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Personally, I think they'd be hard-pressed to hold anything past the Ural Mountains. Vladivostok gives the Soviets a viable warm water port, and holding all of Eastern Europe plus some of Africa will likely stretch the Reich to its limit.

Not to mention that even if Germany wins, Japan is likely defeated by the Allies and occupied as IOTL, meaning a very geographically convenient position for the Soviets.
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