your opinion on operation downfall ? info inside

[Opanashc]

Soviet Union possible could have invaded Hokkaido, but it would have been a small operation. SU did not have the fleet to do an invasion in any kind of force, while most of IJA was grouped against the US invasion from the south. Hard to tell, how it would have turned out.

 

[Direlda]

I did not mean to state or imply that the White Papers were authored in 1950. The WP were a much later production. What I intended was my memory (however shaky and potentially errored) recalled mention of a high level staff conference held somewhere in the '50s to compare and contrast what was believed to be known/true by intel during the invasion planning and later gathered data post war end.

Edited to add: Forgot to say very well done on the info gathering.
Semper Fi.

Ah, I guess I did misread the bit about the dates.

I did find another article that could be relevant. Downfall: The Invasion that Never Was is from the US Army War College Quarterly Parameters. And Chapter 5: Demobilization and Disarmament of the Japanese Armed Forces of Reports of General MacArthur: MacArthur in Japan: The Occupation: Military Phase Volume 1 Supplement might also have relevant information as it lists Japanese troop dispositions. Still having trouble finding information about any high level staff conference. Any suggestions on keywords I should use?

I'm just putting the Master's in Library and Information Science that I'm currently working on to use. Research is fun, after all!

 

[Director]

I believe the Allies could likely have made a success of the Kyushu landings, though the post-War release of accurate information as to what the Japanese were pouring in there certainly throws some doubt on the conclusion. The second invasion, across Tokyo Bay and the Edo plain... if it were done and failed, the Allies would bomb and blockade Japan past the Stone Age into complete collapse. In that situation, even extreme measures - the ones we can't talk about here - might have been used. If it succeeded, a huge chunk of the Japanese population would be gone, the Allied armies would be filled with shell-shocked mass-murdering psychopaths and the prospects for Japan ever being allowed to industrialize or arm again would be small.

Part of what made the American island-hopping strategy successful was that Japan could only approximately calculate where the next blow would fall and why one island would be invaded and others bypassed. But as the US advanced across the Pacific the paths began to converge and there were fewer good choices, and so it became easier to decide where to fortify. It was certain that the Philippines would be a target, but not which island; then a certainty that either Formosa or Okinawa would be hit. And after that - it had to be Kyushu and then Hokkaido, and given the size of the operation, pretty much all strategic surprise as to place and time would be lost. Amphibious operations are the chanciest, most difficult operations of war and charging a well-defended beach has to be the hardest kind of amphibious assault. Even massive firepower and airpower can only take you so far against camouflage, deeply-dug fortifications and tunnels, and fanatical resistance.

I just do not know whether the Allied armies and navies could stand up to the waves of suicide attacks. The incredible ordeal of the USS Laffey makes for instructive reading.

So... yes, indeed, "Thank God for the atom bomb". It is sad to say that a-bombing two cities probably saved more Japanese lives than were lost, and saved the lives of hundreds of thousands of Allied servicemen as well. I think the men who planned Downfall/Olympic/Coronet/whatever did not fully understand what had happened on Okinawa and were too caught up in the 'it has to be done, therefore we can do it' mentality.


My father was one of the US Army soldiers scheduled to go as part of the invasions. He described intelligence briefings where they were shown aerial photographs and detailed intelligence on the Japanese defenses. We now know that a lot was hidden from Allied view. The worst shock might have come from believing you knew the worst and finding out how very wrong you were.

 

[Amur_Tiger]

I'm not an expert on Downfall and the like, but I do recall hearing/reading somewhere (as well as from posters on this forum) that the Soviet invasion of Manchuria had a significant impact on Japan's move to surrender. In-game, if it was just the a-bomb, without the Soviet invasion, Japan could perhaps be a little less willing to surrender and require a degree of invading?

This is my impression as well, I've heard some reports to the effect that the Japanese weren't even entirely aware of what they faced with the A-bomb at the time of surrender, the scale of destruction wasn't out of line with that which they'd already faced during the fire-bombing campaign that preceded it and once a disaster exceeds a certain scale it starts to become hard to get a picture of what happened quickly due to the disaster knocking out communications.

