When Nazi Germany invaded Poland on September 1, 1939, launching World War Two, its army seemed an unstoppable force. The Luftwaffe bombed towns and cities across the country, and fifty divisions of the Wehrmacht crossed the border. Yet only two decades earlier, at the end of World War One, Germany had been an utterly and abjectly defeated military power. Foreign troops occupied its industrial heartland and the Treaty of Versailles reduced the vaunted...