On 22 June 1941, despite the existing non-aggression treaty between Hitler and Stalin, the Germans invaded Russia under codename Operation Barbarossa. The surprise attack was formidable, with some 4.5 million troops advancing along a near 1,800-mile (3,000-km) front. There were three main divisions and targets: Army Group North for Leningrad, Army Group Centre for Moscow and Army Group South for Kiev. Once again, Blitzkrieg tactics proved powerful, and within a week the Germans had advanced over 200 miles (320 km) into Soviet territory, destroyed thousands of aircraft and taken hundreds of thousands of Red Army soldiers prisoner. Army Group North reached Leningrad and began their blockade of the city and Soviet forces had been overcome at Minsk and Smolensk. Yet by September German progress had slowed, particularly after counterattacks at Kiev and Bryansk. In October, Moscow was in sight (‘Operation Tycoon’), but heavy rains and mud stalled the advance, allowing the Soviets to strengthen and plan a counterattack.
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