Others defend Stalin’s greatness on the basis of his role in repelling the Nazi invasion and ultimately defeating Hitler. Yet Stalin’s purge of military leaders during the Great Terror critically weakened the Red Army, his signing of the Nazi-Soviet Non-Aggression Pact in 1939 paved the way for the start of World War II, and his blindness to the Nazi threat in 1941 left the Soviet Union unprepared to resist Hitler’s attack.