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News | June 6, 2024

History Today, June 6: The role of signals intelligence or 'ULTRA' on D-Day

The term D-Day was a shorthand expression first used in World War I to denote the date an operation was to be launched. In the earlier war, officers also used H-HOUR and M-MINUTE, but these were seldom used in World War II. Because of the scope of the 1944 operation and the momentous stakes, in common parlance, “D-Day” has come to refer primarily to the landings in Normandy.

The Germans had occupied France since 1940. When the Americans entered the war in December 1941, U.S. strategic thinking called for an immediate landing in France in conjunction with our British allies, followed by an advance to liberate the country and then press on to Berlin. Britain’s high command argued against this course of action, pointing out, correctly, that the Germans were well dug in, American forces lacked experience in combat against the powerful foe, and neither country had yet assembled the reserve of men and materiel such an effort required.

As a consequence, the Allies battled the Germans in North Africa, Sicily, and Italy - but by spring 1944, the time had come to land in France and carry the battle to the German homeland. Hundreds of thousands of American, British, and Canadian men readied to land on five beaches in Normandy, France, to face well-prepared German defensive positions.

The planning for this operation, codenamed OVERLORD, was complex, but the strategic planning staff had an important asset — SIGINT. This was ULTRA, the product of cryptanalysis of high-grade enemy cryptosystems such as the now-famous ENIGMA machine. Crucial information also was derived from decrypts of reports written by the Japanese ambassador to Germany, who had toured the beachfronts of France in the autumn of 1943.
Those who study intelligence know that ULTRA gave planners access to copious amounts of information about the German weaponry emplaced along the beaches, the order of battle of the defensive units, and the standing orders given to the defenders.

Less well known but no less important was the information on German defensive mines in the English Channel. This was a vital factor, since Britain and the United States were transporting their combat units across the channel in hundreds of ships.

ULTRA provided a great deal of data on German mine laying. Some of it came from communications of the boats actually creating the minefield, some of it came from instructions to German ships about cleared areas for their sailing. The information included types of mines used, as well as boundaries for closed and open channels.

This information allowed the Allies to select mine-free routes for the ships carrying the landing parties and identify areas where minesweeping actions would be a priority.

The official historian of British COMINT in World War II wrote, “Largely with the assistance of SIGINT, though not without much tedious analysis of it . . ., the programme was reconstructed in considerable detail — a fact which proved to be of considerable importance for the success of the landings.”

In addition to ULTRA, U.S. ground forces had tactical COMINT personnel who accompanied deployed troops and provided intelligence from low-level German or Japanese communications.

The U.S. Navy also had tactical COMINT teams aboard ships in the Mediterranean and European Theaters of War, called the Y Service, a term borrowed from British usage. One of the primary missions of these teams was to provide warning of enemy air attacks and to jam German radio-controlled bombs.

Initially, the U.S. Navy had to borrow intercept operators from the U.S. Army or the Royal Navy. In early 1944, the commander of U.S. naval forces in Northwest African waters asked the Chief of Naval Operations to send twenty-four men for training in Y Service operations. He noted that the candidates should be of good intelligence, without family ties in Axis countries, wholly trustworthy, and be thoroughly fluent in idiomatic German; if any had a knowledge of German shorthand, that would be especially desirable.

In March two officers and ten enlisted men were dispatched from the U.S. to Europe for Y Service training, which was to be provided by the British admiralty.

As Allied forces prepared for Operation NEPTUNE, the naval phase of the Normandy landings, seven naval Y teams were deployed. Three of the teams had only British personnel; the other teams had mixed U.S. and UK personnel. It was felt that training alone was insufficient for success; the U.S. had to overcome lack of experience by integration of personnel with its ally.

During the D-Day landings and afterward, the Y teams undertook twenty-four-hour coverage. This began on June 5 and continued through June 18. As one later report put it, “. . . [I]n the case of the Normandy Operation, Y service proved to be of little assistance because of the general lack of enemy aircraft and naval surface craft in the face of overwhelming Allied surface and air power.”

The Y Service teams were disbanded in January 1945. By this time, the German naval and air forces were no longer a threat to U.S. and British movement of troops and support for them from the British Isles to France.

Today is the 80th anniversary of D-Day, still the largest amphibious attack conducted in the face of an armed enemy. The sacrifice in life by British, Canadian, and American troops was heavy on this day in 1944, but the successful landings truly marked the beginning of the end for Adolph Hitler and Nazism.