Invading Russia was a mistake from the beginning. Hitler had men fighting on the Eastern front and now the western front which would later on cause the Germans to be attacked from two directions and create major problems as the war progressed. I believe that the Germans would have had a really good chance of winning WW2 because they lost nearly one million men in Russia if I'm not mistaken and they could have used those men for the defence at D-day and many other important battles.

I remember watching on the Military Channel that Stalingrad was supposed to be taken within a week, back in September of 1942. The Germans kept fighting regardless and eventually winter set in. Since the Germans were not equipped for winter weather and they were in a desperate need of a supply line, it was inevitable that defeat would soon follow. There were also other contributions to their defeat at Stalingrad such as the Soviet snipers making a major contribution plus the two Soviet armies surrounding the German 6th army. One army made sure the Germans couldn't escape and the other army made sure supplies couldn't be brought into the city. Hitler also ordered that the 6th army not leave the city, which was a crucial mistake on his behalf.