Code name for the overall deception plan for the Allied invasion of Europe in World War II. The plan was developed to help hide the real date, time, and details of D-Day, the June 6, 1944, Normandy landings. The code name may have been inspired by a remark made by Prime Minister Winston Churchill: "In wartime, truth is so precious that she should always be attended by a bodyguard of lies."

The complex deception plan was developed by the Twenty Committee and coordinated by the London Controlling Section. The Twenty Committee turned German agents in Britian into double agents, who transmitted more than 250 false messages about the invasion. The portion of the plan specifically related to the landings was Fortitude, which included a nonexistent army, the First U.S. Army Group.

To restrict information about D-Day, beginning in April 1944 military leaves were prohibited outside the United Kingdom, civilian travel along the coast was curtailed, and all outgoing mail--including diplomatic mail--was censored. To deceive the Germans about the location landing, separate deception operations were carried out. In Scotland, the British arranged for radio transmissions from nonexistent divisions seemingly poised for an assault on Norway. A British actor who closely resembled Gen. Bernard L. Montgomery posed as him and went to Gibraltar to spotlight the Mediterranean area as a possible invasion site.

The only known leak occurred in Turkey, where a German spy who was the British ambassador's valet learned the Allied code name for D-Day (Overlord) but nothing else about the invasion.