“By the spring of 1944, some 5,600 people worked on deciphering, translating, or interpreting messages intercepted by legions of radio operators located throughout Nazi occupied Europe.”

By John Prados

THE FIRST-GENERATION histories of D-Day and the breakout from Normandy were written at a time when the success of Allied codebreakers remained a deep secret. Hence the contributions of Ultra, an umbrella term for the product of work against the German codes, was lost to history. Conversely, since the revelation of the program in the 1970s this intelligence source has often been represented as omniscient, leading many to believe that the Allies were supremely aware of every Nazi maneuver. Neither version is correct. There were inherent limitations on Ultra’s potential, but given those boundaries, it’s impressive what advantages the codebreakers provided for the Allied side, particularly during Operation Overlord and the Normandy campaign.

For the European Theatre, codebreaking activities were centred at Bletchley Park, the location of the British Government Code and Cypher School (GC&CS). By the spring of 1944, some 5,600 people worked there on deciphering, translating, or interpreting messages intercepted by legions of radio operators located throughout Nazi occupied Europe.

Few realize the sheer scope of communications produced by the German military. For example, in one single day, the Führer headquarters alone fielded 120,000 telephone calls, sent or received 33,000 messages by high-speed teleprinter (geheimschreiber), and dealt with 1,200 radio messages. These figures from come Hitler’s operations staff chief, Colonel General Alfred Jodl, who, on Jan. 31, 1945, chose to record the numbers for just one 24-hour period. Of course, only radio messages—if intercepted—would be vulnerable to decryption. Teleprinter traffic was secure so long as it went by landline, although sometimes radio transmission became necessary.

The other key drawback was that Ultra could only process information that was communicated. For example, Hitler gathered the German commanders Von Rundstedt and Rommel at Margival on June 17 where they made a strategic decision to respond to the Allied invasion by means of a multi-corps offensive. Many German actions over subsequent weeks concerned gathering the forces for such an attack, finding targets, or checking Allied moves that could undermine the counter-offensive. But the only message traffic about Margival concerned Hitler’s movements or those of his generals – the actual details of the discussions were unknowable to British and American intelligence.

Given those caveats, a survey of Ultra’s revelations during the Normandy campaign shows just how valuable it still was:

• Ultra plus radio direction finding identified the headquarters of the Germans’ Panzer Group West. It was subsequently hit by a powerful air raid on June 10, which wounded its commander, Colonel General Geyr von Schweppenberg.

  

• Hitler suspected that his generals were defying orders to send more troops into the Norman port of Cherbourg, so he demanded a run-down of the units present at the garrison. Ultra intercepted the June 18 response, which provided Allied leaders with a complete order of battle of the Germans at Cherbourg.
• On June 24, Ultra intercepts revealed that the II SS Panzer Corps, with 9^th and 10^th SS Panzer divisions, were arriving at the front in Normandy, but that they had had to detrain in eastern France, almost a week earlier. This bespoke the effectiveness of the French Resistance and Allied air attacks in disrupting German communications.

  

• On July 5, Ultra warned that the powerful Panzer Lehr Division would transfer from the British to the American sectors in Normandy. Allied commanders responded with air attacks on roads throughout the region, delaying the German redeployment. A few days later Ultra warned Panzer Lehr would attack.
• To help General Montgomery’s operations, on July 10 and 14 Ultra provided an overview of German forces defending the Bourguébus sector.

  

• When the Americans moved to break out of their end of Normandy by means of Operation Cobra, Ultra provided a succession of intelligence tips—quickly. It revealed that German stocks of artillery shells were running short and on July 26, it provided a complete order of battle for the defending German LXXXIV Corps. On July 29, it uncovered details of an armoured attack into the U.S. flank. Finally, on July 30 it picked up word that the battered Panzer Lehr Division was being withdrawn.
• During the first week of August, 1944, Ultra reported the concentration of forces for what would become known as the Germans’ Mortain offensive. When the enemy advance appeared to have failed, on August 9, an order from the Führer to continue the attack, even though the units involved risked being caught in the developing Falaise Pocket. Ironically, Hitler sent his order by radio because, after the July 20 attempt on his life, he did not trust the landline networks to transmit his directives.

  

• On August 17, Ultra intercepted six of the ten parts of the message from the German high command in the West to withdraw from Normandy, beginning the Nazi maneuver to escape the cordon at Falaise. That the Allies ultimately could not completely encircle the enemy was a product of tense command decisions, German desperation and field coordination problems, not a lack of intelligence.

Lyman B. Kirkpatrick, an intelligence officer with Omar Bradley’s American army group, once wrote that 70 percent of the best information came from combat intelligence, by which he meant aerial scouts, the Resistance and prisoner interrogation. This list of Ultra accomplishments shows that Kirkpatrick indulged in a bit of deception of historians. Alternatively, the remaining 30 percent includes some pretty incredible intelligence, which went far towards ensuring Allied victory in Normandy.

ABOUT THE AUTHOR: John Prados is a senior fellow of the National Security Archive in Washington D.C., and an historian, author of Normandy Crucible: The Decisive Battle That Shaped World War II in Europe (Amberley).