How did Ultra reveal the locations of German counterattack forces before D-Day?

Through the Ultra intelligence project, Allied code breakers at Bletchley Park were able to decrypt high-level German communications, providing a clear picture of where German counterattack forces were deployed.12

Specifically, the Allies successfully cracked the Tunny cipher machine, which was used for the most sensitive strategic communications between the German high command and generals in the field. By reading these messages, Allied planners gained detailed knowledge of German strategy and their defensive preparations for the anticipated invasion of northern France.2

This intelligence was vital for the success of Operation Overlord, as it allowed the Allies to monitor the effects of their own deception campaigns, such as Operation Fortitude. By intercepting German transmissions, the Allies could confirm that their disinformation—which suggested the invasion would occur at Pas-de-Calais rather than Normandy—was effectively persuading the German high command to withhold reinforcements from the actual landing sites.13