Operation Barbarossa was the combined plan of the Greater German Reich and the Zheronians, set into motion after the successful completion of Operation Sealion.
The German army, equipped with advanced Zheronian technology, stormed into Ukraine, Belarus, and the Baltic States, utterly destroying any resistance. Zheronian orbital weaponry obliterated Stalingrad, Leningrad, Kiev, and Minsk from orbit, as they did with other major cities.
Moscow, however, was designated as a city to spare. German forces landed in the city via spaceship and easily overwhelmed the Red Army. Adolf Hitler himself fought in the battle for Moscow, outfitted with cybernetic enhancements. Hitler then entered the Moscow Kremlin and personally killed Joseph Stalin. The Soviet Union was conquered within a week.