USSR as a Full Ally

As it says on the tin...what if after Germany invades the USSR in June 1941, Stalin casts his lot in with the Allies fully? This would mean the USSR declares war on Japan in December 1941.

Personally, I can't see Stalin doing this, but let's say he does? I know the USSR has a decent army in the Soviet Far East in 1941 that was not withdrawn to fight the Nazis (as the Siberians were). Could they take on the Kwantung Army in 1941? How much would the shutting down of Vladivostok affect Lend Lease?
 
Japan's mainland empire ges taken over sooner, with Manchuria and all of Korea going Red. It wouldn't be an August Storm style disintegration, but a slow and grinding victory. They are horribly overextended, but Japan is even moreso, with its commitments in China and the tight time table it made for conquering the Pacific.

Germany had largely been stopped in the West, so while this might drag the war a bit longer, it still ends with the Axis' defeat.
 
No major advance against Japan as the Far East Army would be kept as a reserve against the Germans, but it would throw a wrench into Japan's plans as they now have to somehow fight the Soviets while conquering the Southern Resource Area and dealing with China. Back in Europe, the Germans might get into Moscow, but I doubt they'll take it. All in all, the war turns into a slower grind in the Eastern Front and a Japanese implosion once the Allies get their act together (so around late 1942).
Post war will be very interesting.
 
Would China not fall to communism as a result of this? That could be an ironic twist. On the other hand, Stalin could give lots of support to Mao.
 
In doing so, Stalin will have eliminated the largest and safest route it used to take in Lend-Lease supplies; the majority was shuttled through the port of Vladivostok aboard Soviet-flagged vessels, which would no longer be protected through neutrality. Traffic would have to be diverted to the Persian Corridor route, but I'm not sure what limits on traffic might exist there, or how adversely it may effect the Soviet war effort.
 
It's completely understandable why people would make a distinction between the 'WAllies' and the Soviets, because of the Japanese-Soviet neutrality, and because of the Cold War that followed, but during the war, they were all the United Nations.

And like Ariosto said, it was not militarily expedient for either side to have the Soviets and Japanese engage in combat directly.

The operations in China were, to the degree that they weren't independent, a drain on the war-effort. And the lend-lease route across the Pacific and Trans-Siberian Railroad was too useful to give up (even if it was just a fraction of the total L-L by tonnage). For Japan, engaging with the Soviets meant giving up China, or if they tried to do both, the most realistic* scenario (all while occupying Southeast Asia, the East Indies, protecting their frontiers, while fighting in the Pacific), they'd be even more over-extended than in OTL.

*I mean realistic as in, most likely to happen. Not realistic as in, most likely to get Japan want it wanted out of the war.


And it's worth bringing up that the Germans declared war on the USA (thinking the license to attack shipping would help them beat Britain before the USA and Britain built up a force to invade Europe), just as the Japanese declared war on the USA (thinking a sucker-punch would somehow be a positive when the time came to talk peace), it wasn't the USA joining the Allies by choice.
 
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