In answering your question, in the same situation with the western Allies at war with Germany, I do not believe Germany could have totally defeated the USSR. They could have reached the AA line but even then, the Soviets could have fought back after recovering losses. Even if Germany defeated USSR, there would have been the necessity to have stationed troops to fight off the guerillas. The western Allies would still have landed in France and even though it may have taken longer, Germany would have been defeated anyway.

Now if Hitler did not declare war on the US, that would have opened another scenario and you can look at a previous posting discussing this.

In a nutshell, Hitler could have conquered the USSR but not the world and the question would have to be asked, would he be able to keep it? Good luck!

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American by birth, TEXAN by the grace of GOD!