Beginning in 1942 the United States , with assistance from Britain and other Allied countries , devoted considerable resources to developing nuclear weapons through the Manhattan Project . In December 1944 the USAAF 's 509th Composite Group was formed under the command of Colonel Paul Tibbets to deliver these weapons once they were complete ; it deployed to Tinian during May and June 1945 . The " Trinity " test of the first nuclear bomb was successfully conducted on 16 July . Four days later the 509th Composite Group 's modified " Silverplate " B-29s began flying practice raids against Japanese cities , each armed with a single high-explosive " pumpkin " bomb ; further practice missions took place on 24 , 26 and 29 July . Japanese fighters did not attempt to intercept these aircraft , and their bombing altitude of was beyond the range of most anti-aircraft guns . Meanwhile , on 24 July President Harry S. Truman approved the use of atomic bombs against Japan , and the next day Spaatz received written orders to this effect . These orders specified that the first attack should be made after 3 August , and named Hiroshima , Kokura , Niigata and Nagasaki as targets . Kyoto , Japan 's former imperial capital , had been included in an earlier version of the target list but Nagasaki was substituted on the direction of US Secretary of War Henry L. Stimson owing to Kyoto 's cultural value ; the city had also been excluded from the urban firebombing raids on the same grounds . On 26 July the United States , Britain and China issued the Potsdam Declaration , which demanded Japan 's surrender after warning that the country would be devastated if the war continued . The Japanese government rejected the Allied demands on 28 July
Trinity
Trinity (nuclear test)
Air raids on Japan