By comparison the Soviet invasion was both long-feared and a new threat and when the invasion of Manchuria kicked off the Soviets proved themselves worthy of the fear the Japanese had of their army, given this knocked out the last of their Imperial dreams, an in particular had the Army joining the Navy in humiliating defeat this wore down much of the remaining resistance to surrender.

 

[Porkman]

One of the other things that doomed the Japanese effort in the Pacific that wouldn't apply for the invasion of the Home Islands is the ability to mass force effectively.

THe Allies invaded the Pacific across three Axes (not counting China). The South Pacific, Central Pacific, and then later Burma. These offensives were three spearheads that spent most of the war not being close enough to eachother to be mutual supporting.

The Japanese should have theoretically been able to use their interior lines to mass forces, gain local superiority at given spearhead, and then defeat them in detail.

They weren't able to do that because the US kept sinking all of the shipping. Moving armies in transports meant that a division could arrive at its destination missing several batallions, all of its artillery, most of its food... etc. As a result, most of the soldiers that the Allies fought in 1944 were underfed, ill equipped and ill.

On the Home Islands, the Japanese would be able to use interior lines and mass in detail against the US. They would have been adequately fed and have enough ammunition. It would have been a different war.

 

[Pugmak]

Ah, I guess I did misread the bit about the dates.

I did find another article that could be relevant. Downfall: The Invasion that Never Was is from the US Army War College Quarterly Parameters. And Chapter 5: Demobilization and Disarmament of the Japanese Armed Forces of Reports of General MacArthur: MacArthur in Japan: The Occupation: Military Phase Volume 1 Supplement might also have relevant information as it lists Japanese troop dispositions. Still having trouble finding information about any high level staff conference. Any suggestions on keywords I should use?

I'm just putting the Master's in Library and Information Science that I'm currently working on to use. Research is fun, after all!

The stuff I was "referencing" was from a decade or so ago. I was just goofing around on the CIA.gov site and stumbled into it. Honestly, I have no clue how to go about finding it again. I've searched through the CIA.gov site since but haven't found the stuff again.

 

[Director]

The Allies pressured the Soviets to invade Japanese holdings partly as a counter to Soviet demands for a second front against Germany.

Once it was apparent that the Soviets were really going to go into Manchuria, Sakhalin and maybe the Japanese Home Islands, the Allies were, um, very unhappy, but couldn't complain because, after all, they had been asking for it. That's when the decision to use the A-bomb as soon as possible was set in stone, and that's when all reservations about Downfall/Olympic/Coronet had to be put aside. It was pretty certain that the Soviets were going to be the first into Berlin; having them take credit for victory in the Pacific would have been just too much.

Had Japan been invaded unsuccessfully by the Allies on Kyushu and/or the Tokyo plain, and had the Soviets been able to get across to, say, Hokkaido, then a post-war divided Japan - like a divided Germany - becomes much more plausible.

I'd agree that the a-bomb was just one more brick-in-the-wall added to the firebombing campaign - what's the loss of comparatively-small Hiroshima and Nagasaki weighed against the virtual elimination of Tokyo, Osaka and the industrial centers? - but I do think I remember the Emperor being told this new weapon was something bigger and worse than the incendiary raids. Could be wrong about that. The surrenders of Germany and Italy must also have had an impact, and I do know that Japan had been floating the idea of a peace treaty through (I think) the Swiss and the Swedes for some time, though with some stipulations. At any rate, the devastation reached a tipping point where the civilian leadership (IE the Emperor) realized that further losses of lives would be large, inevitable and would not avert or even postpone Japan's defeat. After the firebombing raids, there must have been concerns that the futures of the monarchy and of Japanese independence - even civilization - were at risk.

 

[Opanashc]

Part of the reasoning behind Japanese surrender was the fact of SU declaring war. Before then, Japanese leadership could cling to the hope of "Comintern vs Allies" fallout.

 

[hkrommel]

Part of the reasoning behind Japanese surrender was the fact of SU declaring war. Before then, Japanese leadership could cling to the hope of "Comintern vs Allies" fallout.

