June 6 marks the eighty-first anniversary of D-Day, when Allied troops crossed the English Channel, landed under heavy fire on the beaches of Normandy, and launched the successful campaign to liberate Europe from Nazi control. It was one of the riskiest amphibious assaults in military history — not only because of the size of the operation, but also because everyone on the continent knew that it was coming. The German High Command anticipated the timing and location of the Allies’ arrival; but for robust counterintelligence measures, brilliant logistical planning, and good fortune, June 6 could easily have become a day...