Stalins army was caught with its pants down massing at the German border preparing for an offensive in June of '41. It is widely excepted Stalin planned to invade Germany as soon as Germany heavily committed itself to launching an invasion of England. If Germany doesn't attack the Soviets and instead launches an invasion of England in '42, Stalin would backdoor the Germans. I give him 50/50 odds of taking Berlin if Germany is heavily committed elsewhere.
Stalin will not attack the Germans alone, ie, no second front. But he might pick off Romania, Iran, we know he tried Finland, and others if left alone.

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