Particularly taking into consideration the typhoon that hit the marshaling area on the scheduled invasion date.
Was the USSR actually capable of conquering Japan at the end of WWII?
- Thread starter WhatIsAUserName
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Particularly taking into consideration the typhoon that hit the marshaling area on the scheduled invasion date.
Bill Cameron
Banned
Amerigo,
Given the concerns being raised about Downfall by several members of the JCS, the date of the originally scheduled invasion means nothing.
Downfall was either going to be canceled or changed out of recognition. Only MacArthur supported the plan as it stood.
Bill
This is a subject near and dear to my heart, stemming from my interest in theCoronet landings and the attempted coup against the Emperor just before he announced Japan's surrender over the radio. Here's an answer to your questions, based on the information I've gathered:
On June 22, 1941, the Soviet Navy could count 4 battleships, 10 cruisers, 59 destroyers and 218 submarines on its rolls, not a large total by any imagination. In numbers alone, it was a navy on par with Sweden or Brazil. To make matters worse, the navy was divided into four main fleets: Baltic, Black Sea, Northern (Arctic), and Pacific. The Baltic Sea fleet naturally suffered heavily during the war against Germany, and was wholly destroyed in various actions. The Black Sea Fleet also suffered, but owing to an evacuation from the peacetime homeport of Sevastopol to Batumi, was not wholly destroyed. The surviving elements of the Black Sea Fleet, bolstered by landing craft supplied by Lend-Lease, would participate in several regimental-scale amphibious landings during the Soviet advance across the Ukraine and into Romania. The Northern Fleet was the most active Soviet naval force during the war, participating in convoy escorts throughout the duration of hostilities with Germany. Several destroyers were transferred from the Pacific Fleet to the Northern in order to assist in these efforts.
Following the surrender of Germany and the Potsdam Conference, the flow of men and ships from the Pacific to Europe stopped and reversed. The Soviet Union pledged its support for the American war in the Pacific, and would indeed enter the war on August 8, 1945, a few short months following the surrender of Germany, and less than a month before OTL's surrender of Japan.
At the outbreak of war in 1941, the Soviet Pacific Fleet included 2 cruisers, 1 battleship, 12 destroyers, 19 coastal corvettes, 10 mine layers, 52 mine sweepers, 49 submarine hunters (destroyer escorts), 204 torpedo boats, 78 submarines and 1,618 aircraft. By August 1945, this force had been bolstered by Lend-Lease shipments, transfers from the Northern Fleet, and new construction. During the war, Soviet shipbuilding facilities were moribund, and what few ships were built were no larger than destroyers, and very few of those were built. The Soviet Union was, however, the second-largest beneficiary of American naval Lend-Lease. Over 100 American freighters, landing craft, and military vessels (including the cruiser USS Milwaukee), were given to the Soviet Union during the war.
Many of these ships came via a unique Lend-Lease supply line whereby freighters would be loaded with supplies in American ports, given Soviet flags, and sailed directly to Vladivostok, straight through Japanese waters, thus taking advantage of the Soviet cease-fire with Japan. Though this route did not match the importance of the Atlantic convoy routes, it remained the second-largest supply line between the United States and the Soviet Union right up until August 8, 1945. Dozens of ships came across on this route and were given to the Soviet Navy, particularly landing craft, destroyers, and freighters.
These were the ships that OTL's invasions of the Kurile Islands, Sakhalin, and northern Korea would rely upon. On August 18, 1945, the Soviet Union executed three separate amphibious invasions against Japanese forces at these locations and was successful in all three. The Korean invasion, not a land advance through Manchuria (Soviet and Japanese forces in Manchuria had been halted by an Emperor-ordered cease-fire), was what created OTL's North Korea, and that invasion rode on American lend-lease ships.
So where was the Japanese Combined Fleet? In short, sunk. By 1945, the situation had gotten so bad for Japan that American submarines regularly penetrated the Sea of Japan to go hunting. American carriers cruised within sight of the coast, launching strikes with impunity at any target worth hitting. Even American battleships got into the fight, physically shelling the Japanese Home Islands from positions off shore. By August 1945, neither the Japanese Combined Fleet nor its air forces were in any position to resist. What few ships and aircraft still survived were trapped in port by roving American task forces, and in any event lacked the fuel to move more than a few hundred miles. Every surviving surface vessel was thus held in reserve for an enormous suicide run against the American invasion fleet, and virtually the entire air arm of Japan faced a similar situation. To better ensure their ultimate success, they were moved as far south as possible in order to shorten the distance between their bases and the invasion site. This had the effect of leaving Hokkaido uncovered, but it didn't matter -- the blow would fall in the south, or so the Japanese thought.
