Since well before the war Hitler had looked toward the conquest of the Slavic peoples of eastern Europe and the USSR to provide the additional Lebensraum, or "living space," that he believed the German people needed. He hoped to establish German colonies in those regions, to be served by the despised Slavs.

For Hitler the Nazi-Soviet Pact had been merely a temporary arrangement to be abandoned eventually. After the defeat of France the German chancellor began planning an invasion of the USSR. To avoid fighting a two-front war Hitler first tried to make peace with Britain. After that attempt to clear the western front failed, he launched the Battle of Britain but again failed to put the British out of action.

Nevertheless, full-scale preparations for the invasion of the USSR began in December 1940, for Hitler did not believe that he was risking a two-front war. He felt that Britain, having been expelled from the Continent, no longer posed an offensive military threat. He was convinced that the greater menace came from the Soviets, who (in June 1940) had moved uncomfortably close to the Romanian oil fields.

Originally scheduled for mid-May of 1941, the invasion of the USSR, called Operation Barbarossa, was delayed until June 22 by Hitler's campaign in the Balkans. Launching a blitzkrieg with 121 divisions on a 3,200-km (2,000-mi) front from the Baltic Sea to the Black Sea, the Germans employed a three-pronged assault. In the north they moved on Leningrad via the Baltic States. Moscow, the target of the German center, was approached by forces moving east to Smolensk. In the south the invaders marched toward Ukraine and Kiev, where they planned to turn south to the Crimea and cross the Don River to the Caucasus and to Stalingrad on the Volga River. A smaller force of Romanians and Germans attacked in the extreme south.

As justification for the move Hitler accused the Kremlin of treachery, of threatening German frontiers, and of disseminating anti-German and pro-Communist propaganda. He alleged that the invasion was a crusade against Bolshevism; in addition, however, he was attracted by wheat, oil, and mineral supplies that would enable him to defy the British blockade. So certain was he of victory that he did not even bother to equip his troops for winter.

The onslaught took the Soviets entirely by surprise, and the Germans made startling progress. In the first 18 days the attackers advanced 640 km (400 mi), capturing 300,000 prisoners, 1,000 tanks, and 600 guns. During the first 48 hours alone the Soviets lost more than 2,000 aircraft. The northern forces had entered Leningrad province by July 10 and on August 31 were within 16 km (10 mi) of the city. In the center German troops took Minsk on June 30 and Smolensk, only 320 km (200 mi) from Moscow, in mid-July. Progress in the south was slowed by unexpectedly heavy resistance and rainy weather, but the invaders captured Kiev in late September. More than 1 million Soviet prisoners had been taken by the end of that month. The Soviets retreated, adopting a defense-in-depth strategy, but German victory seemed imminent.

The German invasion of the USSR signaled a change in the alliance structure. Despite his aversion to communism, Churchill promised Stalin economic and technical assistance against the Axis. On July 13, 1941, Moscow signed a mutual-aid pact with London. Offers of help also came from Washington. Italy and the Axis satellites Romania, Czechoslovakia, and Hungary allied themselves with Germany. Vichy France broke off its diplomatic ties with Moscow. Britain severed (August 1) relations with Finland, which the Germans had used as one base for their invasion. Sweden had granted permission for German troops to cross its territory but announced its determination to remain neutral. Despite pressure from the USSR and from Britain, with whom it had an alliance, Turkey, too, proclaimed its neutrality. Japan, which had concluded a mutual nonaggression pact with the Soviets in April and was, in addition, a member of the Axis Pact, adopted a policy of watchful waiting.