I tend to stay away from Russian Civil War PODs for the same reasons I stay away from ACW ones; IMHO the right sides won OTL both of them and I don't like wanking the Wrong Side.
A fair case can be made the Mensheviks weren't so bad and should have won, but it's much harder to show what their basis of support would have been.
In this case, handwaving the mechanics of how and why a moderate-socialist regime manages to survive and prosper in the far East but not the heartland of Russia, they still face the stark lack of an industrial base to build on, nor much of a population or agriculture in general. They are probably going to have to be
someone's puppet charity case!
If you don't want that to be the Japanese, the only other plausible candidate I can suggest for a sugar daddy would be--the United States.
OTL, I've read an essay by Trotsky where he's explaining just how dire and perilous the straits the Bolsheviks are in, and he says the only reason they've been able to hang on to the far east is because American and Japanese ambitions have been checking each other--otherwise they'd be helpless against just one or the other.
So if those two powers could agree to a division of formerly Russian spoils, I suppose the two of them working together could step into their respective alloted spheres. For just the Americans to get all of it, you need to show how and why the Japanese are persuaded to back down completely. But at any rate I do know Americans were involved to some degree. I suppose Americans would be more likely than Japanese to foster a Menshevik, rather than royalist or fully colonized, puppet power indulgently.
Quite clearly if the USA has that kind of presence in the Asian mainland, Japan's future course is drastically affected.
I am not sure of the exact timeframes of the American intervention; Woodrow Wilson was President until early 1921, and perhaps all of this was a Wilson admin thing, a carry-over from WWI policy. If the USA had a willing and dependent ally in Siberia, would the Harding/Coolidge admin still pull out, leaving the Siberians with the choices of submitting to Japan, the Soviets, or trying to go it alone? Probably the Republicans would at least try to cut any American aid to the bare minimum judged necessary, then the Depression would choke it even more. By then might your Mensheviks in the east have put their republic on a sustainable basis?
Frankly it makes my head hurt trying to see it. I get particularly dizzy trying to imagine the 1930s and how negotiations and de facto alliances between the Soviets and the Japanese might go; conceivably Stalin gets drawn all the more firmly to the Axis side, partitioning the eastern republic between himself and Japan, and the Russians are left alone against Hitler. (Probably not, Churchill was determined to be active allies with the Soviets once Hitler attacked them and Stalin was hardly going to spurn that help either--it does leave Japan in an odd position though!

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You need someone with more enthusiasm for screwing the Bolsheviks and more faith in a non-radical Russia than I have to do this right I guess. There should be lots of people here who fit that bill though.
