I've never understood why the debate is always posed as "Bomb or invade Japan" when the alternative, assuming the Bomb is not available in time (only reason they wouldn't use it) is clearly the third one of taking the war into China instead. In all of these threads, this option is only given a passing mention.
But, by mid 1945, China is where Japan still had some strength left. The Home Islands themselves were terribly isolated by US Naval and air forces; without access to foreign resources Japan can't even feed itself, let alone field any substantial industrial-based response. Taking the war to their soil is a way of giving them a chance to die honorably and taking horrendous numbers of Allied soldiers down with them, the only way the population there could possibly strike at Allies!
But in China, they still held substantial territory, still had factories with access to some resources, and that's where the bulk of the Army was stationed. These resources and men could not help Japan directly since the US forces would simply sink anything that tried to cross over, but while they held out, the diehards in Tokyo had some sort of cause to rally to.
Meanwhile, in invading China instead of Japan, the Allies--all of them, Soviet as well as Western--could count on positive help from the locals instead of resistance to the death. Substantial forces to link up with existed under both Kuomintang and Communist banners; the common people under the Japanese yoke would also be on the Allied side. If there was no A-Bomb available yet, China seems like the obvious front to deploy Western conventional forces.
Certainly it was the obvious next front for the Soviet forces! OTL Stalin was prepared to send crushing forces down to smash the Japanese in their strongholds of North China.
In fact, that I suppose is the only reason I can think of that an invasion of China before turning to the final task of subduing the Home Islands themselves was not the obvious agenda of all Allied planners. In a race between Western and Soviet forces to secure China first, the Russians had an obvious head start due to geography. For Americans, British, and Free French to seek to meet them halfway they'd have to make landings from overseas or push at insane speeds north over the highlands from Burma to Chunking to link up with Chiang Kai-Shek. Meanwhile the Soviets would be launching massed armored assaults on the Japanese in Manchuria; once these were overrun I can see the Emperor of Japan overriding his war ministers and surrendering right there, before the Western Allies can even get any forces substantially into China itself. If Stalin does not accept a surrender and keeps his forces rolling on south, and if the loose cannon Mao links up with Soviet forces (and other Communist forces were more strongly loyal to Moscow and would of course link up immediately) then before a Western force can even muster in Sichuan or on the south coasts, the Soviet/Chinese Communist forces could be mopping up what Japanese resistance they'd face from behind.
So, I'm left to guess (because on these threads, no one ever comes forth with a less speculative explanation of American thinking that did have them planning in terms of invading Japan rather than China) that the reason Americans were going for the insanely difficult task of invading a hostile Japan rather than liberating a friendly China was that the latter was doomed to Soviet domination in any case, given the geography of the situation, and that the only way Americans could claim to have been the power who defeated Japan would be to attack the Home Islands immediately, damn the cost. There, the Russians would have been in a weak position, lacking amphibious and in fact naval capacity in general; if Japan could be subdued fast enough, the Americans might be able to claim more territory in China than if they tried to secure it directly. "Claim" of course as a fraternal big brother ally of Chiang of course, not as an occupied zone or colony! Just as Stalin would no doubt be in no way occupying his fraternal socialist partner, free peasant/proletarian China! (Actually if he were to invade China before the West could resolve Japan with either bombs or a direct invasion, there's a fair chance he'd have backed Chiang and not Mao or even a more pliant Chinese Communist, and I guess under those circumstances Chiang might have gone along with being Stalin's agent rather than facing the choice of immediate death or exile anyway.)
One notes that in these brutal realpolitik considerations, "humanity" hardly enters anywhere. I'd appreciate it if someone could give some other explanation than the speculative one I have offered, why the American war planners preferred a direct invasion of Japan over continuing the war on much more favorable terms in China. But if my guess is correct, then clearly even conserving American lives took a back seat to considerations of postwar global dominance. Of course no one outside Japan had much regard for Japanese lives during this war.
Under these circumstances I suppose it's for the best that the A-bomb was in fact available.
). In the second place, it was never, never, never, just "invasion or Bomb".