That thought evaporated with Stalin's declaration of war on August 8. In Manchuria, the Soviet Union massed 1,577,225 men, 26,137 cannon, 1,852 motorized artillery, 3,704 tanks, and 5,368 aircraft. Facing these forces was the vaunted Japanese Kwangtung Army, which had survived most of the war with its reputation intact. By the time of the Soviet invasion, though it could muster over 1 million men, the total could only be reached by scraping together every garrison in Manchuria, Korea, and the northern portions of occupied China. The cream of the crop had long ago been deployed to fight American forces in the Pacific or recalled to defend the Home Islands, but the increasing American submarine and mining campaign had put an end to troop movements across the Yellow Sea and even between Home Islands.
Still, at the time of the invasion, Japan could field 1,040,000 men, 6,700 cannon, ~1,000 tanks, 1,800 aircraft, and 1,215 other vehicles. On paper, this sounds like a formidable force. The reality was far less positive for Japan. As I mentioned earlier, the soldiers in Manchuria at the time of Operation August Storm were mostly garrison troops left behind by redeployments across the Pacific. In addition, they had been bloodied by a decade of unending war against China, and thanks to the American submarine and mining campaign, supplies from Japan had dwindled to a trickle. On August 8, most soldiers were living off the land, reliant on Chinese farmers for their daily survival. The 1,000 tanks and 1,800 aircraft of the Kwangtung Army on August 8 were obsolescent and obsolete models unfit for action in the Pacific, let alone against the battle-hardened Red Army. Most of the Japanese tanks were Type 97 Chi-Ha models, which mounted a 57mm cannon and carried 33mm of armor. In comparison, the T-34/85 that made up most of the Red Army's armor strength in August Storm carried an 85mm cannon and up to 90mm of armor, making it virtually invulnerable to anything but a rear hit by the Chi-Ha. To make matters worse, the state of the art for the Red Army was the IS-3 (Iosef Stalin), which carried a 122mm cannon and 200mm of armor. A regiment of these tanks was deployed during August Storm.
In the air, the Japanese total of 1,800 aircraft belies the truth of the matter -- only 50 were front-line fighters similar to the Zero. The remaining 1,750 were trainers, transports, or other obsolete models deemed unfit to face the Americans in the Pacific. In OTL, they were easy meat for an air force that had faced down the Luftwaffe.
(To be continued)
Indeed. I was merely adding additional oomph to the idea that TTL's Olympic/Coronet would bear little resemblance to OTL's plans. A typhoon tends to change the best-laid plans of mice and men.
Possibly, but the American public was also impatient to end the war in 1946. Canceling Downfall would mean a long blockade with American servicemen unable to demobilize and a very unhappy domestic situation.
However that is all moot. If the blockade option was taken instead, a Soviet invasion of Hokkaido becomes a virtual certainty. They could simply wait until Japan is ready to give up and land paratroopers to accept surrender. A delayed surrender would also give the Soviets time to take Korea. Furthermore, if the Soviets are credited with destroying the bulk of the Japanese army on the Asian mainland, it would be impossible to deny their demand for a piece of Japan.
The only way this could be prevented is a speedy Japanese surrender.
(Continued)
Suggested Reading: http://www-cgsc.army.mil/carl/resources/csi/glantz4/glantz4.asp#MI1
http://www.fegi.ru/prim/flot/flot1_13.htm
In the week and a half between the beginning of the Soviet invasion and the formal Japanese cease-fire, Soviet forces had almost advanced to the city of Mukden, in southern Manchuria, from their starting positions along the Amur River. But what has all this to do with a potential invasion of Hokkaido?
