Why couldn't the US persuade Russia, Japan's neighbor to the west, to let it attack Japan from its territory in the later years of WWII?

I cannot provide any documentation for anything said below, and it is speculation, but it doesn't seem like anyone else is providing documentation either, answer- or comment-wise, at least not for anything on point at any rate.

It seems to me that there could easily be at least three threads contributing here:

1. Yes, the non-aggression agreement was nothing of any importance legality-wise to a beast of Satan like Stalin. However, it seems likely (especially as they DID invade and take the Kuriles, half of the region at least) that they wanted to use the area themselves for taking whatever they could of Japan. Consider the Cold War with a North Japan/South Japan along the lines of Korea or Germany. How would that work if the US had already invaded all those areas of Japan that they intended to invade themselves? 'We can't have that: we signed a non-aggression pact with them and this doesn't seem in the spirit of that...' "Mendacity" will never not exist so long as rulers exist. So, they could reserve those areas for ready attack once they have Europe in hand OR they could let the US take those areas and leave only the 3,000,000 strong army in China to attack and wrest things from. Let's weigh those choices...

2. Back to that "Europe in hand" phrase. Whether or not Stalin anticipated rolling straight through Berlin and straight through Eisenhower to the Atlantic or that he simply wished time to take as much of Europe as possible, including at least some chunk of Germany, for all the nefarious reasons that it turns out DID apply, the simple fact is that a faster end to the Pacific war likely meant reinforcement of US efforts in Europe kind of forcing the US hand vis-a-vis allowing the Soviets to take Berlin (and with that promise in hand, to dally about taking all of Eastern Europe first rather than driving 100% at Berlin), perhaps leading to all of Germany and much of Eastern Europe to end up in US hands, not Soviet hands. Not really on point, Soviet/Stalin-wise. I admit, evidence can suggest fairly strongly they knew their limits very well (420,000 trucks shipped to the USSR during US participation being just one indication they were not ready to "drive to the Atlantic" - and not even just the trucks... I'm sure they considered the shipping capacity that did that... and it was just one piece) and intended to finesse themselves into the US hanging back a bit even as they dallied into owning Eastern Europe but that goal simply could not be aided by an end of war in the Pacific.

3. So, then to the whole idea of US troops by the billion on actual Soviet soil. I'm sure they, as a nation and people, had no trouble at all remembering (from school lessons if not alive at the time) US troops running about the USSR in its very early days when men like Stalin didn't know if they'd end up as masters of a nation or as failed bank robbers who took a chance at the big time before dying violently... running about talking about protecting US money interests (I mean property rights, sure, property rights) in pretty much the same way we said such in every weakling country in Central America (even in Japan, kind of, with Matthew Perry's mission). Consider how many Americans regard Muslims in general (and in particular when those folks get the chance, they're not shy) 20 years after 9-11. What if instead of that attack, some Muslim country had sent 5,000 soldiers to the US and kept them here for 3-4 years, fighting and killing American soldiers who came near to them? Or if they'd taken advantage of the hippie unrest in the 60's to do so? Just how would we then take them being allies in whatever 20 years later (now) and suggesting we let them send half a million troops to the US? NOT that Papa Joe was doing daily press conferences and dodging awkward questions. But still. I must say it would seem like that was very, very simply "not on."

To go off point, but still connected, I understand the question to be saying the US actually did ask for this. Surprised, to say the least, but if so, then so. Surprised because how, precisely, would this have really helped at all? We'd STILL have had to cross water and land an army against as massive a defense as the Japanese could provide. And that would have been at least threefold: 1) Whatever forces were massed in the south in real life could have been massed in the north instead as if we had not been island hopping northward, their south would have faced no threat, 2) They could still have thrown 4 year-olds and grandmothers into the defense as they planned AND TRAINED FOR. 'This is how you use a leaf rake to defend against a tank', and 3) If basing in the USSR in place to stage said attack/s, that 3,000,000 men in China, Manchuria, and Korea could have pretty easily attacked before we did too awfully much.

And more (like it or not, their navy needed attritted way, way down for any amphibious attack to work, and removing all the supplies they could bring up from Southeast Asia due to said attrition to both their navy and merchant fleet, and the British seeming to value hugely the cutting off from supplies of the armies aiming at India... and more even then...), but those three reasons above mean I'm very surprised we ever asked for the opportunity. What, really, would have been the useful point of being given permission? But if we asked, we must've evaluated it as worth doing I guess.

