Operation Barbarossa, the German invasion of the Soviet Union, began the largest and most costly campaign in military history. Its failure was a key turning point of the Second World War. The operation was planned as a Blitzkrieg to win Germany its Lebensraum in the east, and the summer of 1941 is well-known for the German army's unprecedented victories and advances. Yet the German Blitzkrieg depended almost entirely upon the motorised Panzer groups,...