The biggest blow to Japanese morale was the shattering of their notion of their homeland being invincible. They would not have surrendered if it weren't for Hiroshima and Nagasaki destroying that attitude. The bombings of Hiroshima and Nagasaki severely damaged their military capabilities (Nagasaki being one of their last and largest military ports as well as a major manufacturing center, particularly for ordnance (about 90% of the city's workforce was employed by Mitsubishi for its various wartime manufacturing facilities), and Hiroshima being a major logistics, communications, transportation and command center for all forces in the southern part of Honshu), but they were also warning shots. What if the US decided to nuke Tokyo, or Kyoto next? That was unacceptable.

Even with Manchuria taken and the IJA decimated, they wanted to hold out on the Home Islands, because that's what mattered to them far more than Manchuria or Korea. Plus they had plenty of soldiers on the Home Islands, losing their army in Manchuria was hardly the death blow some make it out to be. The only way to get them to surrender was either Downfall or dropping nukes. Thankfully (for many different reasons) Truman opted for the latter.

 

[Porkman]

The biggest blow to Japanese morale was the shattering of their notion of their homeland being invincible. They would not have surrendered if it weren't for Hiroshima and Nagasaki destroying that attitude. The bombings of Hiroshima and Nagasaki severely damaged their military capabilities (Nagasaki being one of their last military ports and Hiroshima being a major logistics and command center for all forces in the southern part of Honshu), but they were also warning shots. What if the US decided to nuke Tokyo, or Kyoto next? That was unacceptable.

Even with Manchuria taken and the IJA decimated, they wanted to hold out on the Home Islands, because that's what mattered to them far more than Manchuria or Korea. Plus they had plenty of soldiers on the Home Islands, losing their army in Manchuria was hardly the death blow some make it out to be. The only way to get them to surrender was either Downfall or dropping nukes. Thankfully (for many different reasons) Truman opted for the latter.

The conventional bombing of Japan had already shattered any notion of Japanese Invincibility. The Tokyo firebombing killed more people than either atomic bomb. They were used to death from the skies in 1945.

 

[hkrommel]

The conventional bombing of Japan had already shattered any notion of Japanese Invincibility. The Tokyo firebombing killed more people than either atomic bomb. They were used to death from the skies in 1945.

Not to High Command. There is an undeniable psychological disconnect between thousands of conventional bombs and hundreds of bombers, versus one bomber, one bomb, and an entire city decimated.

 

[Vonboe]

Normally I stay quiet when I see these kind of threads pop up on the forum, but I must say my stomic turns when I see people say stuff like

Thank God for the atom bomb

with no intervention from mods. I don't really care for people's "reasoning" for making these statments, just like I don't wanna hear people make excuses for concentrations camps, rape of nanking, chemical weapons etc. Only wish you people knew how much impact those horrific bombs have made on the japanese people, culture and mindset. So don't give me your half-minded "but what if" crap.

 

[Porkman]

Not to High Command. There is an undeniable psychological disconnect between thousands of conventional bombs and hundreds of bombers, versus one bomber, one bomb, and an entire city decimated.

They didn't know that. They didn't get a detailed after action report that told them that there was only one plane and one bomb.

It took them 2 days to get qualified people on the ground to actually investigate Hiroshima.

When Truman told them 16 hours later that it was a single bomb, they were pretty sure it was a trick.

The atomic scientists had to confirm it 2 days later.

Even then they didn't contemplate unconditional surrender until Soviet intervention on the 9th. Then they got bombed again on the 9th .

Coincidentally, the Soviet Union invaded Manchuria on the 9th.

 

[Amur_Tiger]

Not to High Command. There is an undeniable psychological disconnect between thousands of conventional bombs and hundreds of bombers, versus one bomber, one bomb, and an entire city decimated.

In practical terms what's the difference?

