ULTRA. Section 1 deals with the intelligence derived from decrypts of signals enciphered on the German
ENIGMA and
Geheimschreiber cipher machines; section 2 covers the intelligence gleaned from the codes of the Japanese armed forces. The former therefore describes the influence of ULTRA intelligence on the war in the northern hemisphere while the latter is almost wholly concerned with the war in the Far East. The two articles have been labelled UK and USA to indicate the main—but far from the only—nationality of those involved in decrypting these signals, and in translating and assessing their value.
Both the Germans and Japanese used many codes and ciphers, some of which were never broken. The Japanese possibly had more than 50; the Germans had nearly 200 ENIGMA variants alone as well as several Geheimschreiber and hand cipher systems.
See
MAGIC for intelligence derived from diplomatic signals enciphered on the Japanese cipher machine, codenamed
PURPLE. See also
signals intelligence warfare,
SIGABA and
Typex.
1. UK
ULTRA was the British security classification chosen in 1940 to denote the new and highly secret intelligence produced by the decryption of intercepted German (and some Italian) radio messages enciphered in the ENIGMA machine cipher or transmitted on the Geheimschreiber, or, in the case of Italian signals, enciphered in the C38m's machine cipher. The C38m was very similar to the ENIGMA. The ENIGMA was used by all three German armed services, the
SS, the
Abwehr, and the German state railways.
Translations of naval ENIGMA messages teleprinted from the British Code and Cypher School at
Bletchley Park to the Admiralty's
Naval Intelligence Division, and copies of the signals sent to commands abroad based on army and Luftwaffe messages (but not translations of the actual decrypts), can be seen in the Public Record office in London and in the National Archives in Washington. It is upon them that this article is mainly based.
ENIGMA was supposed to be unbreakable, and this, together with the portability of the enciphering machine, encouraged its widespread use by the Wehrmacht. There was a consequent dilution in the quality of the signals personnel, of the Luftwaffe in particular (see
FLIVOS), who operated the machine, and in their observation of the proper security procedures. This greatly assisted decryption. The degree of dilution and other variables, such as the occasional capture of a month's ENIGMA key-lists, varied from service to service and from time to time. The number of decrypts from a particular service or theatre of war therefore varied from nil upwards, and so did the usefulness of the derived intelligence, though on the whole both gradually rose throughout the war. Thus, for example, fewer than 200 army/air signals were sent to commands abroad in the six months ending in November 1941, and only about 8,000 by June 1942, but 15,000 by October 1942. By the end of the war almost 100,000 had been sent, together with more than half a million of the usually far shorter naval teleprints to the Admiralty for operational action.
The seeds of what was to become ULTRA were sown by an anti-Nazi German, a far-sighted French intelligence officer, and a trio of brilliant young Polish mathematicians. In the late 1920s the German armed forces bought up the rights to the ENIGMA machine, which had failed in the commercial market for which it was designed, and were developing it for military purposes even before the Nazis rose to power; machine ciphering was then a novelty. In 1931 Hans-Thilo Schmidt (cover name Asche), a member of the German defence ministry staff, gave some sheets of the ENIGMA instruction manual to Captain
Bertrand of the French military cryptographic bureau; it was the first of several such contacts. Bertrand offered to share his new and potentially valuable knowledge with the British and the Poles. The British, believing the cipher to be as insoluble as was claimed, showed little interest, as did Bertrand's own superiors; the Poles, on the other hand, accepted Bertrand's proposal with alacrity, though with little hope of being able to exploit it.
However, using Asche's guidelines and a commercial ENIGMA, Marian Rejewski—he was one of the three bright young Poles—worked out the theory of the machine, and by the late 1930s the Poles were reading a great deal of the traffic passed in the new cipher. Seeing the threat to their independence, in July 1939 they presented both the British and the French with one of the machines they had reconstructed, together with details of the knowledge they had accumulated.
By this time British cryptographers were alert to the possibility of breaking ENIGMA and to the military advantages of doing so. A group of university mathematicians was established at Bletchley Park, and before long they had advanced beyond the point the Poles had reached, in spite of new procedures introduced by the Germans on the outbreak of war. These procedures were often ignored and the resultant breaches of security played into the hands of the cryptographers; hence the repeated remark by Gordon Welchman, one-time head of the army/air cryptographic section at Bletchley Park, ‘We were lucky.’ A system which should have been invulnerable, as the British
Typex was, proved not to be so. Army and navy ENIGMA were more difficult to break, but entry into the Luftwaffe version eased the way into them.
