Operation Bodyguard was the overarching deception strategy for the Normandy invasion, named after Winston Churchill's observation: "In wartime, truth is so precious that she should always be attended by a bodyguard of lies." It coordinated dozens of sub-operations designed to make the Germans defend everywhere at once.
Beyond the well-known Fortitude South (fake army at Dover), Bodyguard included: Fortitude North (a fictional attack on Norway, using fake radio traffic from Edinburgh), Zeppelin (threats to the Balkans), Ironside (a fake attack on the Bay of Biscay), and numerous smaller operations. Each employed combinations of double agents, fake radio traffic, dummy equipment, and diplomatic pressure.
The deception extended to neutral countries: diplomats were told to buy maps of non-target areas, fake invasion currency was "accidentally" leaked, and even General Montgomery had a body double (actor M.E. Clifton James) who appeared publicly in Gibraltar and North Africa to suggest the invasion would come through the Mediterranean. The overall effect was spectacular: on D-Day, the Germans had 58 divisions spread across Western Europe, but only 18 were in Normandy—and they held back reserves for weeks.