Best POD for Soviet Europe?

What POD , preferably during WW2, can lead to a Soviet occupation of all continental Europe? D-Day Failure perhaps?
 
Barbarossa imploding much earlier for whatever reason (post-poned into 1942, August lunge for Moscow which fails, Soviets prepare better... whatever works) combined with the US returning to isolationism after the war ends are probably your best bets.
 
Id say having the communist movements outside of the former Russian Empire do better post WW1, could lead to a larger Soviet presence in Continental Europe.
 
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