ULTRA became the code word the Allies used to identify intelligence produced by decrypting enemy communications during World War II. The British Admiralty first used the term in May 1940 to enable commanders to evaluate the source of the intelligence dispatched to them. By 1945, ULTRA was in official use by American, Australian, Canadian, and British intelligence agencies and field armies and navies worldwide. ULTRA's sources were mainly decrypted German and Japanese military and naval ciphers, although some codebreaking successes were achieved against the Italian armed forces as well. In general terms, the Germans relied on numerous machine‐generated ciphers, while the Japanese resorted to multiple book‐based cipher systems. Success in reading one code did not translate into a wholesale breakthrough into all existing code systems. Nor did breaking one code guarantee continual solution of the system because the enemy introduced changes on a routine basis to preclude just such a possibility.

The first German code penetrated was the German Luftwaffe (Air Force) system. It relied for security on an ENIGMA machine to encrypt messages. The British began their uninterrupted reading of this cipher in May 1940 and used its secrets to great advantage during the Battle of Britain. Of greater importance was the breaking of the German U‐boat cipher. Some penetrations were made in the system in 1940, but not until April 1943 did sustained and timely decryption of German submarine messages became possible. ULTRA's contribution to victory in the Atlantic cannot be overstated. By enabling the Allies to know where German submarines lurked, ULTRA allowed commanders to reroute convoys around the wolf packs or to direct aircraft and warships to attack the submarines. Depriving the U‐boats of their greatest advantage, stealth, ULTRA made possible the great trans‐Atlantic convoys that first kept Britain in the war, then fed the buildup for the D‐Day landing, and finally nourished the Allied drive across Western Europe.

ULTRA's role in the ground war in the west was a mixed one. The Allies deciphered few German Army messages until the summer of 1942. By reading the porous Luftwaffe ciphers, however, they gained significant intelligence about ground dispositions. Yet it required a frustrating learning period to develop a system to distribute ULTRA where it was most needed—into the hands of the field commanders. In 1941, for instance, ULTRA revealed in precise detail the German plan to attack the Mediterranean island of Crete. Without a distribution system for the precious intelligence, London could order immediate action, but field commanders hesitated, either unsure about the reliability of the source or fearful of betraying it by acting on its revelations. The story in the seesaw North Africa Campaign of the western desert against Erwin Rommel's Afrika Korps between November 1941 and June 1942 was initially similar until the British deployed ULTRA intelligence analysts to field commands and institutionalized a system for distributing ULTRA to operational headquarters.

Perhaps ULTRA's greatest contribution to victory in the west was its cumulative accretion of details about the German order of battle. This priceless intelligence, communicated unwittingly in the Germans' own words, enabled the Allies to make accurate assessments of German strengths and weaknesses; to exploit German preconceptions; and sometimes to disrupt German plans.

In European war, ULTRA was never distributed to field commanders below army level. Each army headquarters had an assigned special liaison officer or special security officer. They received ULTRA intelligence from Bletchley Park in Buckinghamshire, northwest of London, and hand‐carried it to the army commander as well as his immediate staff. A series of Anglo‐American arrangements concluded in 1942 and 1943 also made ULTRA the universal code word for such intelligence and established the distribution system used in the western theater throughout the war. Furthermore, the major Allies divided the ULTRA world between themselves. The British took responsibility for ULTRA in the China‐Burma‐India theater; the Americans in the Pacific; and both shared the Atlantic, Mediterranean, and European theaters.

In the Pacific, the situation was far less centralized. American, British, and Australian naval cryptanalysts worked against Imperial Japanese Navy book codes. The U.S. Army's cryptanalytic arm, the Signal Intelligence Service, attacked Japanese Army code systems and foreign ministry communications from Arlington Hall in the Washington, D.C., suburbs. German ciphers were the main British targets, but Bletchley Park and the Wireless Experimentation Center in India also played major roles in solving the Japanese Army Air Force's ciphers and air‐ground codes. And a combined American, Australian, British, and Canadian organization, Central Bureau, served as Gen. Douglas MacArthur's independent cryptanalytic agency. Since the Japanese Army and Navy used different cryptologic systems, there was little exchange of technical cryptanalytic data. ULTRA intelligence, however, was exchanged on a routine basis.

Before the Japanese attack on Pearl Harbor, in December 1941, both the U.S. Army and Navy were devoting most of their slim cryptanalytic resources to solving Japanese diplomatic ciphers, notably the foreign ministry's PURPLE machine. The U.S. Navy, together with British cryptanalysts at Singapore, was analyzing the Japanese naval cipher and indeed had broken into the system in September 1940. A change to the key register in 1941 prevented further exploitation of the initial success. Thus, at the time of Pearl Harbor, the Allies were not breaking any Japanese high‐level military ciphers.

