During my readings of WW2, a common theme was Stalin badgering Churchill to open a Second Front because he felt that Russia was fighting the Germans all by themselves. But from winter of 1941 on, those Russians were chewing the Germans up inch by inch.
Invading Russia was probably the biggest mistake Hitler made.
If Hitler had invaded Russia 3 or 4 monthe earlier as the original plan...the Germans would have not been bogged down in the very harsh russian winter and may have destroyed the Red Army and the commies. But Hitler delayed the invasion to help the Italians finsh off the italians weak effort against Greece. I read that little tidbit in “The rise and fall off the third reich” A master (to a point) political manipulator and conniver but a very poor general. Many German generals felt the war was over as soon as it began.