As I mentioned earlier while we understand and recognize a nuclear weapon for what it is fairly easily given the city-devastating raids that were already happening, the Japanese would have had a hell of a time actually recognizing the nuclear bomb for what it was. Confirming that it was really just one bomber would have been impossible over the time frame given, the vast majority of witnesses would be dead and the rest too traumatized by the experience to provide a reliable account. By comparison the implications of the Soviet invasion of Manchuria and Sakhalin and the complete capitulation of the Japanese forces they faced were clear, negotiating through the Soviets wasn't going to work, instead the Soviets were now poised to land on Hokkaido within a week or two. This had 'unmitigated disaster' written all over it where the nuclear bomb hits would have been more or less a continuation of the bombing already done. At the time, atomic weapons had much more of an effect on the thinking of fellow allied powers then anyone else and one could argue that that was the point.

Japanese doubts about the nature of the bombing of Hiroshima

Spoiler

The Japanese Army and Navy had their own independent atomic-bomb programs and therefore the Japanese understood enough to know how very difficult building it would be. Therefore, many Japanese and in particular the military members of the government refused to believe the United States had built an atomic bomb, and the Japanese military ordered their own independent tests to determine the cause of Hiroshima's destruction.[78] Admiral Soemu Toyoda, the Chief of the Naval General Staff, argued that even if the United States had made one, they could not have many more.

Reaction to the 'twin shocks' of Manchuria

Spoiler

When the Russians invaded Manchuria, they sliced through what had once been an elite army and many Russian units only stopped when they ran out of gas. The Soviet 16th Army — 100,000 strong — launched an invasion of the southern half of Sakhalin Island. Their orders were to mop up Japanese resistance there, and then — within 10 to 14 days — be prepared to invade Hokkaido, the northernmost of Japan’s home islands. The Japanese force tasked with defending Hokkaido, the 5th Area Army, was under strength at two divisions and two brigades, and was in fortified positions on the east side of the island. The Soviet plan of attack called for an invasion of Hokkaido from the west. The Soviet declaration of war also changed the calculation of how much time was left for maneuver. Japanese intelligence was predicting that U.S. forces might not invade for months. Soviet forces, on the other hand, could be in Japan proper in as little as 10 days. The Soviet invasion made a decision on ending the war extremely time sensitive.[83]

These "twin shocks"—the atomic bombing of Hiroshima and the Soviet entry—had immediate profound effects on Prime Minister Suzuki and Foreign Minister Tōgō Shigenori, who concurred that the government must end the war at once.[84] However, the senior leadership of the Japanese Army took the news in stride, grossly underestimating the scale of the attack. With the support of Minister of War Anami, they started preparing to impose martial law on the nation, to stop anyone attempting to make peace.[85] Hirohito told Kido to "quickly control the situation" because "the Soviet Union has declared war and today began hostilities against us."[86]

The Supreme Council met at 10:30. Suzuki, who had just come from a meeting with the Emperor, said it was impossible to continue the war. Tōgō Shigenori said that they could accept the terms of the Potsdam Declaration, but they needed a guarantee of the Emperor's position. Navy Minister Yonai said that they had to make some diplomatic proposal—they could no longer afford to wait for better circumstances.

News of the Nagasaki bombing arrived later that day.

To be clear I don't think that the Soviets were critical in defeating Japan, they simply provided some last minute time pressure on the Japanese to surrender now to the Americans before they find themselves cut up like Germany had been.

 

[hkrommel]

Even then they didn't contemplate unconditional surrender until Soviet intervention on the 9th.

You're telling me they didn't even discuss the possibility until then? I didn't know sources were so exhaustive.

Coincidentally, the Soviet Union invaded Manchuria on the 9th.

Of course, they tried to grab as much territory as possible.

Regardless of how long it took them to confirm (which really has no bearing on my point since it was confirmed before the surrender), it would have taken an invasion or nukes to make them surrender. They weren't surrendering from firebombings, unfortunately for everyone.

Normally I stay quiet when I see these kind of threads pop up on the forum, but I must say my stomic turns when I see people say stuff like with no intervention from mods. I don't really care for people's "reasoning" for making these statments, just like I don't wanna hear people make excuses for concentrations camps, rape of nanking, chemical weapons etc. Only wish you people knew how much impact those horrific bombs have made on the japanese people, culture and mindset. So don't give me your half-minded "but what if" crap.

Look, nukes are totally within forum rules to discuss, unlike the other things you seem to be mentioning to try and get this thread closed.

