The 500,000 death toll was a vast overestimation, and it was highly unlikely that an actual invasion of mainland Japan was going to be totally necessary for Japan to surrender. Generals such as Eisenhower and Lemay believed that an actual invasion of Japan was not necessary.

I would like to recommend J Samuel Walker's work, Prompt and Utter Destruction. He does a great job describing the decision to use the bomb, the morality behind it, and the reaction by America's military leaders. He concludes that "The answer seems to be that it shortened the war and saved the lives of a relatively small but far from inconsequential number of Americans. It might also have saved many Japanese lives, though this was not an important consideration for us Policy makers. Was the sufficient reason to wipe out two Japanese cities with weapons that delivered unprecedented military power and unpredictable diplomatic consequences? There is no definitive answer."