The Allies used various forms of intelligence to plan for D-Day, including human intelligence, signals intelligence, and imagery intelligence. By gathering and analyzing this information, the Allies were able to make informed decisions about the timing, location, and strategy of the D-Day invasion.
One key intelligence tool that was used was Ultra, a top-secret program that allowed the Allies to intercept and decode German messages. By breaking the German military's encrypted codes, the Allies were able to gain valuable information about German troop movements, defenses, and intentions.