Operation Barbarossa - Wikipedia

On June 22, 1941, under the code-name Barbarossa, Germany invades the Soviet Union. From the article:

"In the two years leading up to the invasion, Germany and the Soviet Union signed political and economic pacts for strategic purposes. Nevertheless, the German High Command began planning an invasion of the Soviet Union in July 1940 (under the codename Operation Otto), which Adolf Hitler authorized on 18 December 1940. Over the course of the operation, about four million Axis powers personnel, the largest invasion force in the history of warfare, invaded the western Soviet Union along a 2,900-kilometer (1,800 mi) front. In addition to troops, the Wehrmacht employed some 600,000 motor vehicles, and between 600,000 and 700,000 horses for non-combat operations. The offensive marked an escalation of the war, both geographically and in the formation of the Allied coalition.

Operationally, German forces achieved major victories and occupied some of the most important economic areas of the Soviet Union, mainly in the Ukrainian Soviet Socialist Republic, and inflicted, as well as sustained, heavy casualties. Despite these Axis successes, the German offensive stalled in the Battle of Moscow and the subsequent Soviet winter counteroffensive pushed German troops back. The Red Army absorbed the Wehrmacht's strongest blows and forced the unprepared Germans into a war of attrition. The Wehrmacht never again mounted a simultaneous offensive along the entire Eastern front. The failure of the operation drove Hitler to demand further operations of increasingly limited scope inside the Soviet Union, such as Case Blue in 1942 and Operation Citadel in 1943 – all of which eventually failed.

The failure of Operation Barbarossa proved a turning point in the fortunes of the Third Reich.[26] Most importantly, the operation opened up the Eastern Front, in which more forces were committed than in any other theater of war in world history. The Eastern Front became the site of some of the largest battles, most horrific atrocities, and highest casualties for Soviet and Axis units alike, all of which influenced the course of both World War II and the subsequent history of the 20th century. The German armies captured 5,000,000 Red Army troops, who were denied the protection guaranteed by the Hague Conventions and the 1929 Geneva Convention. A majority of Red Army POWs never returned alive. The Nazis deliberately starved to death, or otherwise killed, 3.3 million prisoners, as well as a huge number of civilians through the "Hunger Plan" that aimed at largely replacing the Slavic population with German settlers.[27] Einsatzgruppen death squads and gassing operations murdered over a million Soviet Jews as part of the Holocaust.[28] "