22-06-1941 Operation Barbarossa.
Operation Barbarossa, German: Unternehmen Barbarossa, was the code name for Nazi Germany’s ‘s invasion of the Sovet Union, which began on 22 June 1941. Over the course of the operation, about four million soldiers of the Axis powers invaded Soviet Russia along a 2,900 kilometer front, the largest invasion force in the history of warfare. In addition to troops, the Germans employed some 600,000 motor vehicles and between 600–700,000 horses. The operation was driven by Adolf Hitler’s ideological desire to conquer the Soviet territories as outlined in his 1925 manifesto Mein Kampf (“My Struggle.”) It marked the beginning of the rapid escalation of the war, both geographically and in the formation of the Allied coalition. 
Prior to the invasion, the two countries had signed political and economic pacts for strategic purposes. Hitler authorized an invasion of the Soviet Union on 18 December 1940 for a start date of 15 May 1941, but this would not be met, and instead the invasion began on 22 June 1941. Operationally, the Germans won resounding victories and occupied some of the most important economic areas of the Soviet Union, mainly in Ukraine, while sustaining heavy casualties. Despite these successes, the German offensive stalled on the outskirts of Moscow and was then pushed back by a Soviet counter offensive without taking the city. The Germans would never again mount a simultaneous offensive along the entire strategic Soviet-German front. The Red Army repelled the Wehrmacht’s strongest blow and forced Germany into a war of attrition, for which it was unprepared.
Operation Barbarossa’s failure led to Hitler’s demands for further operations inside the USSR, all of which eventually failed, such as continuing the Siege of Leningrad, Operation Nordlicht, Operation Blue, and Operation Citadel, among other battles on occupied Soviet territory.
Barbarossa was the largest military operation in world history in both manpower and casualties. Its failure was a turning point in the Third Reich’s fortunes. Most importantly, the operation opened up the Eastern front, to which more forces were committed than in any other theatre of war in world history. Regions covered by the operation became the site of some of the largest battles, deadliest atrocities, highest casualties, and most horrific conditions for Soviets and Germans alike, all of which influenced the course of both World War II and the subsequent history of the 20th century. The German forces captured millions of Soviet prisoners who were not granted the protection stipulated in the Geneva Conventions. Most of them never returned alive. Germany deliberately starved the prisoners to death as part of its “Hunger Plan”, aiming to reduce the population of Eastern Europe and then repopulate it with ethnic Germans. When the notorious Russian winter (nicknamed “General Winter”) set in, German advances came to a halt. By the end of this, one of the largest, deadliest military operations in history, Germany had suffered some 775,000 casualties. More than 800,000 Soviets had been killed, and an additional 6 million Soviet soldiers had been wounded or captured.
