Operation Barbarossa, the largest military operation of all time, was Nazi Germany's invasion of the Soviet Union which began in summer 1941. It was planned for earlier in the year, but was delayed due to Germany's need to conquer Greece first after Italy's disastrous effort to attack that country had met with failure.
The operation against the Soviet Union was a 'Blitzkrieg' ('lightning war') that saw the German Wehrmacht and its allies advance cover greater distances in less time than any of their other campaigns. Of course it would also need to, due to the vast land area that made up the Soviet Union and Russia in particular. The Luftwaffe worked in co-ordination with the Panzer Divisions to ensure as rapid an advance as possible. The attack aimed to reach the capital of the USSR, Moscow, before the dreaded Russian winter set in, but failed to do so. The Russians threw back the German armies at that time; their first military success.
The Germans began their offensive anew in 1942 after the winter had passed and again met with success, but the Russians were bolstered by supplies from the Allies including some from the United States and their continued mobilization of their own great manpower reserves. Also, the addition of the T-34 tank gave them a weapon that was in many ways superior to any other tank that had been seen in the war. The Battle of Stalingrad in late 1942/early '43 was the turning point in the War on the Eastern Front, and a Nazi victory became practically impossible after that as the size of the Soviet forces and their fighting ability continued to grow.