It serves to illustrate a point: That the Japanese forces in Manchuria were wholly outnumbered, outgunned, and outfought by the 1.7 million men of the Soviet Red Army in Operation August Storm. If we imagine fighting continued beyond OTL's cease-fire and surrender date, the defeat the Japanese faced in OTL easily turns into a rout. The Red Army, with the Kwangtung Army vanquished, will face no serious opposition beyond the weather, terrain, and logistics involved with covering such a wide swath of territory. And once Japanese forces in northern China are defeated (likely before Christmas), those 1.7 million men will be free to advance on Hokkaido, a few dozen miles offshore.
Of course, the Soviet Union did not have the sealift capability for even a tenth of those men, but 170,000 men aren't necessary to secure a port on Hokkaido to bring more troops in -- 17,000 will do, and that amount is one percent of the entire Soviet force. What about the Japanese, you ask? As I stated earlier, the vast, vast majority of Japanese forces are either in Kyushu, where the American invasion is expected, in Manchuria being killed, or grounded due to lack of fuel. Fewer than three divisions are on the entire island of Hokkaido, and no more can be moved due to the American mining and submarine campaigns, which have cut the Home Islands off from each other.
In OTL, the Soviet Union executed three widely-spaced amphibious operations in the space of four days. On August 18, 9,000 Soviet troops landed on the Japanese islands of Shumshu and Paramushir in the Kuriles. On August 19, 10,000 soldiers landed on the southern half of Sakhalin Island, joining Soviet forces advancing from the north. On August 22, 7,000 Soviet soldiers landed at Wonsan in Korea. Together, these three operations stretched the Soviet sealift capability in the Pacific to their limit, but if we combine the soldiers carried by these three operations, we get a total of 26,000.
In OTL, of course, the rapid advance of Soviet forces was aided by the imminent collapse of the Japanese government and surrender orders issued by the Japanese government. Most Japanese chose to fight on until September 2, however, so we do have somewhat of a basis for comparison. While in TTL, the Soviet advance through the Kuriles, Manchuria, and Sakhalin may be slower, it will be no less successful. And if you agree with Bill Cameron's conclusion that the Kyushu invasion plans will be altered, pushing back the American invasion, there's even less chance that Hokkaido will be reinforced. For the Japanese, the main threat is from the south, regardless of what the Soviet Union is doing in the north.
30,000-60,000 Japanese troops on Hokkaido are no laughing matter, of course, but the big advantage for the Soviet invasion planners is that the Japanese can't concentrate their forces. They must defend every bit of coastline, and so disperse their forces accordingly. The Soviet invasion force, meanwhile, is concentrated, and has complete air superiority. Even if the Japanese just happen to have a full division right in the invasion area, the Soviet Union will still be able to transport no fewer than 25,000 soldiers on the first day of the invasion. More will soon follow, owing to the closeness of Soviet ports, and those are just the amphibious forces. We haven't even talked about the airborne troops, who in OTL dropped in Manchuria on places like Port Arthur and Darien, seizing locations in advance of the ground forces and sowing confusion and disorder among the Japanese.
The Soviet Union, between seaborne troops and airborne landings, can expect to put 1-2 divisions ashore on Hokkaido in the first 24 hours of the invasion. The Japanese, in the face of complete Soviet air superiority, cannot mass and counterattack quickly enough to defeat the beachhead in those 24 hours, and by the next day, it will likely be too late. 48-72 hours from the initial landings, the Soviet forces will have large amounts of armor ashore, and once Soviet tanks make landfall, the Japanese have nothing that can stop them. The fall of Hokkaido will be assured.
Ironically, the invasion couldn't happen without American help. The threat of an American invasion is what drew the Japanese south. American bombers and subs prevented the Japanese from reinforcing the north, and Soviet invasion forces would surely land on Hokkaido in LSTs provided by American Lend-Lease.
Suggested Reading: http://www-cgsc.army.mil/carl/resources/csi/glantz4/glantz4.asp#MI1
http://www.fegi.ru/prim/flot/flot1_13.htm
In the week and a half between the beginning of the Soviet invasion and the formal Japanese cease-fire, Soviet forces had almost advanced to the city of Mukden, in southern Manchuria, from their starting positions along the Amur River. But what has all this to do with a potential invasion of Hokkaido?