Operation Downfall was not some pipe dream or something that was never going to happen like Operation Unthinkable. It was planned for, soldiers were slotted for invasion. They even made enough Purple Heart medals (the medal the US awards for being wounded or killed in action) that to this day the US hasn't manufactured any more. We still had 120,000 in stock in 2003. That's how high the casualty estimates went.

What we are discussing here are the possibilities the Allies were faced with in 1945. Nukes were one, invasion was another. The Japanese showed no signs of surrender previous to this. Both actions were justified, and horrible at the same time. Either one was necessary to end the war. Downfall would have resulted in far greater loss of life and devastation, and the effects would have been much further reaching. What OP is asking for are our thoughts on the matter, and how the operation would have gone. That is completely within the bounds of discussion here, as are the alternatives to the operation as per the forum rules.

I'm sorry the Japanese got nuked. WWII sucked for everyone involved. However, not discussing the war and the decisions that were made simply because they were hard decisions, decisions that resulted in destruction, is how we lose our history as a human race, and how we cease to understand how the world works.

To be clear I don't think that the Soviets were critical in defeating Japan, they simply provided some last minute time pressure on the Japanese to surrender now to the Americans before they find themselves cut up like Germany had been.

Sure, they provided pressure, but I've heard people (Soviet stronk-types) argue that the invasion of Manchuria was the reason for Japanese surrender. I was mostly trying to preempt that nonsense. Also, they did confirm the nuclear attack and that the Americans were telling the truth (that it was one bomb) a couple days after Hiroshima IIRC.

 

[Vonboe]

You're telling me they didn't even discuss the possibility until then? I didn't know sources were so exhaustive.



Of course, they tried to grab as much territory as possible.

Regardless of how long it took them to confirm (which really has no bearing on my point since it was confirmed before the surrender), it would have taken an invasion or nukes to make them surrender. They weren't surrendering from firebombings, unfortunately for everyone.



Look, nukes are totally within forum rules to discuss, unlike the other things you seem to be mentioning to try and get this thread closed.

Operation Downfall was not some pipe dream or something that was never going to happen like Operation Unthinkable. It was planned for, soldiers were slotted for invasion. They even made enough Purple Heart medals (the medal the US awards for being wounded or killed in action) that to this day the US hasn't manufactured any more. We still had 120,000 in stock in 2003. That's how high the casualty estimates went.

What we are discussing here are the possibilities the Allies were faced with in 1945. Nukes were one, invasion was another. The Japanese showed no signs of surrender previous to this. Both actions were justified, and horrible at the same time. Either one was necessary to end the war. Downfall would have resulted in far greater loss of life and devastation, and the effects would have been much further reaching. What OP is asking for are our thoughts on the matter, and how the operation would have gone. That is completely within the bounds of discussion here, as are the alternatives to the operation as per the forum rules.

I'm sorry the Japanese got nuked. WWII sucked for everyone involved. However, not discussing the war and the decisions that were made simply because they were hard decisions, decisions that resulted in destruction, is how we lose our history as a human race, and how we cease to understand how the world works.



Sure, they provided pressure, but I've heard people argue that the invasion of Manchuria was the reason for Japanese surrender. I was mostly trying to preempt that nonsense. Also, they did confirm the nuclear attack and that the Americans were telling the truth (that it was one bomb) a couple days after Hiroshima IIRC.

You can talk about nukes all you want, but I'm pretty sure glorifying civilian mass-murder is againts the forum rules.
Also I haven't reported this thread yet, but please try and show some respect.

 

[Opanashc]

Sure, they provided pressure, but I've heard people (Soviet stronk-types) argue that the invasion of Manchuria was the reason for Japanese surrender.

It was not THE reason, it was the straw that broke the camel's back.
1) Now SU and Allies were fighting the same enemy again, thus any possibility of them fighting each other instead was reduced. Remember, that Nazis clung to the same hope - USSR and Allies will have a falling out.
2) If SU did invade the islands, there was darn-all Japan had in place to stop them.
3) By Japanese reasoning (quite accurate), if SU occupies the islands - Emperor is gone. Snowball chance in Hell they let Hirohito stay. Hence, better to surrender to the Allies, who could be willing to keep the Emperor in place.
Once again - Soviet invasion was not the sole reason for Japanese surrender - it was the last straw on the camel's back.

