Japan would have lost, horribly.
And it would have been a blood bath for the invaders. I personally question how much of Japan or Japanese culture would have survived such an invasion.
If the US had landed in Kyushu as originally planned, they would have been fighting against about 1,000,000 well fortified troops; troops that were well dug in.
The Japanese would have been heavily bombed and heavily shelled for months, and it still would have been hell for the invaders and death for the defenders. Just like the battle of Okinawa, the Japanese would have lost inflicting horrible casualties.
By the planned invasion date, Nov 1945, there would likely have been mass starvation in Japan. The Japanese had been on short rations for years, there were zero food imports, the US Navy had destroyed a large portion of the fishing fleet and the 1945 rice harvest was expected to be poor.
By the summer of 1945 it [Japanese civilian food ration] was about 1,680 calories per capita. Coal miners and heavy industrial workers received higher-than-average rations, the remaining populace, less. The average diet suffered even more drastically from reductions in fats, vitamins and minerals required for balance and adversely affected rates of recovery and mortality from disease and bomb injuries. Summary Report (Pacific War)
Japan had very limited air capability – 10,000 planes sounds impressive, but they had very limited fuel and very few fully trained / experienced pilots.
The assumption is that the planes would have been used as kamikaze against the invasion force. Given that the kamikaze were not particularly effective late in the war, and with the growing size of the US carrier force, superior US long range fighter planes, the vast supply of fuel for US planes, the large pool of trained US pilots, it is expected that would have been a very expensive slaughter of the less experienced Japanese pilots.
Japan had virtually no remaining navy, so absent Japanese airplanes (torpedo bombers) the US navy could sit within range of shore and shell targets – this was done.
Also, all of Japan was within range of B29 bombers, and the US was building air bases on Okinawa so the pace and intensity of bombing of targets in Japan would have continued to accelerate up to the invasion date. The army air corp was concerned about running out of targets in Japan – prior to Hiroshima they had already completely destroyed 41 Japanese cities and severely damaged 60 plus more. One assumes they would have burned Kyoto, which had become a manufacturing center. Presumably the bombing would have completely destroyed Japan’s remaining rail lines / rail hubs, bridges, harbors, etc.
Civilians would have been involved in defenses – school children were given sharpened pieces of bamboo to use as spears – you can image how that would work against GIs with semi-automatic rifles and machine guns. One can also imagine that in response to guerilla attacks, troops would burn or destroy Japanese villages and cities.
The US was discussing moving the first landing to Honshu – near Tokyo – rather than Kyushu – this would have meant dramatically less air cover for the invasion fleet, but far fewer fortifications, and the bulk of the Japanese troops and artillery were hundreds of miles south in Kyushu. Japan would have had a shortage of fuel, trucks and rail to move troops long distances. The area around Tokyo is the largest flat area in Japan – the Kanto plain – in November it would be dry or frozen and would have been one of the few places in Japan that US armor could have been used en masse.
Fortunately Japan surrendered prior to any invasion, prior to widespread famine.
– George Sawyer