People say that that invasion was more important than the atomic bombs in causing Japan to surrender. But why is that?
By 1945, wasn't shipping to/from japan basically crippled by submarines (according to wikipedia, down to about 1/4 of 1941 numbers)? So it seems like losing Manchuria/the rest of China wouldn't really reduce the amount of goods flowing to Japan by that much. Besides, if a lot of supplies were flowing from Manchuria, wouldn't it be the target of allied bombers anyways which would wreck the major ports?
Maybe Japan still wanted to hold onto an empire and the fall of Manchuria ruined those dreams? But it seems like at this point it was mainly trying to fight off an invasion and not that interested in keeping an empire any more, and in any case, it already more or less lost its pacific territories, and surely could not expect to keep China for that much longer, despite the failings of Chaing Kai Shek and whatnot.
Wikipedia says "Japan realized the Soviets were willing and able to take the cost of invasion of its Home Islands, after their rapid conquest of Manchuria and southern Sakhalin". But the Soviets didn't really do any major amphibious landings before right? So there's a good chance they'd fail miserably. And for all they knew Stalin didn't care about the Home Islands and just wanted Manchuria. And the costs weren't that high for the Soviets, they lost about 30-40K soldiers in the invasion, which seems nothing compared to the invading force of 1.6 million.
But in any case Japan would face millions of troops even without the soviets there, since apparently Allied causality estimates ran from half to four million.
So Japan must have assumed that either the non-Soviet allies were weak fighters, or they would run out of willpower after suffering lots of causalities. I think they did think the former at the beginning, but by now they would be thoroughly disabused of the notion. As for the latter, that seems like a bit of a desperate hope and if they thought that, then wouldn't they also decide that the soviets would probably fail to launch an amphibious invasion.
So basically it seems that the Soviet invasion of Manchuria would have little effect on Japanese morale, supplies, or thoughts of defending the Home Islands so there's no particular reason this would cause them to surrender. But apparently that isn't the case.
And yes the atomic bombs were kind of just a more powerful bomb and a few hundred bombers could have the same effect, but then I'm confused at why they're surrendering at all. And a few hundred atomic bombs (for all they knew, the US had that many) would surely matter.
You've asked a good question. It's one of the most-studied issues in American WWII history, and it's frequently asked here.
My favorite source for answering it is Richard Frank's Downfall, but /u/t-o-k-u-m-e-i pointed out Tsuyoshi Hasegawa's Racing the Enemy about three months ago in another thread, and I think his choice may be better.
Personally, I believe the shock of the Soviet invasion (Frank describes on p. 288 how Japanese officials reacted with shock and incredulity to the initial announcement) and the invasion's rapid success greatly contributed to the Japanese decision to surrender.
Remember, at the time, the Japanese government was considering the Soviet Union as a go-between for peace negotiations. Imagine the shock when the Soviet Union suddenly turned around and invaded. "Because the Soviets have now joined the war, I think it is necessary for us to decide to terminate the war quickly," Togo insisted in a meeting Aug. 9 at Prime Minister Suzuki's house.
You ask about the Soviet invasion plans, and that's something I do know a fair bit about. In short, from everything we know, it probably would have succeeded -- and with fairly low casualties at that.
Marshal Vasilevsky planned to assault the island from a base in Sakhalin Island. Soviet plans called for a lead division to seize a single small beachhead containing one port with a single rifle regiment. The rest of the regiment's division would follow on and expand the beachhead. Two more divisions would serve as a follow-on wave.
There would be no specialized landing craft, as the Americans used in the South Pacific. It would resemble Dieppe, not D-Day. Soldiers would come ashore in small wooden boats, carried on destroyers, minesweepers and auxiliaries, a plan that was ultimately used to seize the Kuriles.
A freighter or two might even be rammed ashore in order to provide a speedy disembarkation for that first regiment.
It would've been chaotic and slapdash, but it absolutely would have worked.
The Soviet Union estimated Japan had three divisions and a brigade to defend all of Hokkaido. In reality, Japan had two understrength divisions and one brigade, all of which were oriented toward the American threat from the east. Airpower and naval support were almost nonexistent. Hokkaido's mountainous terrain would have prevented the rapid deployment of soldiers to the beachhead, and while the Soviets surely would have been disorganized, they would have faced only militia at the shoreline -- if anything.
On August 18, however, Truman sent a terse message to Stalin, rejecting the Soviet leader's proposal that Soviet soldiers accept the surrender of northern Hokkaido. Vasilevsky's invasion preparations were complete on August 22, but Stalin abruptly ordered the operation's halt.