Plausible. But pretty niche alt history.

It would likely mean no US entry into World War II, no Communist takeover of China and no effective insurgency movements in European colonies in Southeast Asia. Japan may well have never left the League of Nation, broken with the British or formed an alliance with Germany. That means that there would not have been a Soviet spy master in Tokyo getting information about German plans and activities from Nazi briefings of the Japanese.

Plus, the entire economic development of Australia in the second half of the 20th Century would be massively changed.

I think what you're suggesting would have made a land confrontation between the (still likely victorious) Soviet Union and Japan happen earlier and result in Soviet victory eventually. A bloody land war in Siberia doesn't sound like a particularly amazing prospect for the Empire of Japan.

Totally agree that it would entirely change the potential shape of the second world war.

The Japanese Empire defeated a much stronger Russian Empire. I am not at all certain that the Red Army could have easily defeated the Japanese Imperial Army in Siberia if the Soviet Union had to build the transportation infrastructure to supply a transcontinental campaign after the Japanese had a decade or more to prepare. The Green Uprisings would have likely pushed Siberian peasants into a Japanese puppet Buryat kingdom under Semyonov. Not to mention that stealing peasant grain and livestock in southern Siberia was relatively important in avoid mass starvation in the cities of European Russia during the early 1920s. With fewer productive agrarians to steal from the urban Bolshevik regime would have had to squeeze even harder west of the Urals, which presumably would have made the Green Uprisings even worse.

I think we can observe that:
i) the fact that something is militarily possible does not make it politically possible.

For example, the US had the strength and resources to overthrow the Soviet regime in the Russian Civil War. I mean. A landing in St. Petersburg was not so infeasible. Clearly American politics was not ready to make such a commitment even if it'd pay off in the long run.

In Japan there was a close competition between Navalists and the Army fundamentally similar to that in Great Britain since 700 with the difference that the British parliament owned a military while it was the Japanese military to own the parliament, to paraphrase Mirabeau.

I don't see how the UK or the US could have recognized the Japanese annexation of parts of Siberia. On the other hand, it doesn't seem that Japan could withstand international pressure to do so.

ii) It's true that an important element of Japanese expansionism was a self-perception of weakness and lack of some fundamental resources. But oil was discovered just in Western Siberia in the sixties.

On the other hand, the ambiguous question: the lack of resources was an incentive for a risky and aggressive foreign policy, but are we sure that Japan would have ceased to be a revisionist power had it felt less economically insecure?
I sincerely doubt. Look at a juicy target like Dutch Indonesia. It was the Western power that held Japan back, not some special internal constraint.
A war in the Pacific has probably been inevitable.

The Americans were in Siberia with the Japanese at the time. The root issue with my alternative proposal is that Semyonov had spent the Russian Civil War extracting rents on railroad traffic and so would not have ever thought to rip up the railroad on the retreat from Omsk. He was just a too much of a scoundrel to do strategically wise things.

Keeping the Bolsheviks out of Siberia would have been enough to get the Brits and Americans to close their eyes to a a Buryat puppet state under the Japanese thumb. Much as Manchuria ultimately did keep the Japanese busy, my proposition is that Siberia would have absorbed much of Japan's expansionist energy.

Like to suggest something for this thread. Gopher's alt history scenario has been running for about a week. How about we get some more alt history suggestions, +1 what you think we should explore and then we spend a week roughly. Might fill in the time while the server gets fixed. Or maybe its just another pointless idea.

What if...

The US responds to the rise of Totalitarianism in the early 1920s by drafting, passing and ratifying the 22nd Amendment to the US Constitution 25 years early after having just ratified the 17th and 18th Amendments. Thus FDR cannot seek a third term as President. I would presume that "Cactus Jack" Gardner would then become President and be the man who won World War II and confronted Hitler and Tojo. Gardner, who was willing to try the improvisation of the First New Deal in desperation at the crises of the Great Depression, was an unequivocal opponent of the Second New Deal while helping to organize opposition to the attempts at Court Packing. FDR would have left office with the US still mired in the Great Depression and may well be remembered as a popular and well-meaning failure.

How would American Domestic politics have been altered by a very different man being the hero of anti-fascism and the man who ended the Depression? Many of the men who turned out to be Communist spies would likely have been purged out of a Gardner Administration fairly quickly for purely political reasons thus resulting in no Hiss-Chambers Controversy, no Richard Nixon and no Joe McCarthy. Eisenhower's entry into politics would have unquestionably been different, even if just as a function of the electoral calendar.

How would the Cold War have unfolded with a President more vigorous than FDR and more experienced than Truman at the helm during the key moments when Stalin was seizing various advantages? At least part of the Marshall Plan was about shipping abundant American food stuffs to Europe for the benefit of voters in swing states amid the dislocations of de-mobilizations, so the saving of Europe from post-war starvation would have still happened. Gardner's advisors would have been more Jesse Jones and less Henry Wallace.

Additionally, Gardner spoke with JFK on the phone the morning that he was assassinated. Gardner lived seemingly forever and would have hovered over American politics as a towering hero for 20 years in a manner that no popular former American President has during the 20th Century. Teddy Roosevelt cast a significant shadow but he still died in 1920, only 11 years after he left office and he was more of an outsider than Gardner would have been. Men like Truman, Nixon, Ford, Carter, Bush and Clinton did not exist post-White House as national heros. And only Johnson and possibly Nixon were as firmly embedded in the machinery of national politics prior to becoming President.