No, it was the fact that there would be at least 1 million US casualities for a US invasion of Japan. The Japanese casualities weould be worse, as civilians would be asked to man the beaches, thus resulting in a tremendous loss of lives.
As to the selection of target, there were six selected, the choice was made at the last moment, owing to cloud cover
a big part of the bombs was believe it or not was revenge, the war was really over and the japs were negotiating their surrender terms. Saw it on the history cahnnel.