Before the outbreak of hostilities on the Eastern Front between Nazi Germany and the Soviet Union, the Germans and the Soviets had signed a non-aggression pact in August 1939, which provided for the division of the Baltic States, Finland, Poland and Romania into "spheres of influence." The Invasion of Poland and the Winter War were the military result of that agreement. However, given that the Soviet Union embodied the ideological and racial enemy of Nazism, that "peace" could not last long.
The Invasion of the Soviet Union by Germany and her allies, code-named Operation Barbarossa, began on June 22, 1941. During the summer and autumn, following the effect of initial surprise, massive successes of the Wehrmacht in front of the Red Army resulted especially after successive encirclement of hundreds of thousands of Soviet soldiers.
Bundesarchiv, Bild 101I-136-0883-25A / Cusian, Albert / [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/de/deed.en)], via Wikimedia Commons
However, the German supply lines had been significantly extended, their losses and attrition had begun to be felt, and the weather became an ally of the Russians. Blitzkrieg reached its limits during the Battle of Moscow, the Soviets launching in early December a counteroffensive that shocked the Germans.
By spring 1942 the Germans had managed to stabilize the front in the center, and in summer the initiative passed on their side again, this time in the southern part of the front. The Wehrmacht's target was the Caucasus petroleum, but it was necessary to secure the river banks of Don and Volga, which led to the Battle of Stalingrad.
In the fall of 1942, an urban battle of an intensity unseen before was fought for Stalingrad, but after three months, when the city was almost entirely in German hands, the Red Army launched a shattering offensive on the German flanks, defended by two Romanian armies, surrounding the German 6th Army.
By WebMining 296 (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/deed.en)], via Wikimedia Commons
This moment was considered the turning point on the Eastern Front as the strategic initiative passed on the Soviet side. The Germans relief attempt failed, and the Soviets continued their offensive until February 1943, defeating the Italian and Hungarian armies, and regaining vast areas of the lost territory.
In the summer of 1943, the Germans made a last major attempt to change the tide in the East, attacking the salient which had formed around the Russian city of Kursk. Informed of the German intentions, the Soviets had prepared a powerful defense in depth, so the German offensive in the Battle of Kursk was not successful.
The Soviets resumed their attacks towards west, with an overwhelming numerical superiority until the end of the war. In the summer of 1944, the Red Army launched Operation Bagration against Army Group Center, inflicting losses on the Germans almost similar to those of the Battle of Stalingrad.
Although it did not have anymore strategic offensive capabilities, the Wehrmacht still proved a formidable opponent in defensive battles. In early 1945, the Soviets paid a heavy price for victories in Budapest and East Prussia. The same scenario of fierce German resistance was repeated in late April, when the Battle of Berlin was fought, which brought the end of the Third Reich. The war waged by the Soviet Union was fundamental to the Allied victory since the overwhelming proportion of the German military deaths were recorded on the Eastern Front.