Because of its prewar experience, the U.S. Navy was able to solve portions of the Japanese Navy's main administrative code, JN‐25, in early 1942. Intelligence gleaned from this source proved crucial to the Allies' deflection of the Japanese naval thrust against Port Moresby, Papua New Guinea, in May 1942, and decisive in the Battle of Midway the following month. Sustained and timely reading of the naval cipher, with occasional periods of blackout, typified the next three years of the naval war in the Pacific. ULTRA uncovered numerous Japanese seaborne reinforcement schemes for eastern New Guinea and the Solomons. In March 1943, ULTRA forewarning led to the Battle of the Bismarck Sea, where the loss of sixteen ships crammed with Japanese reinforcements shifted the strategic initiative to the Allies in the southwest Pacific. ULTRA was especially valuable to the American submarine campaign in the Pacific. The Allies' ability to read the Japanese Army and Navy water transport codes enabled them to pinpoint convoy routes, times, and locations for Japanese convoys.

In the early days of the war, Central Bureau received responsibility for solving Japanese naval land‐based aircraft codes and ciphers. Within nine months of its establishment in April 1942, Central Bureau cryptanalysts had solved the naval land‐based air cipher; the army air force's air‐ground code; and the Japanese weather cipher. This intelligence enabled Allied air commanders to marshal their forces to meet enemy raiders and later to catch Japanese aircraft on the ground.

Allied success in the great air battles over Guadalcanal, the central Solomons, and Papua, New Guinea in late 1942 and early 1943 owed much to alerts provided by ULTRA. Probably the most notable example was to shoot down the aircraft carrying Adm. Isoroku Yamamoto after a decrypted message betrayed the admiral's itinerary. Other major success included the destruction of the Japanese air forces in eastern New Guinea, at Wewak in August 1943 and at Hollandia in March 1944.

The main Japanese Army codes, both strategic and tactical, resisted all cryptanalytic attempts to break them. Then, in January 1944, Australian troops pursuing the retreating Japanese in eastern New Guinea discovered a steel box containing the complete cryptologic library of the Japanese 20th Infantry Division. These captured codebooks enabled the Allies to read the Japanese Army's main code system until April 1944, when changes appeared. By that time, however, MacArthur had capitalized on this newly available source to put forces ashore some 200 miles behind the main Japanese units in eastern New Guinea. His surprise landings at Hollandia and Aitape in April 1944 severed Japanese forces in New Guinea, completely isolated Rabaul, and opened the route for his return to the Philippines.

In the Philippines and the Central Pacific campaigns, ULTRA enabled U.S. submarines to interdict Japanese reinforcements. Heavy losses of troops and supplies intended for Saipan, Okinawa, Luzon, Leyte, and Iwo Jima weakened those garrisons, although the Japanese still exacted a terrible toll of U.S. casualties in those grim struggles. ULTRA's contribution to British Gen. William J. C. Slim's brilliant campaigns in India and Burma is less certain, awaiting further declassification of pertinent documents. There can be no doubt about ULTRA's prowess in the summer of 1945. By that time, the Allies could solve all major Japanese Army and Navy code systems. ULTRA uncovered the massive Japanese reinforcement of Kyushu, the next target on the Allied drive to Tokyo, thereby certainly influencing the decision to use atomic bombs against Japan.

The release of ULTRA documents in 1977 opened the way for numerous historical reassessments of generalship and strategic decision making in World War II. Revisionists initially emphasized the spectacular intelligence coups ULTRA provided; but subsequent scholarship, by placing codebreaking triumphs in a larger context, makes more modest claims for the cryptanalytic warriors. The continuing declassification of records and firsthand published accounts is helping to fill still significant gaps about this marvelous intelligence source. Even without complete documentation, however, ULTRA surely shortened the war in east and west, enabling the Allies to win it with fewer losses than might otherwise have been the case.
[See also World War II: Military and Diplomatic Course; World War II: Changing Interpretations; World War II, U.S. Naval Operations in: The North Atlantic; World War II, U.S. Naval Operations in: The Pacific.]

Bibliography

Ralph Bennett , ULTRA in the West: The Normandy Campaign of 1944–45, 1979.
F. H. Hinsley, et al. , British Intelligence in the Second World War: Its Influence on Strategy and Operations, 3 vol. in 4 parts, 1979–88.
W. J. Holmes , Double Edged Secrets: U.S. Naval Intelligence Operations in the Pacific During World War II, 1979.
Thomas Parrish , The ULTRA Americans: The U.S. Role in Breaking the Nazi Codes, 1986.
David Kahn , Seizing the Enigma: The Race to Break the German U‐Boat Codes, 1939–1943, 1991.
Edward J. Drea , MacArthur's ULTRA: Codebreaking and the War against Japan, 1942–1945, 1992.
F. H. Hinsley and Alan Stripp, eds., Codebreakers: The Inside Story of Bletchley Park, 1993.

Edward J. Drea

ul·tra / ˈəltrə/ inf. • n. an extremist.• adv. very; extremely: the play was not just boring, it was ultra boring.

Ultra in the Second World War, codename given to the project at Bletchley Park in which British crytographers broke the German Enigma code.