German Invasion of the Soviet Union(begins June 22, 1941)–In June, 1941, Nazi Germany made the ultimately fatal mistake of invading the Soviet Union. The actual invasion was delayed by the Axis offensive in the Balkans (Hitler had to save Mussolini from himself–the Fascist Italian invasion of Greece went very badly) and in the process the Germans also invaded Yugoslavia and coerced the Hungarians, Romanians and Bulgarians into joining the Axis. While the Balkan offensive was successful for the Axis, it did delay the timetable for attacking the Soviets, which also limited the time available for defeating the Russians before winter set in.
The invasion of Russia by a combined Axis force of 3,800,000 troops (Germans, Italians, Romanians, Hungarians, and Bulgarians) began on June 22, 1941. Despite huge gains in the first few weeks of the invasion, the attackers were unprepared for the savage Russian winter and the equally savage resolve of the Soviet forces. With material assistance from the Western Allies (including the United States after Pearl Harbor), and the opening of secondary fronts in Italy and France, the Soviets gradually forced the Axis armies back into Central Europe and seized Berlin in the spring of 1945.