/u/restricteddata has previously answered
Why weren't the bombings of Hiroshima and Nagasaki considered war crimes? The United States wiped out hundreds of thousands of Japanese civilians. Was this seen as permissible at the time under the circumstances?
Did the atomic bombings in Japan indirectly prevent the deaths of more Japanese citizens than they killed?
Did the US have to nuke Japan in WWII?
/u/t-o-k-u-m-e-i has previously answered Was it the bomb, or the Soviet declaration of war that ended WWII?
More remains to be written
EDIT: Added more answers about the A-Bomb's role in ending WWII.