The Nazi invasion of the Soviet Unions Operation Barbarossa on June 22nd 1941 was one of the biggest military operations in history. The invasion was not only military but also ideological and racial.
Hitler's policy of Lebensraum and his anti-semitism and general meant this was a war fought with no mercy. Losses were massive, an estimated 27 million Russians lost their lives, most of them civilians. As a consequence the pressure on the Soviet Union was immense as they were absorbing virtually on their own the power of the German military along with their Italian, Romanian and Hungarian allies.