Spies spend as much time feeding false information to their enemies as they do gathering information. This keeps them guessing, forces them to miscalculate military capabilities and commit forces to the wrong area. A steady stream of misinformation can even damage the real information the enemy has, because they will begin to doubt the authenticity of their own intelligence gathering activities.
One method of spreading misinformation is the double agent. Imagine that a U.S. scientist is recruited by the Russians to supply American military technology. The United States becomes aware that this scientist is spying for the Russians. Instead of arresting him, they allow him to continue feeding information to the Russians. However, they make sure that the blueprints, technical readouts and other data he has access to are altered. The Russians are now getting technical information that is useless. They might spend millions of dollars on research into flawed technology. Thus, the scientist is an unwitting double agent.
Alternately, the United States could confront the scientist and threaten him with a long prison sentence (or even the death sentence, the penalty for treason). To avoid this, he agrees to intentionally turn double agent. Not only does he knowingly supply the Russians with false information, but he works to gain information from his Russian controller. He might provide the United States with the names of other Russian spies, or intelligence on the level of Russian scientific research.
There is always the potential for this same scientist/spy to turn triple agent. That is, he informs the Russians that the Americans caught him. Now, the Russians know to disregard the technical information that he provides, and in turn, they supply misinformation back to the Americans. If it seems confusing, imagine trying to keep it all straight when a mistake could cost you your life. Some agents have even gone beyond triple agent, playing the two sides against one another and creating such a tangled web that historians have no idea whose side the spy was really on.
Operation Fortitude was one of the grandest and most successful misinformation campaigns ever conducted. The goal of Fortitude was to fool the Germans into withholding their strongest military units or putting them in the wrong place when the Allies invaded Normandy in 1944. Wooden and cardboard airplanes, fake fuel depots and even dummy troops were massed in southern England to make the Germans think the attack would come from there, rather than at Normandy in the north. A completely fictional U.S. Army group was created: FUSAG (First U.S. Army Group), which even had General George Patton leading it. False radio traffic supplemented the deception. The most important element, however, was the misinformation provided to the Germans by double agents. Information supplied by a double agent code-named Garbo convinced Hitler that the attack would come from the south.
To keep up the pretense and delay the arrival of German reinforcements in Normandy as long as possible, the day of the invasion even featured a fake landing force with loudspeakers playing the sounds of a giant fleet moving across the English Channel, with radar-reflecting balloons and metal strips dropped by planes creating the radar signature of a large invasion. Once the attack at Normandy was underway, Garbo told his German handlers that it was just a feint meant to draw German troops away from the "real" attack to the south.
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