It serves to illustrate a point: That the Japanese forces in Manchuria were wholly outnumbered, outgunned, and outfought by the 1.7 million men of the Soviet Red Army in Operation August Storm. If we imagine fighting continued beyond OTL's cease-fire and surrender date, the defeat the Japanese faced in OTL easily turns into a rout. The Red Army, with the Kwangtung Army vanquished, will face no serious opposition beyond the weather, terrain, and logistics involved with covering such a wide swath of territory. And once Japanese forces in northern China are defeated (likely before Christmas), those 1.7 million men will be free to advance on Hokkaido, a few dozen miles offshore.
Of course, the Soviet Union did not have the sealift capability for even a tenth of those men, but 170,000 men aren't necessary to secure a port on Hokkaido to bring more troops in -- 17,000 will do, and that amount is one percent of the entire Soviet force. What about the Japanese, you ask? As I stated earlier, the vast, vast majority of Japanese forces are either in Kyushu, where the American invasion is expected, in Manchuria being killed, or grounded due to lack of fuel. Fewer than three divisions are on the entire island of Hokkaido, and no more can be moved due to the American mining and submarine campaigns, which have cut the Home Islands off from each other.
In OTL, the Soviet Union executed three widely-spaced amphibious operations in the space of four days. On August 18, 9,000 Soviet troops landed on the Japanese islands of Shumshu and Paramushir in the Kuriles. On August 19, 10,000 soldiers landed on the southern half of Sakhalin Island, joining Soviet forces advancing from the north. On August 22, 7,000 Soviet soldiers landed at Wonsan in Korea. Together, these three operations stretched the Soviet sealift capability in the Pacific to their limit, but if we combine the soldiers carried by these three operations, we get a total of 26,000.
In OTL, of course, the rapid advance of Soviet forces was aided by the imminent collapse of the Japanese government and surrender orders issued by the Japanese government. Most Japanese chose to fight on until September 2, however, so we do have somewhat of a basis for comparison. While in TTL, the Soviet advance through the Kuriles, Manchuria, and Sakhalin may be slower, it will be no less successful. And if you agree with Bill Cameron's conclusion that the Kyushu invasion plans will be altered, pushing back the American invasion, there's even less chance that Hokkaido will be reinforced. For the Japanese, the main threat is from the south, regardless of what the Soviet Union is doing in the north.
30,000-60,000 Japanese troops on Hokkaido are no laughing matter, of course, but the big advantage for the Soviet invasion planners is that the Japanese can't concentrate their forces. They must defend every bit of coastline, and so disperse their forces accordingly. The Soviet invasion force, meanwhile, is concentrated, and has complete air superiority. Even if the Japanese just happen to have a full division right in the invasion area, the Soviet Union will still be able to transport no fewer than 25,000 soldiers on the first day of the invasion. More will soon follow, owing to the closeness of Soviet ports, and those are just the amphibious forces. We haven't even talked about the airborne troops, who in OTL dropped in Manchuria on places like Port Arthur and Darien, seizing locations in advance of the ground forces and sowing confusion and disorder among the Japanese.
The Soviet Union, between seaborne troops and airborne landings, can expect to put 1-2 divisions ashore on Hokkaido in the first 24 hours of the invasion. The Japanese, in the face of complete Soviet air superiority, cannot mass and counterattack quickly enough to defeat the beachhead in those 24 hours, and by the next day, it will likely be too late. 48-72 hours from the initial landings, the Soviet forces will have large amounts of armor ashore, and once Soviet tanks make landfall, the Japanese have nothing that can stop them. The fall of Hokkaido will be assured.
Ironically, the invasion couldn't happen without American help. The threat of an American invasion is what drew the Japanese south. American bombers and subs prevented the Japanese from reinforcing the north, and Soviet invasion forces would surely land on Hokkaido in LSTs provided by American Lend-Lease.
I personally don't believe the blockade option is workable for simple political reasons. As you mentioned, the public mood in the wake of Germany's surrender is to finish the job and bring the boys home as soon as possible. A blockade is a solution, but it's a long-term one, and the people at home wouldn't have the patience for it. People need to see tangible, physical action that they can understand. It's why body counts were so popular in Vietnam and now in Iraq. People need things they can count and see -- a blockade doesn't do that.
