The final item of preparation for the operation came on 29 July 1945 . Orders for the attack were issued to General Carl Spaatz on 25 July under the signature of General Thomas T. Handy , the acting Chief of Staff of the United States Army , since General of the Army George C. Marshall was at the Potsdam Conference with the President . The order designated four targets : Hiroshima , Kokura , Niigata , and Nagasaki , and ordered the attack to be made " as soon as weather will permit after about 3 August . " The mission profile for both atomic missions called for weather scouts to precede the strike force by an hour , reporting weather conditions in code over each proposed target . The strike force consisted of a bombing aircraft , with the aircraft commander responsible for all decisions in reaching the target and the bomb commander ( weaponeer ) responsible for all decisions regarding dropping of the bomb ; a blast instrumentation aircraft which would fly the wing of the strike aircraft and drop instruments by parachute into the target area ; and a camera ship , which would also carry scientific observers . Each mission had an additional " spare " aircraft pre-positioned on Iwo Jima to take over carrying the bomb if the strike aircraft encountered mechanical problems . The six combat crews of the Hiroshima mission were briefed on their targets , operational flight data , and the effects of the bomb on 4 August 1945 . Their pre-mission briefing on 5 August , under the terms of Operations Order No. 35 , covered details on weather and air-sea rescue . The Order described the bomb to be used as " special "
Potsdam Conference
Potsdam Conference
509th Composite Group