The Soviet forces defeated the Wehrmacht through a combination of political, tactical, strategic, and logistical factors:
1) Hitler made several strategic blunders that weakened the Wehrmacht, including interfering with its structure and issuing brutal orders targeting civilians.
2) Stalin's purges had decimated the Red Army officer corps but survivors later proved effective commanders.
3) Stalin was able to rally the Soviet people to total war through patriotic speeches and brutal policies, mobilizing the entire nation and limiting German gains.
4) The Wehrmacht's blitzkrieg tactics were ineffective in prolonged campaigns, and it suffered heavy attrition and equipment losses on the vast Eastern Front.