The Soviet T-34 was one of the finest tanks of the Second World War and the mainstay of Soviet armoured units throughout the war. Most nations underestimated the scale and quality of Soviet tank production before the Second World War and the Germans were no exception. They were certainly not prepared for the T-34, which they encountered during Operation Barbarossa (the German invasion of Russia) in 1941. Its combination of firepower, mobility, protection, and ruggedness led German Panzer General Paul von Kleist at the time to call it "The finest tank in the world." Another legendary Panzer tactician and general, Heinz Guderian, also confirmed the T-34's "vast superiority" over existing German armour of the period.