When was Stalin on his knees begging for peace? I don't remember reading that anywhere. In fact Stalin gave orders that no Russian soldiers were to retreat. Seriously, when was Stalin on his knees begging?
While I agree that Germany could have defeated Russia, Stalin was not begging for peace. Not at Stalingrad or at any other time.
He was, by the time the Germans had 90% of the city Stalin was very close to begging for peace. The changing situation kept him from doing so. His personal secretary recollected all this in a documentary not so lng ago.