Did Truman know Hiroshima was a city?

[Neil]

Yes, but the inability of the Soviets to effectively attack it is a point against.

It'd be like people in 2002 worried about Saddam attacking some small town in Nebraska.  Sure, a Republican Guard division with tanks and air support could wreak havoc against the unprepared farmers of the Bible Belt.  But how are they going to get there, and even if they could manage to teleport their troops and war machines there, how are they going to keep them supplied?

When you're considering military history, and especially military history in the industrial age, the most important questions that you have to ask are how are your troops going to get where you want them to fight and how are you going to keep them supplied.  When it came to an invasion of the Home Islands, the Soviets didn't have an answer to those questions, at least not in 1945, and probably not even in 1946.  The most profound effect of the Soviet declaration of war wasn't military, but diplomatic.  It forced the moderate faction, who had maybe been willing to consider some sort of negotiated peace, to realize that there would be no peace negotiated through a third party as there had been in 1905.  The choice was between either unconditional surrender or the annihilation of the kokutai by devastating bombardment and invasion.

When looking at reasons for Japan's surrender however the reality doesn't really matter. As Sheilbh says at the time Soviet strength was massively over-estimated. That Soviet tanks would be rolling down the streets of Tokyo anytime soon we know with the full knowledge of hindsight just wasn't going to happen. But that's not how things looked to the Japanese leaders. This seemed a very real and terrifying threat, especially combined with the belief in simmering unrest (not unfounded) and paranoia of communism.
See also operation sea lion and how seriously it was taken by the UK....

Its besides the point, but to address the tangent, I do think a Soviet invasion of Hokkaido was possible. Many in the Soviet leadership felt it was possible (though yes, others disagreed) and it was mostly diplomatic considerations that led to it being cancelled.
They were hitting Japan in the rear, they would be going up against virtually no defenders. I'm somewhat more questionable the plan would succeed in its entirety, but that they could seize and hold the Soya peninsula is very possible.
This wouldn't be a comparable operation to Pacific island hopping. We're looking at a significantly larger island and very different terrain that would be much more amenable to the Soviets than any Japanese forces that would be redirected that way.
The analogy of Iraq invading the middle of the US doesn't work. We're talking about a major power invading an island 20km from territory they hold, at the sparsely inhabited northern fringe of a pretty linear country.

The Japanese were aware that Soviet sea power was limited.  And the Japanese knew the importance of seabourne supply to any army, let alone one that was going to be waging an aggressive war.  They also knew the difference between a land war (the kind they fought in China, and were able to keep supplied) and amphibious warfare (the kind they fought in the Pacific, and were strangled by).  The Japanese had real experience with these problems.  The Soviets did not, and had no solutions for the problems that they were going to face.  The Japanese also had enough information about Soviet forces to know that they didn't have those solutions. 

Either you haven't thought about how a Soviet invasion would go or you're operating from a standpoint of religious faith in the Red Army.  Maybe you're thinking about how they beat the Germans.  But how are the Soviets going to invade an island and occupy it when they don't have any tanks, artillery, transport, ammunition or food?  And because the Soviet sealift capacity was so small, any force they could land would be heavily outnumbered by the two Japanese divisions tasked with holding the island.  Even with the poor state of supply in Japan in 1945, they'd still be better off than the Soviets in that situation. 

Just waving around the term 'great power' doesn't actually do anything.  As strong as the Soviets were, their strength was built in a way that focused on the immediate problem, the war with Germany.  They weren't equipped yet to fight Japan, and building that capacity was no small thing.  Look at how long it took the Allies, with vastly greater capabilities, to put together the equipment and expertise needed for Torch.  The Soviets couldn't even invade the Kuriles without American naval and logistical support, which would not be available for any operation against Hokkaido. 

[Neil]

Given the war its common sense that there wouldn't be many Japanese forces in position to defend against an invasion from Japanese Karafuto prefecture.

But we know that there were two divisions in Hokkaido, which were more than adequate to deal with whatever the Soviets could land, especially once the Soviets ran out of supplies. 

[grumbler]

The Japanese, when they made the decision to surrender, actually thought that they were doing very well against the Soviets.  The Kwantung Army was holding off the Soviet Fifth Army and believed that that was the only force they were facing.  The reality that the Soviets had launched a huge right hook from the Transbaikal was unknown to them, though this force would eventually crush the Kwantung Army. 

A key factor influencing events this day [Aug 9-10] and for some time thereafter was Tokyo's ignorance of the dimensions and progress of the Soviet onslaught.  This sprang directly from the Kwantung Army's drastically erroneous initial estimate of the Soviet forces in eastern Manchuria [3 div and 3 tank brigade versus the reality of 15 divisions and 8 tank brigades].  Moreover, during August 9 the Kwantung Army and Tokyo had no inkling of the huge Soviet mechanized thrust into western Manchuria.  No wonder that on that afternoon Imperial general headquarters stated that "the scale of these attacks is not large."

(Frank, Downfall, p. 289)

We need to keep in mind what was known to the Japanese at the time of the decisions.  Later recollections would be colored by the knowledge that the Soviets were attacking Manchuria in overwhelming numbers (and quality, for that matter, given that half the Kwantung Army was made up of hastily-recalled reservists and the Soviets were deploying well-trained and equipped forces).

The records seem to show that shock of the Soviet DoW wasn't so much the military aspect (Japan had already written off Manchuria and the Kuriles/Sakhalin) but the abrupt puncturing of their fantasy that the Soviets would ride to their rescue and force some kind of compromise peace a la the Portsmouth Treaty.

