1941–1942 invasion of the Soviet Union by Nazi Germany and its allies
Operation Barbarossa was the invasion of the Soviet Union by Nazi Germany and many of its Axis allies, starting on Sunday, 22 June 1941, during World War II. It was the largest and costliest land offensive in human history, with around 10 million combatants taking part, and over 8 million casualties by the end of the operation. The operation, code-named after Frederick I "Barbarossa" ("red beard"), a 12th-century Holy Roman Emperor and Crusader, put into action Nazi Germany's ideological goals of eradicating communism and conquering the western Soviet Union to repopulate it with Germans.
https://en.wikipedia.org/wiki/Operation_Barbarossa