The Germans' belief that ENIGMA guaranteed the security of their communications had two consequences for the British and their Allies.
First, the cryptographers' awareness that the complexities of the cipher could easily be increased and that it might thus be put beyond their reach, or that a new and unbreakable cipher might be substituted if serious doubt were cast on ENIGMA, demanded extreme care to preserve the secret that it had been broken. Careless talk could cost more than lives, and so might military action which could only have resulted from Allied knowledge of information transmitted in ENIGMA. By 1945 many thousand British and American men and women were engaged in the successive stages between interception and use in battle, and not the least remarkable feature of ULTRA's history is that none of them ever leaked the secret, but kept it inviolate for the next 30 years. Similarly, the loyalty and security-mindedness of the French and Polish personnel who remained silent under the stress of enemy
occupation of their countries are quite beyond praise. In the field, commanders accustomed themselves to shaping their actions so as not to rouse suspicions that they were using ULTRA, after a few sharp rebukes in the early days reminded them of the overriding claims of security (but see
Medenine).
Secondly, because ULTRA was the result of unsuspected eavesdropping on German official correspondence, information derived from it was absolutely reliable factually. Information from other sources always required subjective evaluation before it could be acted on: had an agent been duped or misinformed, did a prisoner-of-war really know as much as he suggested under interrogation, and so on. ULTRA was free from this confusing element. But because it was necessarily only a random selection of German correspondence—the Germans naturally preferred to use landlines where they existed—it always needed to be interpreted and set in its probable context. This need was particularly acute in the early days, when intelligence staffs were inexperienced and few keys were being broken, but it always remained paramount for the full utilization of decrypted intelligence even when experience had been gained and new keys had widened and deepened the foundations of ULTRA intelligence.
So necessary was it to protect the guarantee of ULTRA's reliability—that it was the product of supposedly impossible decryption—that in the early days it was passed off as the work of an omnipresent spy called ‘Boniface’ who had access to German military secrets at all levels from the lowest to the highest. As an agent, ‘Boniface’ had to be represented as subordinate to ‘C’, the head of
MI6, and the information he provided cast in the form of an agent's report. The illogicality of this arrangement not only led to friction between MI6 and the service ministries but also had the unfortunate side-effect that, because so few were allowed to know the truth, it discredited ULTRA in the eyes of military commanders, who could not of course believe any agent to be entirely trustworthy. The chosen disguise, in fact, was so successful that for a time it reduced the effectiveness of the intelligence it was designed to protect. As more officers were indoctrinated, after suitably severe warnings, the pretence was silently dropped.
ULTRA is a collective, not a singular, noun. It was not the product of a single cipher, but of several, all closely related. Almost 200 ENIGMA keys were known by 1945, not all of which were broken; some (the Luftwaffe general key, for instance) were in use throughout the war, others (the variant employed in the
Norwegian campaign, for example) disappeared when the reason which had called them into existence no longer applied. In one form or another, ENIGMA was an integral component of the Wehrmacht command system at all levels, from an airfield reporting the number of serviceable fighters it housed at the end of each day, to Berlin's directions for the conduct of the war to the U-boat fleet, or to individual theatre commanders.
Over the years, the cryptographers progressively widened their hold, and (as will be seen) this was reflected in the improving quality and value of the derived intelligence. Thus there was no question of one gigantic effort uncovering the whole ENIGMA mystery. On the contrary, the cryptographers' work was never complete; they waged a continuous, fluctuating, and ultimately victorious struggle to enlarge and maintain their mastery over keys which underwent regular monthly, daily, or sometimes even thrice-daily modifications.
