The Two Women Who Cracked the Abwehr Enigma: How Mavis Batey and Margaret Rock Helped Secure D-Day (a long-form post) Ahead of D-Day, Mavis Batey and Margaret Rock played a leading role in cracking the complex Enigma code of the Abwehr, the German Secret Service. Unlike other machines it had four rotors instead of the standard three, and they rotated randomly, with no predictable pattern. Hut 6 had been unable to break it. On 8 December 1941 Mavis Batey broke a message on the link between Belgrade and Berlin, allowing the reconstruction of one of the rotors. Within days Knox and his team had broken into the Abwehr Enigma, and shortly afterwards Mavis broke a second Abwehr machine. The significance of their work cannot be overestimated. MI5 and MI6 were running double agents as part of the ‘Double Cross System’ and feeding deception and fake intelligence to the Germans. Being able to decode the messages between the Abwehr and their agents ensured that the double agents were really working for the Allies. It also provided a way to monitor whether the Germans had accepted the deception. It ensured the success of the D-Day landings because the Double Cross agents fed information to the Germans that the invasion was to occur at Pas de Calais and not at the real landing points in Normandy. Brigadier Bill Williams, Montgomery’s chief intelligence officer, said that without cracking the Abwehr Enigma, the deception operation could not have been mounted as the Germans would have moved their reinforcements from Calais to Normandy. The successes of D-Day would have had a very different outcome. (end).