Ghost Army

 
The Ghost Army was a United States Army tactical deception unit during World War II officially known as the 23rd Headquarters Special Troops. The 1100-man unit was given a unique mission within the Allied Army: to impersonate other Allied Army units to deceive the enemy. From a few months after D-Day, when they landed in France, until the end of the war, they put on a "traveling road show" utilizing inflatable tanks, sound trucks, fake radio transmissions, scripts and pretence. They staged more than 20 battlefield deceptions, often operating very close to the front lines.
Their story was kept secret for more than 40 years after the war, until it was declassified in 1996. The unit was the subject of a PBS documentary "The Ghost Army" in 2013.
History and deployment.
Inspiration for the unit came from the British units who had honed the deception technique for the battle of El Alamein in late 1942, as Operation Bertram. The unit had its barracks at Camp Forrest, Tennessee, and was fully formed at Camp Pine, New York (now Fort Drum), before sailing for the United Kingdom in early May 1944. In Britain they were based near Stratford upon Avon, and troops participated in Operation Fortitude, the British-designed and led D-Day deceptions of a landing force designated for the Pas-de-Calais. There was a multitude of people that were involved in the creation of the Ghost Army, but Ralph Ingersoll is credited with its creation, along with his superior, Colonel Billy Harris.
Some troops went to Normandy two weeks after D-Day, where they simulated a Mulberry harbour at night with lights intended to draw German artillery from the real ones. After this the entire unit assisted in tying up the German defenders of Brest by simulating a larger force than was actually encircling them.
As the Allied armies moved east, so did the 23rd, and it eventually was based within Luxembourg, from where it engaged in deceptions of crossings of the Ruhr river, positions along the Maginot Line, Hürtgen Forest, and finally a major crossing of the Rhine to draw German troops away from the actual sites.
Recruiting.
Ghost soldiers were encouraged to use their brains and talent to mislead, deceive, and befuddle the German Army. Many were recruited from art schools, advertising agencies and other occupations that encouraged creative thinking. In civilian life, ghost soldiers had been artists, architects, actors, set designers, and engineers. 
Although the 23rd Headquarters Special Troops consisted of only 1,100 soldiers, the contingent used equipment pioneered by British forces such as dummy tanks and artillery, fake aircraft, and giant speakers broadcasting the sounds of men and artillery to make the Germans think it was upwards of a two-division 30,000-man force. The unit's elaborate ruses helped deflect German units from the locations of larger allied combat units.
The unit consisted of the 406th Combat Engineers (which handled security), the 603rd Camouflage Engineers, the 3132 Signal Service Company Special, and the Signal Company Special.
Tactics.
Visual deception.
The visual deception arm of the Ghost Army was the 603rd Camouflage Engineers. It was equipped with inflatable tanks, cannons, jeeps, trucks, and airplanes that the men would inflate with air compressors, and then camouflage imperfectly so that enemy aerial reconnaissance could see them. They could create dummy airfields, troop bivouacs (complete with fake laundry hanging on clotheslines), motor pools, artillery batteries, and tank formations in a few hours.
Many of the men in this unit were artists, recruited from New York and Philadelphia art schools. Their unit became an incubator for young artists who sketched and painted their way through Europe. Several of these soldier-artists went on to have a major impact on art in the postwar US. Bill Blass, Ellsworth Kelly, wildlife artist Arthur Singer, and Art Kane were among the many artists who served in the 603rd.
Sonic deception.
The 3132 Signal Service Company Special handled sonic deception. The unit coalesced under the direction of Colonel Hilton Railey, a colorful figure who, before the war, had "discovered" Amelia Earhart and sent her on her road to fame.
Aided by engineers from Bell Labs, a team from the 3132 went to Fort Knox to record sounds of armored and infantry units onto a series of sound effects records that they brought to Europe. For each deception, sounds could be "mixed" to match the scenario they wanted the enemy to believe. This program was recorded on state-of-the-art wire recorders (the predecessor to the tape recorder), and then played back with powerful amplifiers and speakers mounted on halftracks. These sounds were audible away.
Radio deception.
"Spoof radio", as it was called, was handled by the Signal Company. Special Operators created phony traffic nets, impersonating the radio operators from real units. Different Morse Code operators each have their own individual style of sending; the Signal Company operators mimicked a departed operator's style so that the enemy would not detect that the real unit and its radio operator were long gone.
Atmosphere.
To complement existing techniques, the unit often employed theatrical effects to supplement the other deceptions. Collectively called "atmosphere", these included simulating actual units deployed elsewhere by the application of their divisional insignia, painting appropriate unit insignia on vehicles and having the individual companies deployed as if they were regimental headquarters units. The same few covered trucks/lorries, with just two troops in the visible seats near the rear to appear to be full of motorized infantry, would be driven in a loop to look like long convoys. "MPs" (military police) would be deployed at cross roads wearing appropriate divisional insignia and some personnel would dress as divisional generals and staff officers visiting towns where enemy agents or scouts were likely to see them. A few actual tanks and artillery pieces were occasionally assigned to the unit to make the "dummies" in the distance appear more realistic.