It's the 70th anniversary of the Normandy landings.
For Nazi Germany, it was not "if" an invasion of continental Europe would come. It was a matter of "when" and "where."
For the Allies, invasion required deception. At the Teheran Conference in December 1943, British war planners developed a scheme known as "Operation Bodyguard." The goal of the plan was to generate fake radio traffic, use double-agents, and leak false information to try to convince Nazi Germany that the planned Allied invasion of Europe would occur any place except Normandy. Operation Bodyguard was the plan they conceived. Operation Bodyguard had three components. Fortitude North was designed to give the Nazis the impression that Norway was the planned invasion site. Fortitude South was designed to give the impression that the invasion would come at Calais in France. Zeppellin was designed to give the impression that the invasion would come in Romania or the Balkans.
Fortitude South, however, got the most attention from Allied intelligence. A major component of Fortitude South was its attempt to physically fool German intelligence through visual elements. This generated "Quicksilver" -- a plan to actually create a dummy First U.S. Army Group around General George S. Patton. The German High Command simply refused to believe that the Allies would put all their armies under the command of one general -- and a British general at that. Thus, they assumed that there would be an American army group, and began looking for it. Once Allied intelligence services understood that the Germans had made this assumption, they decided to build on it and create a fake First U.S. Army Group. Fake barracks made out of canvas and sticks, fake airfields painted on the ground, fake airplanes painted on the ground or made of cloth and stakes, fake tanks made out of balloons, and fake radio traffic were used to convince the Nazis that the invasion would be targeted at Calais. Although the Germans were never fed fake documents, German spies (every single one of whom had been turned into a double-agent) reported fake troops movements, fake tank and airplane production, and fake troop training and coastal assembly. Once the Allied invasion of Normandy got close, "Quicksilver" gave way to "Cover." This operation involved a feint rather than deception. All Allied bombing of the French coast would intensify just before the invasion. But "Cover" sent Allied bombers in large numbers over Calais rather than Normandy to convince the German High Command that Calais was the planned invasion route.
Finally, the night of the invasion, "Cover" sent British and American planes at low altitude over the English Channel near Le Havre where they precision-dropped large amounts of tinfoil strips in the air. The airborne tinfoil fooled German radar into thinking that a large fleet was steaming toward Le Havre rather than toward Normandy. The same night, dummies were dropped via parachute over over Le Havre and Isigny, leading the Germans to believe that a paratroop assault was occurring there.
A effort similar to but far less elaborate that "Quicksilver" was instituted under Fortitude North. Called "Skye," this deception effort attempted to convince the Germans through fake radio traffic alone that Norway was the true object of the invasion.