[grumbler]

Given the war its common sense that there wouldn't be many Japanese forces in position to defend against an invasion from Japanese Karafuto prefecture.

But we know that there were two divisions in Hokkaido, which were more than adequate to deal with whatever the Soviets could land, especially once the Soviets ran out of supplies.

Especially when kamikaze attacks would wipe out the Soviet invasion force before it arrived.

There is a reason the Soviets deployed their amphibious forces out of range of the kamikazes.  They had no defenses against them.  Dunno why the Soviet fans don't even acknowledge this.

[Razgovory]

There's no leap or speculation at all. I've already provided a prominent quote that sums up this strain of thought on the Japanese side. From Fumimaro Konoe, who took these concerns to the emperor in early 1945:

Regrettably, I think that defeat is inevitable. What I shall say is based on this assumption. Defeat will be a blemish upon our imperial system, but public opinion in Great Britain and the United States up to now has not gone so far as change in this imperial system(of course there are extremist opinions among some, and it is difficult to gauge what sort of change may take place in the future). Thus, if it were only a matter of defeat, I think it would not be necessary to be concerned about the imperial system. More than defeat itself, what we must be most concerned about from the standpoint of preserving the imperial system is the communist revolution which may accompany defeat.

You really don't see it, do you?

[Neil]

Given the war its common sense that there wouldn't be many Japanese forces in position to defend against an invasion from Japanese Karafuto prefecture.

But we know that there were two divisions in Hokkaido, which were more than adequate to deal with whatever the Soviets could land, especially once the Soviets ran out of supplies.

Especially when kamikaze attacks would wipe out the Soviet invasion force before it arrived.

There is a reason the Soviets deployed their amphibious forces out of range of the kamikazes.  They had no defenses against them.  Dunno why the Soviet fans don't even acknowledge this.

I'm being generous and assuming that the Soviets achieve some sort of surprise that allows them to minimize their losses on their way to the beaches.  But yes, even in that best case situation, they're going to lose everything and be unable to land further troops or supply anything that they landed. 

[Sheilbh]

Incidentally on Frank, one interesting point is that in his discussion of Racing the Enemy, says it has convinced him of the central importance of Hirohito#s invervention:

Thus, Hirohito took the first indispensable step on the path to Japan's surrender: he became the legitimate authority to make the political decision that the war must end.  Racing the Enemy convinces me that Hirohito's sacred decision, not the atomic bombs or Soviet intervention, was the single most shattering blow to the leaders of the "war party."  One popular Japanese historian, Hando Kazutoshi, maintains that Soviet entry killed any hopes of the politicians for a negotiated end to the war while the atomic bombs finished the military's vision of a fight to the finish. [31]  I believe Hasegawa concurs with the first part of this formulation.  In Downfall, I concurred with the second part of Hando's formulation insofar as the senior officers in Tokyo were concerned.  I believed those senior officers recognized that with atomic bombs, the U.S. would not need to attempt to invade and if there was no invasion, they really had no strategy other than national suicide.  [32]  Racing the Enemy, however, convinces me that the emperor's intervention takes primacy even above the atomic bombs in collapsing the will of the militarists in Tokyo.

I actually quite like that formulation by Kazutoshi - but if Hasegawa is correct as Frank now thinks it focuses attention less on the other parties in the Japanese government and more on what was the motivation for Hirohito's intervention. Of course I'm not sure if that's correct.

[grumbler]

Incidentally on Frank, one interesting point is that in his discussion of Racing the Enemy, says it has convinced him of the central importance of Hirohito#s invervention:

Thus, Hirohito took the first indispensable step on the path to Japan's surrender: he became the legitimate authority to make the political decision that the war must end.  Racing the Enemy convinces me that Hirohito's sacred decision, not the atomic bombs or Soviet intervention, was the single most shattering blow to the leaders of the "war party."  One popular Japanese historian, Hando Kazutoshi, maintains that Soviet entry killed any hopes of the politicians for a negotiated end to the war while the atomic bombs finished the military's vision of a fight to the finish. [31]  I believe Hasegawa concurs with the first part of this formulation.  In Downfall, I concurred with the second part of Hando's formulation insofar as the senior officers in Tokyo were concerned.  I believed those senior officers recognized that with atomic bombs, the U.S. would not need to attempt to invade and if there was no invasion, they really had no strategy other than national suicide.  [32]  Racing the Enemy, however, convinces me that the emperor's intervention takes primacy even above the atomic bombs in collapsing the will of the militarists in Tokyo.

I actually quite like that formulation by Kazutoshi - but if Hasegawa is correct as Frank now thinks it focuses attention less on the other parties in the Japanese government and more on what was the motivation for Hirohito's intervention. Of course I'm not sure if that's correct.

I got that same impression from Frank's Downfall:  that the emperor's personal intervention was what made the die-harders accede to surrender.  When the Emperor said that the US response to the August 10th Japanese request for conditions satisfied him, there was no way forward for the war party. 

Obviously, my point all along is that it is clear that, to the Emperor, the a-bombs were the key, since they made a US landing unnecessary and a Japanese face-saving battlefield victory impossible.  Kido and Togo are pretty clear on this, as is the actual message the Emperor sent out to the nation.

Soviet entry into the war was indeed instrumental in convincing the peace party to abandon conditions for peace.