As far as I'm aware, this is one of those great what-ifs of history that is great because we know fairly well what would have happened. It seems that the allies didn't actually expect Japan to surrender and had well drawn out plans for an invasion of mainland Japan.
I'm not a historian; basically all I know I learned from some fascinated reading of the article on Operation Downfall on Wikipedia.
At the time of Japan's surrender, Russia has recently declared war on them (waiting to do so until Nazi Germany had been defeated). Highlight from Wikipedia: "Unknown to the Americans, the Soviets also considered invading a major Japanese island—Hokkaido—by the end of August 1945"
These were the allies stated assumptions (also from Wikipedia):
"That operations in this area will be opposed not only by the available organized military forces of the Empire, but also by a fanatically hostile population."
"That approximately three (3) hostile divisions will be disposed in Southern KYUSHU and an additional three (3) in Northern KYUSHU at initiation of the OLYMPIC operation."
"That total hostile forces committed against KYUSHU operations will not exceed eight (8) to ten (10) divisions and that this level will be speedily attained."
"That approximately twenty-one (21) hostile divisions, including depot divisions, will be on HONSHU at initiation of [Coronet] and that fourteen (14) of these divisions may be employed in the KANTO PLAIN area."
"That the enemy may withdraw his land-based air forces to the Asiatic Mainland for protection from our neutralizing attacks. That under such circumstances he can possibly amass from 2,000 to 2,500 planes in that area by exercise of rigid economy, and that this force can operate against KYUSHU landings by staging through homeland fields."
The article is long and absolutely fascinating. I'd recommend anyone interested in this what-if scenario read the whole thing.