Of course, they tried to grab as much territory as possible.

And the Allies were not? Pfft.
SU agreed, that it will attack Japan no later than 3 month after the end of the war in Europe. Germany surrendered on 9 May, Moscow time, SU attacker Japan August 8, Moscow time. 3 month, to the day.

 

[TheDerpyBeagle]

While it's true that the atomic bombs did limit casualties, there was still nothing moral about them. The main US reason for choosing those cities was that they were not heavily damaged by US bombing like Tokyo was, and the US wanted to test the full effects of the bombs against an urban environment. But I agree with Vonboe, being thankful for the most destructive weapons ever conceived by humans to this day should not be condoned.

 

[Amur_Tiger]

While it's true that the atomic bombs did limit casualties, there was still nothing moral about them. The main US reason for choosing those cities was that they were not heavily damaged by US bombing like Tokyo was, and the US wanted to test the full effects of the bombs against an urban environment. But I agree with Vonboe, being thankful for the most destructive weapons ever conceived by humans to this day should not be condoned.

I think we should be careful about justifying the dropping of the atomic bombs as limiting casualties as that's only really true within a fairly narrow set of circumstances. Namely it's only true for the options available to the US & UK to push the war to a close at that particular moment in time. The western allies knew at that point that the Soviets were coming and could help add further pressure on the Japanese, they could have waited but instead all evidence is that they were in a rush to conclude things before the Soviets involved themselves.

Sure, they provided pressure, but I've heard people (Soviet stronk-types) argue that the invasion of Manchuria was the reason for Japanese surrender. I was mostly trying to preempt that nonsense. Also, they did confirm the nuclear attack and that the Americans were telling the truth (that it was one bomb) a couple days after Hiroshima IIRC.

Well just like the idea that 'the atomic bombs did limit casualties' the idea that 'the invasion of Manchuria was the reason for Japanese surrender.' is true, but from a very narrow set of circumstances, it's true in so much as it was the last major negative development for the Japanese that pushed them from 'hold out for better terms' to 'peace now before we're half Soviet' . This is of course the problem with this entire conversation as the opposing views are both true to an extent but only in their own circumstance and each side sees the other view as either a general policy of excusing civilian casualties or a general view that the Soviets played a major role in defeating Japan generalizing either of these statements into a broader context makes them quickly wrong.

 

[hkrommel]

glorifying civilian mass-murder is againts the forum rules.

I agree, and from what I've seen nobody is glorifying that. They're referencing it as the preferable alternative (for all parties) to Operation Downfall.

The main US reason for choosing those cities was that they were not heavily damaged by US bombing like Tokyo was, and the US wanted to test the full effects of the bombs against an urban environment.

They were also chosen as military targets, and the main reason that Nagasaki was not heavily damaged was because its geography made it difficult to locate via radar at night. Minor points, but the "main reason" was not simply to test its effects. I wouldn't go so far as to say there was any "main reason" other than forcing a Japanese surrender.

While it's true that the atomic bombs did limit casualties, there was still nothing moral about them

Limiting casualties is about the understatement of the century. We're talking a death toll that would make Stalin take notice.

Also, perhaps decisions in war aren't made with morality primarily in question? First, you have to establish what morality you're working off of, and what the basis of that morality is. Second, you have to understand that in most wartime situations, the "moral" decision (again, assuming there is an established, objective, universal morality) is simply not available. Nobody is saying that Hiroshima and Nagasaki were "good" by most standards of morality (though it was by the Utilitarian standard), but it's rather a question of necessity and the "best" way to end the war.

And the Allies were not? Pfft.
SU agreed, that it will attack Japan no later than 3 month after the end of the war in Europe. Germany surrendered on 9 May, Moscow time, SU attacker Japan August 8, Moscow time. 3 month, to the day.

Since when did I say the Allies weren't doing that? This is the problem with Soviet stronk people.

