On June 6, 1944, 150,000 Allied troops landed on the beaches of Normandy and suffered an astonishingly low rate of casualties. D-Day was a stunning military accomplishment, but it was also a masterpiece of trickery. Operation Fortitude, which protected and enabled the invasion, and the Double Cross system, which specialized in turning German spies into double agents, deceived the Nazis into believing that the Allies would attack at Calais and Norway rather than Normandy. It was the most sophisticated and successful deception operation ever carried out, ensuring that Hitler kept an entire army awaiting a fake invasion, saving thousands of lives, and securing an Allied victory at the most critical juncture in the war.