Germany wanted to invade the Soviet Union because from their they could expel/exterminate the Jews, Slaves, and Bolsheviks and create more "living space" for resettled Germans, Hitler believed this would be easy because of their financial problems. They were able to capture the Russian heartland, put Leningrad under siege, and have troops outside of Moscow but Germany had underestimated the Russian troops. They had a lot more people than they realized, a better industrial capacity and the Russian winter was hard for the ill-prepared German troops. Although this did stall the troops in the spring the German forces regrouped and inflicted heavy loses to the Soviet forces. This was important because it brought the Soviet Unions much needed force for the Allies and allowed for German defeat at Stalingrad.