The '500,000 casualty' figure dates to a decade after the war. The US had a great deal of experience fighting the Japanese and our planning for Olympic and Coronet (based upon 'Iceberg', the invasion of Okinawa) was for under 100,000 casualties. The silly debate over whether or not we should have 'dropped the bomb' is a post-war, and cold-war event. The fact was that Japan had started the war, we had built the bomb, and we were going to nuke them. There was very little debate about using the bomb, and it would have taken a decision to NOT drop it. That people have continued to spin the facts to suit their sides in what became an argument over the 'morality' of nuclear war just shows the power of propaganda over fact.
U.S. House Speaker Boehner brushes off call for his ouster (Reuters)
from the article: MI chief: Iran has enough nuclear material for bomb