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Liaison Units whose role was not merely to protect secrecy, but also to ensure that no initiative or preemption of German action revealed Allied foreknowledge. If a prospective naval target was located at sea through cryptanalysis, whenever possible reconnaissance aircraft overflew the enemy before an attack to mask Ultra’s role. From 1942 onwards, Bletchley Park became an industrial centre, with 6,000 staff working in a hutted township, processing a flood of messages in shifts around the clock. The heart of its operation was Colossus, the electromechanical bombe which dramatically speeded exploration of multiple mathematical possibilities. The codebreaking teams were dominated by some hundreds of brilliant academics, most of them mathematicians and German-speakers. The most influential personalities, both in their early thirties, were Alan Turing, sometimes described as the father of the computer, and Gordon Welchman. Some young men performing vital, and perforce absolutely secret, roles at Bletchley were chided by outsiders for their absence from the front. One received a letter from his former headmaster, asserting that his doggedly civilian status disgraced his old school.

The picture of enemy operations provided by Ultra was always incomplete, but it offered a reliability no intelligence provided by spies could match. For instance, the Allies could launch D-Day on 6 June 1944 confident that the enemy was still oblivious of their objective and timing. Churchill permitted some Ultra information about the Eastern Front to be passed to Moscow. Stalin was never officially informed of the Bletchley Park operation, but Moscow was well briefed by British traitors, who supplied their NKVD handlers in London with a steady flow of decrypts.

Full Anglo-American intelligence-sharing sharing began only in 1943. The United States had broken the Japanese diplomatic cipher before the war, but their handling of Ultra never matched the interservice integration achieved by the British, partly because of army-navy rivalry. The U.S. Army ran its own decryption operation at Arlington Hall, Virginia, eventually employing some 7,000 staff. The navy team, based in bleak subterranean quarters at the Fourteenth Naval District, Pearl Harbor, was led by Cmdr. Joseph Rochefort, the awesomely brilliant Japanese-speaker, cryptanalyst and intuitive thinker who contributed so much to victory at Midway. Rochefort’s men read some messages in the Japanese navy’s JN-25 operational cipher soon after the outbreak of war and achieved fragmentary breaks at vital moments in 1942, which proved the most important Ultra achievements of the Pacific war. Thereafter, however, for some months JN-25 defied Rochefort’s team, leaving naval intelligence dependent upon coast watchers and traffic analysis. In 1943 the operational code was again broken, and thereafter provided a stream of data for the rest of the war.

Bletchley Park and Arlington Hall played key roles in breaking Japanese army codes in 1943, the first being that of military attachés overseas. Captures of Japanese codebooks laid open bulk military signal traffic in 1944. Whereas in January that year Arlington Hall read fewer than 2,000 of the enemy’s army messages, in March this increased to 36,000, decisively influencing MacArthur’s New Guinea strategy. Interception of Japanese communications faltered during the 1944–45 Philippines campaign, when the army’s main codes changed, causing a further interruption of decryption. In general, U.S. naval operations were more importantly influenced by Ultra than were those of the armies in the Pacific campaigns. No codebreaking achievement could eliminate the difficulties of assaulting strongly defended enemy positions. But the collective contribution of U.S. and British cryptanalysts to the war effort was greater than that of any other such small body of men in history. Their operations provided the supreme example of the Western Allies’ imaginative integration into the war effort of their cleverest civilian intellects.

IN THE AUTUMN of 1942, Churchill was passionately