Japan wasn't going to accept undonditional surrender, the US casualty estimate for the invasion of Japan was about 500,000 allied troops I believe, and the Japanese casualty would be even higher. The atomic bomb actually gave the Emperor an excuse to accept the terms of surrender over the objections of the hard liners in the military. Cruel as it may sound, the 2 atomic blasts actually saved both American and Japanese lives. If an actual invasion of the Japanese mainland was necessary, it is very possible that the Russians would've taken over part of northern Japan and you would have a situation very similar to East/West Germany.