It would, however, work spectacularly well if implemented. By September 1945, Japan was on the verge of total starvation, and in OTL, only prompt shipments of food from the United States averted a massive humanitarian disaster (another one of those lovely historical ironies). If we do a blockade, or even just delay an invasion for six months, the consequences for Japanese women and children are disastrous. The Japanese will suffer a starving winter in 1945, then, when the 1946 planting season is lost due to continuous American air attack, the spring and summer of 1946 will turn deadly. We're talking starvation deaths in the millions, and possibly the tens of millions, even if Japan surrenders as early as early summer 1946. Emergency food shipments would help, but would a United States made weary by an additional year of war be as willing to send food to the starving Japanese? It's a bleak picture.
Bill Cameron
Banned
Amerigo,
First, thanks for the material you posted.
A blockade would be unpopular so another offensive would need to be launched. How about Formosa, supposedly so that the Chinese could be helped? It had been the USN's plan from nearly the beginning
Let's not forget the rest of Japanese-occupied Asia. I've an article I'm still looking for concerning Truman's decision to drop the bombs. It's sort of a revision of the accepted revisionist theory. The author quotes certain historians who've calculated that 250,000 civilians in Japanese-occupied Asia were dying each month and that any argument for a longer, "No Bomb Needed, They Would Have Surrendered In A Few Months Anyway", WW2 needs to take those extra deaths into account.
Bill
First, thanks for the material you posted.
A blockade would be unpopular so another offensive would need to be launched. How about Formosa, supposedly so that the Chinese could be helped? It had been the USN's plan from nearly the beginning
Let's not forget the rest of Japanese-occupied Asia. I've an article I'm still looking for concerning Truman's decision to drop the bombs. It's sort of a revision of the accepted revisionist theory. The author quotes certain historians who've calculated that 250,000 civilians in Japanese-occupied Asia were dying each month and that any argument for a longer, "No Bomb Needed, They Would Have Surrendered In A Few Months Anyway", WW2 needs to take those extra deaths into account.
Bill
Indeed. Korea, in particular, suffered from a lack of food. In OTL, several million people died of starvation and disease before aid could arrive and the 1946 harvest collected. In many ways, a Soviet occupation might be preferable to the alternative from the Korean POV, at least in the short term.
Let me say that the Soviet Union had little in the way of capacitiy and training to mount an amphibious invasion of the Japanese home Islands. The US Navy transfered a limited amount of equipment to the soviets and attempted to train them but it was in the opinion of the trainers a hopeless case. Any attack by soviet forces would result in a fantastic ammount of loses to the defending Japanes.
the soviets were at the end of there manpower, they needed to rebuild there country, and they now have half of europe to absorb, the real reason stalin attacked japan was to get back territory that russian lost in the first war with japan. He couldnt have gotten into japan or at least not enough of japan to even allow him to get an occupation zone the usa wouldnt have allowed
I won't restate the details of the Soviet capabilities, AV has done that quite nicely indeed.
In this thread https://www.alternatehistory.com/Discussion/showthread.php?t=70594&page=3 the relative strengths of the USSR and IJA/N are discussed at some length.
The Soviet military machine, in 1945, was quite simply the most fearsome conventional force the world had seen to that time. In armor, artillery, and infantry it was unmatched. In tactical air it was qualitiatively inferior to the West, although not by as much as some in the West liked to imagine (the Yak-9 was a very nice fighter, albeit limited by somewhat short range compared to Western counterparts), but miles ahead of the IJA/N.
By late 1945 the Japanese were, in many ways, defenseless. As has been pointed out, they had pushed virtually all their surviving assets to Kyushu to oppose the expected American invasion. The forces left in the north, even on Honshu, were mostly militia quality and completely non-mechanized. Air support was non-existant, even on Kyushu, with the few operational fighters dedicated to sheparding/defending the kamakazi swarms. There was NOTHING left to oppose a northern Soviet invasion, even if it was an airborne attack to secure a port to sail in freighters. The IJN was gone, not reduced, GONE. Nothing existed to stand in the way (the few DD's left were all hiding in the far south husbanding their fuel for last stand against the Americans). Even if a naval presence could be assembled, it wouldn't have mattered. The Red Army could put more paratroopers onto Hokkaido than the Japanese had troops & once they secured an airhead or bridgehead it was over.
