Operation FORTITUDE, the D-Day deception plan, was a near perfect plan used by the Allies during World War II to deceive the Germans as to the time and place of the Normandy invasion. This short research paper studies the methods and techniques used by the Allies, specifically the British Security Services, in the near flawless execution of the deception plan. This paper also proposes that the plan was so well executed because it was a "closed loop" plan. That is to say that the British controlled not only the information going forward to the Germans, but they were also in the enviable position of being able to determine the exact extent of the Germans' belief in the veracity of the information that they were given. This was due to two factors: the British had complete control of all German agents in England by the second year of the war, and the British were able to read encrypted German message traffic, often as fast as the intended recipients. In the final analysis, the Germans were completely outmaneuvered in the intelligence department during the Second World War. Through sloppy work on their part and the amazingly well manufactured deception story put forth by the Allies, the Germans were essentially blind while trying to defend the Normandy beaches. The research for this paper was conducted solely using open-source material. Many of these were secondary sources, though others were recently declassified operational documents from the British and United States historical records.