Operation Dragoon followed on 15th August ’44 in the South of France.
In my post on French Hagelin cipher machines , there was a statement by cryptanalyst Buggisch of the Army’s Signal Intelligence agency Inspectorate 7/VI , [Ticom I-92,p3] : ‘’ ……. Buggisch spoke especially of the successful solution of C36 in 1943, on de GAULLE traffic to CORSICA. He also said that the Southern France landings were largely given away as to date and strength of force by broken C36 traffic.’’
Buggisch was telling the truth. From HW 40/7 ‘’ German Naval Intelligence successes against Allied cyphers, prefixed by a general survey of German Sigint’’ , p29 :
In the Mediterranean area the Germans continued to derive a certain amount of information from low grade French traffic. On 11th August, 1944 a German Army B reports seen in Special Intelligence quoting a Free French signal, thought to be made in Hagelin, which gave details of the allocation of shipping space for the eminent Allied landing in Southern France. The time lag in issue was only about 10 hours, and on the basis of this B-report the German Admiral commanding South coast of France was warned on 14th August of the probability of a landing in his area in the near future .