Operation Overlord
Allied intelligence and counterintelligence efforts were successful. Operation Fortitude before the invasion kept German attention focused on the Pas-de-Calais. High-quality German forces were kept in that area, away from Normandy, until July. Prior to the invasion, few German reconnaissance flights took place over Britain, and those that did saw only the fake camps. Ultra decoding of German Enigma (machine) communications had been helpful as well, which exposed German plans and showed that the Allies' trick was still believed.