdam Conference on 31 July
Potsdam Conference
Potsdam Conference
Twentieth Air Force