Decryption of the Luftwaffe general key on a regular and current basis began on 21 May 1940, which has been called the birthday of ULTRA. This was too late for it to have any influence on the fighting which led to the
fall of France, then approaching its climax. In any case the British armed services were as unprepared to use high-grade intelligence as they were in other respects, and no arrangements secure enough for the distribution of such intelligence had been made in advance. The same situation had prevailed in Norway, where the special key being used by the Germans was successfully broken. However, the reading of Luftwaffe traffic currently from May 1940 had one immediately important consequence: the revision downwards of previously inflated estimates of German air strength, which had hitherto been very inaccurate, because no sound information on which to base them had been available. During the coming
battle of Britain it was of the greatest assistance to know how long the Germans could maintain their offensive at the current rate of aircraft loss, and future estimates of aircraft numbers could usually reflect truth rather than guesswork.
From July 1940 onwards ULTRA's chief service was to give general warnings of the state of preparations for the invasion of England (see
SEALION) and in September to furnish strong indications that invasion had been postponed; but no precise details of German invasion intentions were ever revealed. This was doubtless because of the extensive use of landlines for the most important orders. Except in the desert, where they did not exist, landlines were always to restrict the completeness of ULTRA intelligence.
A special ENIGMA key came into use for the autumn and winter
Blitz of British cities. The inner meaning of the cryptic messages which were enciphered in it defied interpretation until a young MI6 scientist attached to the air ministry, Dr. R. V. Jones, realized the way in which bombers were being directed to their targets by radio beams (see
electronic navigation systems), and was able to counteract the beams sufficiently to reduce substantially the damage the bombers using them were causing. Four years later the same key gave clues to the development of the V-1 and V-2 (see
V-weapons), and in combination with evidence from
photographic reconnaissance and reports from the
resistance in France and Poland made it possible to foresee and, to some extent, counter the V-weapon attacks of the summer and autumn of 1944.
With the postponement of SEALION in 1940, and the move of German bombers away from France to take part in the invasion of the USSR (see
BARBAROSSA) the volume of ENIGMA traffic declined for several months. The lull gave the opportunity for an enlargement of staff which was already plainly desirable. At its fullest extent, this staff combined men and women, civilians and members of the uniformed services, into a single whole, subdivided into a number of mutually supportive sections on the sole basis of individuals' qualifications and abilities in different aspects of the work. From December 1943 onwards, American officers were also drafted into it, ‘some to remain at Bletchley, some to be trained in ULTRA before heading the US equivalent of the British
Special Liaison Units.
Soon after regular daily decryption of the Luftwaffe key came on stream in the summer of 1940, it became clear that the best results would be obtained if the functions of decryption, translation, and intelligence servicing were formally separated, while of course remaining closely interconnected. This represented a radical departure from previous practice and undoubtedly raised the quality of the end product, intelligence for operational use.
The new arrangements were just settling into place when the
Western Desert campaigns erupted into violent action with the arrival of
Rommel in February 1941. A new Luftwaffe ENIGMA key for the Mediterranean had just been broken, and it was soon obvious that for the first time enough information was likely to be available to provide field commanders with urgent intelligence of great if sometimes ephemeral value (Rommel's tactical plans, or orders for aircraft movements, for instance). A special radio channel, exclusive to ULTRA, was opened between London and Cairo. The intelligence staff at Bletchley Park was authorized to draft and transmit signals over it on their own initiative and these were then disseminated by Special Liaison Units to those cleared to receive them. By 1944 there were two or three dozen such stations operating at the same time.
Both these innovations, of fundamental significance for the future of ULTRA, are associated with the name of Group Captain F. W. Winterbotham, but the precise extent of his contribution has never been disclosed.
Decryption and translation of naval ENIGMA material were the responsibility of a separate section at Bletchley, but the drafting of signals to fleets at sea was the province of the Admiralty. This was because the Admiralty was an operational headquarters as well as an organizational and administrative centre, and consequently had functions which the other service ministries were compelled to delegate to commanders in the field.
Hitler invaded Greece and Yugoslavia in April 1941 (see
Balkan campaign), only a few weeks after the break into the new Mediterranean key had sharpened interest in the Western Desert war. Regular intercepts, decrypted with unprecedented speed, justified the new radio link, which was extended briefly to Yugoslavia and GHQ Greece, but only a very few signals could be delivered quickly enough to protect the British force as it retreated southwards. Determined to press home his advantage, in April Hitler sanctioned a project to capture
Crete—where the tiny British garrison was reinforced only by men exhausted by their hasty retreat through Greece—with parachutists and glider-borne detachments. So completely were the arrangements revealed by the Luftwaffe key that the whole plan was in British hands a fortnight before the invasion on 20 May. This was the first convincing demonstration of what ULTRA could do. It would also have been ULTRA's first operational test had not the material weakness of the defenders loaded the dice too heavily against them. In spite of the forewarning, Crete fell on 31 May.
