This is not a question that truly answers how/why Nazi Germany was defeated.
The real answer, as someone else posted, was the industrial might of the US.
To better answer your question, though, in terms of red army v US army...if the US army had not committed to Europe, would the red army have been able to push back against the Nazis? I don't think so. The Germans essentially had a one-front war until the US went in to Italy and Normandy. Sure, they over-extended their supply lines and did the usual against Russia (underestimated the Russian winter), but it was the western/southern invasions by the US (and UK/Canada) that forced Germany to fight a two-front war.
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