The logical place where US forces could have been based would have been Vladivostok. It is close to northern Japan, but as others have stated getting any US equipment or personnel there without the Japanese doing anything about it would have been difficult.

There are a number of other issues concerning Vladivostok:

• Vladivostok has been been the home of the Soviet and now Russian Pacific fleet.

• From late December, Vladivostok is iced in for about 2 months.

• During summer, the approach to Vladivostok is complicated by fog. Between June and August Vladivostok has an average of 51 percent foggy days. During the 9 months of the year when the port is open it has an average of 21 percent foggy days.

• The Soviets were particularly secretive about their military assets

• In 1921, the Soviet Union closed Vladivostok to foreigners. It wasn't re-opened to foreigners until 1992.

• Vladivostok was occupied by foreign forces (US and Japanese) in 1918 after the Russian Revolution.

• A US base potentially could have been established on the Kamchatka Peninsula, but it is remote and it would would have similar issues of launching raids on Japan there would have been if raids were launched from the Aleutian islands.

Roughly fifty percent of Lend Lease supplied to Russia was transported across the Pacific from U.S. and Canadian ports by Soviet vessels. Japan allowed this, by treaty obligation to the Soviets, in exchange for strict Soviet neutrality. The Soviets were never going to risk this in any way, by blatantly allowing a Western Allied build up on its soil, while it remained dependent on U.S. Lend Lease aid.

Further, I can't see the Western Allies risking the safe delivery of this aid either. Well into 1944 they remained terrified that the Soviets would negotiate a separate peace with Germany, and free up Nazi manpower to deploy into France and Italy.

There are several reasons

• Up until late June/July of 1944 Germans still occupied large parts of Soviet territory like Belarus, parts of Ukraine and Baltic republics. Only after Bagration (started on June 22/23) and Lvov–Sandomierz (started in July, ended in August) we could say that main part of USSR was liberated, although some German pockets like Courland remained until the end of war.

• In addition to this, Soviets really faced manpower shortages late in the war. So much that they had to conscript 17 year old boys, but also various "elements" from recently liberated territories. There were many Ukrainians, Belarus, Lithuanians, Latvians, Estonians ... not really willingly fighting in the Red Army, some of them actually working for Germans previously. But they simply switched sides and have gone with the flow, better to be a Red Army solider than face NKVD and SMERSH back home. Of course, Soviet officers understood that very well, but they had no other choices, so they were also going with the flow as long as they were advancing and German defeat was nearing. Soviets also armed Poles (who traditionally hated Russians and Moscow) and Bulgarians and Romanians (recently switched sides) just to keep war momentum going forward. Finland also got away relatively easily because Soviets did not want to waste time and resources. All of this is important, because simply saying, until Germany was defeated, Soviets could not afford war with Japan. In fact, USSR promised on Yalta to attack Japan 3 months after the end of war in Europe.

• Another thing to consider was could US and UK afford those 50 divisions ? Obviously, not without either stripping their forces in Europe (not in USSR interest) or from other places in Pacific which would slow down offensives there. Of course, stationing US divisions on Soviet soil would breach neutrality. Japan would have no other choice than to try to prevent this, both on sea and on land. Kwantung Army was gradually weakened during the war, now the process would be reversed. Like it or not, they would have to attack Soviet forces in Far East which were also gradually weakened during the war. Logistics would also play its part. Soviets didn't have enough food even for themselves, Japanese would do their best to block Vladivostok etc ... Overall, buildup would be slow and comparatively ineffective compared to actual historical campaign of island hopping.

• Ideological differences would also come to effect. Soviets generally mistrusted Western Allies and same could be said in vice versa (especially British). Soviets reluctantly agreed to shuttle bombing, but 50 divisions would be too much, especially in the Far East. There was no way to ensure these divisions would leave in the end of the war, and Far East intervention from Civil War could repeat itself. Of course, exposing population to capitalism of US/UK forces would also be detrimental to Soviet power - workers and peasants could learn about much better living conditions in capitalist countries.

• Finally, when Germany collapsed, as mentioned before, Soviet Union did not need those 50 divisions. As promised, they did attack Japan exactly 3 months after end of war in Europe. Soviet invasion of Manchuria crushed Japanese forces with relative ease. Atomic bombings did force Japan to surrender, but in alternate scenario without them, this new front in Manchuria and Korea would simply be too much for Japanese. US and UK of course knew that, and it could be even said that Soviet attack was some kind of backup plan in case that atomic bombs do not achieve desired result.