Operation Barbarossa (June 1941)

On June 22, 1941, Germany invaded the Soviet Union, breaking the Nazi-Soviet Pact. This was the largest military operation in history, involving over 3 million Axis troops along a 1,800-mile front. The Soviet army was initially shattered. Within months Germany was at the gates of Moscow and Leningrad. The Soviet leadership relocated industrial plants eastward and called for a Great Patriotic War.

The German advance stalled in winter 1941, partly because of stiffening Soviet resistance, partly because German planning had assumed a short campaign and lacked winter equipment. From this point forward, the Eastern Front became the largest and bloodiest theater of the war.