The quote Joe cites does show one thing. As of June 1945 the emperor wanted to end the war. He was searching for a way out that would save some face for Japan–a negotiated treaty of some sort. My understanding is that he was trying to work through Stalin who was our ally against Germany but not at war with Japan. What this implies is that the use of the bomb was not a last resort. Truman decided to target large civilian population centers with atomic bombs because he couldn’t get an unconditional surrender, not because there was no other way to end the war. What those other ways might have been we’ll never know because Truman wouldn’t discuss it.