A few things
1. The Tactic Blitzkrieg which was(as some may know) very affective and shaped modern warfare
2. The German's Tanks where the best in the world and in a 1v1 between a T-34 and a Panzerkampfwagen(I know it's a long word) the Panzer would win
But on a smaller note the USSR had the world's largest army so they had manpower over the German's
