Russia couldn’t fight back as it was without US help.
The worst case scenario is a Germany with its eastern flank secure, never mind a bigger death toll of the Jews, Gypsies, and many other minorities.
Ultimately the Germans had far overreached the capacity of their smaller nation to play in the really big leagues against the US and the ussr. Technological excellence could only get them so far. Even when everyone had been weakened by the Depression.
US armed forces in WWII became legendary. Solzhenitsyn in a different context later bemoaned "the accursed capacity of our (Russian) people for suffering."