It is easy to talk about war weariness, given the presence of it in America and Western Europe, but this ignores the realites of the USSR. Until Stalin was tired of war, the USSR wasn't tired, simple as that. As far as having to reorganize, it was already organized. IOTL the Red Army moved EIGHTY divisions (or the entire operational strength of the U.S. Army at its peak) from Europe to the Far East in three months (May to August 1945) while consolidating its position in Eastern Europe.
The Soviets were many things, few of them admirable, but as far as the 1945 Red Army, from command staff to grunt, there was much to admire, and to respect.
In this thread https://www.alternatehistory.com/Discussion/showthread.php?t=70594&page=3 the relative strengths of the USSR and IJA/N are discussed at some length.
The Soviet military machine, in 1945, was quite simply the most fearsome conventional force the world had seen to that time. In armor, artillery, and infantry it was unmatched. In tactical air it was qualitiatively inferior to the West, although not by as much as some in the West liked to imagine (the Yak-9 was a very nice fighter, albeit limited by somewhat short range compared to Western counterparts), but miles ahead of the IJA/N.
By late 1945 the Japanese were, in many ways, defenseless. As has been pointed out, they had pushed virtually all their surviving assets to Kyushu to oppose the expected American invasion. The forces left in the north, even on Honshu, were mostly militia quality and completely non-mechanized. Air support was non-existant, even on Kyushu, with the few operational fighters dedicated to sheparding/defending the kamakazi swarms. There was NOTHING left to oppose a northern Soviet invasion, even if it was an airborne attack to secure a port to sail in freighters. The IJN was gone, not reduced, GONE. Nothing existed to stand in the way (the few DD's left were all hiding in the far south husbanding their fuel for last stand against the Americans). Even if a naval presence could be assembled, it wouldn't have mattered. The Red Army could put more paratroopers onto Hokkaido than the Japanese had troops & once they secured an airhead or bridgehead it was over.
It is easy to talk about war weariness, given the presence of it in America and Western Europe, but this ignores the realites of the USSR. Until Stalin was tired of war, the USSR wasn't tired, simple as that. As far as having to reorganize, it was already organized. IOTL the Red Army moved EIGHTY divisions (or the entire operational strength of the U.S. Army at its peak) from Europe to the Far East in three months (May to August 1945) while consolidating its position in Eastern Europe.
The Soviets were many things, few of them admirable, but as far as the 1945 Red Army, from command staff to grunt, there was much to admire, and to respect.
I can't argue with the facts that CB and AV has assembled. By 1945 the Japanese would have have been outmatched in all ways - even naval. However, those Japanese who were in the north would more than likely have resisted fanatically in a suicidal effort. Yes, the Red Army fought fanatically against the Nazis, but how much of that was because of what the Nazis did to the Russians in 1941-43? There was a complete hatred of the Germans. Would the average Soviet soldier or low grade officer have exhibited the same fanaticism and dedication when sent against a nation which had done nothing to them? In a defensive role, Finland, after all, did a pretty good job against the USSR twice in the 1940s with far less military capability and craziness than the Japanese could have mustered in 1945. I tend to think the average Ivan would have wondered why the hell he was being shipped off to get blown up by a kamakazi in a cave after he'd just won the Great Patriotic War. This might well translate into a less than effective effort.
"In the Soviet army it takes more courage to retreat than advance."
-Joseph Stalin
It really didn't matter how much you wanted to fight in the Red Army. You just did.
It should, however, be noted that the Japanese were a traditional enemy, with humilation of the 1905 War still a living memory, as well as being the last foreign army to leave Russian (nee Soviet) soil post Russian Civil War, holding part of the Kamchatka Peninsula until 1925. There was also the quasi-war that had occurred in Manchuria circa 1938, where the Japanese had been the aggressor (and the loser).
Motivation would have been easier than some imagine.
Indeed it would, but the Soviets have a fantastic amount of manpower to soak up the damage. No one is saying that this is going to be a pretty, textbook landing on the scale of D-Day. This is going to be a hard-fought, improvised, no-holds barred invasion. There are going to be soldiers stacked upon one another aboard minesweepers, torpedo boats, destroyers, and anything that can possibly float. Crates are going to be thrown overboard haphazardly, supply will be on an ad-hoc basis, done if and when the invasion succeeds. The Soviets will take massive casualties in terms of the size of the invasion, and it's going to be far more bloody than it has to be.
