Battle of the Bulge
Before the offensive the Allies were not aware of German troop movement. During the liberation of France, the French resistance had provided information about German movements. Once they reached the German border, this information was not available. In France, orders had been relayed within the German army using radio messages coded by the Enigma machine.  These could be picked up and decoded by Allied code-breakers  at Bletchley Park, to give the intelligence known as ULTRA.