FT. MEADE, Md., –
The landings in the Normandy region of France on June 6, 1944, D-Day, were ( and are) the largest amphibious assaults ever undertaken.
The planning for this operation involved many factors, although COMINT was a major source of intelligence information used by the planners. 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 autum 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 compat 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 mine field, 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.
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."