Actually an invasion of Japan might have ended up in the murder-suicide of the entire population. No less a person than Kurosawa Akira recorded as much in his memoirs. The Japanese public were basically told, from the beginning of "The Pacific War," that the Emperor would, in the event of an invasion of Japan, order the suicide of all Japanese. I say murder-suicide because parents would have been ordered to dispatch their children before taking their own lives. Most families had knives for this purpose, and usually kept in a place of honor in their residence.
However, the results of the two atomic bombings of Japan are staggeringly horrific. So much so that it took until 1968 before footage taken by the Japanese Army's Documentary Corps was released as the documentary Hiroshima-Nagasaki August 1945. I am just finishing up a class on the history of documentary film, and I've watched some pretty hard-to-take footage. The clips from this documentary are perhaps the hardest to watch of anything I've seen.
Thermonuclear weapons are absolutely horrifying. We must endeavor never, ever, to use them again. However, considering what the Japanese were prepared to do to themselves and to their children if an invasion occurred, the atom bomb saved an entire civilization from self-destruction.