But for all its ill-preparation, it's going to work. Hokkaido is simply too big to defend effectively with the forces the Japanese had on the ground at the time, and as soon as the Red Army gets tanks ashore, it's game over. Short of suicide satchel charges or a tank somehow blundering into the range of a coastal defense gun, the Japanese don't have a single thing that can reliably stop Soviet armor. As in Manchuria, the Soviets' greatest enemy will be the terrain and the logistics involved with the invasion.
In OTL, the Soviet Union proved itself to be -- if not a master of -- than quite capable at amphibious operations. In OTL, there were over a dozen Soviet amphibious landings either in the Black Sea or the Pacific. Almost all were at the battalion scale or smaller, but regardless of their size, they worked. At Hokkaido, the Soviets will be going up against their toughest task yet -- but thanks to numbers, paratroop landings, complete air superiority, and the sheer will to win regardless of casualties, they're going to carry the island.
What happens after that depends on what you think of Stalin. If you think he was a reckless empire-builder, you're going to imagine the Red Army continuing on to northern Honshu. If you believe that Stalin only occuped Eastern Europe to serve as a shield against future invasions, you're going to imagine the Red Army stopping at Hokkaido, and potentially withdrawing from the island at the end of hostilities. After all, there's no use for it in Stalin's grand scheme.
I tend to take the middle ground. I don't see Stalin ordering the Red Army into Honshu. To do so would to antagonize the Americans, and in OTL, until the Soviet Union had its own atomic bomb, Stalin was reluctant to do anything that would risk war. The Berlin Blockade, the only violation of this reluctance, came about because Stalin believed the Allies wouldn't go to war over Berlin. When he realized he couldn't play the gambit through, he ended the blockade.
If we extrapolate that to Japan, I imagine a conquest of Hokkaido and the Red Army continuing operations -- not in Japan, but in mainland China. The defeat of the Kwangtung Army won't completely eliminate the Japanese presence on the continent, and by continuing to fight in China, Korea, and so forth, he'll be able to set up the same ring of client states to shield the Soviet Union from invasion that were present in Europe. Furthermore, this will avoid further antagonizing the United States, which will likely take a dim view of the Soviet invasion of Hokkaido, despite public opinion praising it as the road to a shorter war.
But I don't think this would result in a wholly Communist-dominated China. I'm venturing into Hendryk territory here, but this is my best guess. Stalin and Mao didn't get along at the best of times, and the 1950 treaty of friendship and alliance only came about after several months of negotiation, despite this being the high-water mark in relations between the two countries. With a Soviet advance into Northern China and Korea, Stalin and Mao are going to be closer together than at any point in OTL. At some point, Mao is going to step on Stalin's metaphorical toes, and Stalin is going to have to ask himself if Mao is worth all the trouble -- China might be worth all the trouble, but Mao certainly isn't.
With Mao out of the picture, Stalin can divide China up, in effect creating a return to the warlord years. Multiple small states are far less threatening than one large one, and it won't matter if some of the small states are democratic, since they won't be near the Soviet Union. Stalin would have his Democratic Republics of Manchuria, Korea, Mongolia, Xinjiang, and so on, and be doing it without antagonizing the United States on a large scale. If the Republic of China continues to exist in the south, so what? Stalin has gotten what he wanted -- a secure border, and many new friendly allies. Now, when the Soviet Bomb is ready, and the Soviet Union rebuilt, he can look forward to meeting the Americans on far more even terms.
Good question. I think it goes back to the idea of whether you believe Stalin was an empire-builder or just looking out to defend his country. If he's just trying to create a ring of satellites, "liberating" the Ainu gives you a lot more plausible backing for setting up a satellite state. If, however, you want to cast your portion of Japan as the "true government," then the Democratic Republic of Japan idea makes a lot more sense.
Either way, with a longer Pacific War, you'll probably see a lot less resistance to the dismemberment of Japan than there was to Germany -- After all, the war against Japan was "personal" to the United States in a way that the war against Germany never was. If we pile tens of thousands of additional casualties taken in an invasion of Japan atop that idea...