On June 6, 1944, after nearly two years of preparation, American, British, Canadian, and Polish troops crossed the English Channel and landed in Normandy, often in the face of heavy German resistance. The goal of Operation OVERLORD, commonly known as D-Day, was to open a second front against Hitler and liberate occupied Europe from the Nazis. Intelligence played a vital role in the invasion’s success.
Reading Enemy Communications
The ability to decode a variety of German communications was important to preparations for what was sure to be an extremely dangerous operation. For years, the Allies had read traffic the Germans encoded on their Enigma machines. But just weeks before the landings, British cryptanalysts began using a high-level German cryptomachine codenamed TUNNY. From its decoded messages, Allies identified previously unlocated units and learned of German defense plans.
In addition, the Japanese ambassador to Berlin, Lt. Gen. Hiroshi Oshima, reported often to Tokyo on German Army operations. His messages, and those of the Japanese military attaché in Germany, used a code that American cyptologists had solved in 1940. He unknowingly provided detailed information on German strategies and deployments in Normandy.
Intelligence as a Guide
Senior Allied commanders received detailed information on German strength and activities from signals intelligence (SIGINT) and photo-reconnaissance. Officers not cleared for communications intelligence (COMINT) still received orders based on that secret knowledge. Cryptology supported Allied planners in charting the German order of battle, understanding Hitler’s defense plans, and determining the best places to strike enemy forces. Last-minute information from the TUNNY system, for example, was crucial in determining final drop zones for British and American paratroopers.
Even with good intelligence, victory on D-Day was by no means certain. The Germans knew the Allies were preparing to attack them in France. Prior to the air and naval assault, the supreme Allied commander, General Dwight Eisenhower, prepared two statements for release to the public–one if the landings were successful, and a second if they were not. The courage, skill, and determination of the Allied forces carried the day.
Sound Information Saves Lives
Operation OVERLORD, and the freedom it did so much to bring, came at a high human cost. But the casualties could have been much worse. Intelligence—particularly from German communications—was vital to success. It was a strong contributor to the good planning that made the invasion of France possible, and it helped save Allied lives.