Nazi Germany's assault on the Soviet Union in the summer of 1941, Operation Barbarossa, was the largest invasion in history. Almost 3.5 million men smashed into Stalin's Red Army, reaching the gates of Leningrad, Moscow and Sevastopol. But not all of this vast army was German; indeed, by the summer of 1942, over 500,000 were Romanians, Italians, Hungarians, Slovaks and Croatians – Hitler's Axis al...