It was not THE reason, it was the straw that broke the camel's back.
1) Now SU and Allies were fighting the same enemy again, thus any possibility of them fighting each other instead was reduced. Remember, that Nazis clung to the same hope - USSR and Allies will have a falling out.
2) If SU did invade the islands, there was darn-all Japan had in place to stop them.
3) By Japanese reasoning (quite accurate), if SU occupies the islands - Emperor is gone. Snowball chance in Hell they let Hirohito stay. Hence, better to surrender to the Allies, who could be willing to keep the Emperor in place.
Once again - Soviet invasion was not the sole reason for Japanese surrender - it was the last straw on the camel's back.

1. Never going to happen. Even the Japanese knew that.
2. Assuming they had the amphibious capacity to do so, and assuming they even successfully took Hokkaido, they weren't going to be able to cross to Honshu successfully. The Soviets simply did not have the equipment and experience the Allies had in amphibious warfare, and Downfall was going to be incredibly difficult even for them.
3. Sure, assuming the Soviets can occupy the islands, and even the most generous estimates in that regard only give them Hokkaido. The only Soviet amphibious operation (excluding ones in the Black Sea) was the invasion of the Kuril Islands against minimal resistance, which then mostly surrendered as a part of Japan's general surrender.

In August 1945 the Soviet Pacific Fleet consisted of 2 light cruisers, 11 destroyers, 2 torpedo boats, and a collection of submarines, minelayers, minesweepers, and patrol boats. Though the Japanese forces were in shambles (by July only 1 capital ship hadn't been sunk by the United States), the Soviets could mount a limited amphibious operation at best, with little to no support from the Western Allies (they didn't want the Soviets pulling a Germany with Japan). The Soviets knew this, which is why they opted for the Kuril Islands operation after quickly scrapping the Hokkaido invasion plan.

Also, considering that Nagasaki happened after the Soviet invasion, wouldn't that be the last straw? Essentially Japanese High Command was faced with two options: risk the Americans dropping more atom bombs, perhaps this time on Kyoto, or face an invasion the likes of which the world had never seen. The Soviets could perhaps hold some of Hokkaido, but their force composition was not ready for a full-scale amphibious invasion of a landmass that size, particularly since the beaches they would be landing on and the ports they could maybe capture (on the northern part of the island) would limit the amount of troops they can land and supply. The Japanese also had about 4 full strength divisions on the island IIRC, and the US had laid quite a few mines around the island that the Soviets would have to find and sweep, since they were not aware of their location, and perhaps their existence. That would be a rude awakening for the landing force. They weren't going to land the killing blow unless they were given time to build up their open sea amphibious and overseas supply capabilities. The Allies on the other hand, were readying the Marines and the veterans of Overlord.

I think we should be careful about justifying the dropping of the atomic bombs as limiting casualties as that's only really true within a fairly narrow set of circumstances. Namely it's only true for the options available to the US & UK to push the war to a close at that particular moment in time. The western allies knew at that point that the Soviets were coming and could help add further pressure on the Japanese, they could have waited but instead all evidence is that they were in a rush to conclude things before the Soviets involved themselves.

I think Allied leaders correctly saw that the war needed to end, and end soon. The Cold War was on the horizon, and the US had to focus its resources on building up its allies against the Soviets, who now controlled Eastern Europe. It was of the utmost importance that the war end quickly. Besides, I don't think it was any more "moral" to continue to blockade and bomb them until they finally surrendered, with devastating results, do you?

it was the last major negative development for the Japanese that pushed them from 'hold out for better terms' to 'peace now before we're half Soviet' .

Nagasaki happened after the invasion started, so there's that. It was after that when Hirohito instructed the Supreme Council to accept the Allied terms of surrender.

Did the Soviets help? Yes. Was the help that significant? Considering that they didn't really have the capability to pose a credible threat to the entirety of Hokkaido (even if they did land enough men to gain a beachhead, the Japanese aren't going to ignore that, and they would have very limited supply capacity due to the quality of ports and infrastructure on the north side of the island), let alone Honshu, I don't think so. I don't think it pushed the Japanese over the edge. It certainly troubled them but I don't think there was any danger of the Soviets actually invading (especially since it would break the Potsdam Agreements and Truman specifically told Stalin to stay out of Japan).