Although it was only dimly recognized at the time, Greece and Crete had shown that tactical intelligence could not be ULTRA's strongest suit. This was because in most cases it was impossibly difficult to carry out the whole process—from interception to the delivery of a signal to the officer who could use it—before the land battle had moved on. Very occasionally the process could be completed in about three hours, but six hours may have been nearer the average. When, at Churchill's insistence,
Wavell launched attacks in the desert which quickly collapsed, this conclusion should have been drawn.
Auchinleck, Wavell's successor as C-in-C
Middle East Command, resisted similar demands more stubbornly, but ULTRA was still too novel and too mysterious to command the attention it was beginning to deserve. Half the ground gained by Auchinleck's CRUSADER offensive of November 1941 was lost in the new year, partly through misinterpretation of ULTRA and partly through military error, although some German army traffic was now being read for the first time. A truer, but not yet fully appreciated, pointer to the future had been given six months earlier: analysis of successive supply returns showed that the navy and the RAF, assisted by a combination of Italian naval decrypts with German reports of Luftwaffe cover of trans-Mediterranean shipping, had almost strangled Rommel's air component by reducing its petrol stocks by 90% between May and October. Military strategy, often based on logistical analysis like this, was gradually recognized as potentially ULTRA's greatest contribution to victory.
Government cryptographers, some with over 20 years' experience, formed the nucleus of the cryptographic party at Bletchley, and the university mathematicians who joined them found little difficulty in adapting their skills to new requirements. The translators possessed first-class degrees in German. But since in 1939 hardly anyone in England had experience of intelligence at the level of ULTRA, the new intelligence officers received no guidance and had to discover the rules of their craft for themselves as they went along; small wonder that their first steps sometimes faltered. The same applied in the field. The necessary skills had been learned at both ends by mid-1942. The defensive
battle of Alam Halfa at the beginning of September first showed how much progress had been made; less happily, it also contributed to the foundation of the ‘Montgomery legend’.
Montgomery had taken command of the Eighth Army only a fortnight earlier; but he never openly admitted that his victory was due to ULTRA as well as to his own genius.
Rommel could have broken through to Cairo and the Nile in July had his men not been exhausted by their efforts, and it was obvious that he would make another attempt as soon as he could. On 15 August he explained to Hitler, via ENIGMA, what he would do, announcing that his tanks would advance at the end of the month. A translation was in Montgomery's hands 48 hours later. It showed that Rommel would swing south round the end of the British line and then strike north to isolate the Eighth Army from its base, but that the Alam Halfa ridge would then bar his way. This was exactly what Montgomery had prophesied after his first reconnaissance of the front line two days earlier; the confirmation not only gave him time to strengthen the defences of the ridge still further, but also invited the navy and the RAF to redouble their assault on Rommel's Mediterranean supply line. They sank so many tankers that on 1 September petrol shortage immobilized the
Afrika Korps in front of Alam Halfa, where it was bombed mercilessly. Rommel withdrew to his starting-point three days later, having gained nothing.
The forewarning given by ULTRA resembled that over Crete, but with the immense difference that this time there were resources enough to take full advantage of it. This first victory of a British army over the Germans after three years of retreat and failure silenced those who had doubted the value of intelligence in general and ULTRA in particular, and the frosts of former scepticism soon melted away. The second
battle of El Alamein in October was by comparison a lesser intelligence triumph, and when the move south of 21st Panzer Division (which opened the way for the decisive breakthrough) was reported by the
Y service several hours before ULTRA, the latter's relative tactical incapacity was underlined.
Decrypts of the Italian naval ENIGMA cipher had laid the foundations of the Mediterranean fleet's victory over the Italians at
Cape Matapan in March 1941, and continued to assist air and sea attacks on the Axis supply route to Libya until it was withdrawn in that summer. The Admiralty had always recognized the enormous advantage the breaking of German naval ULTRA would bring, if it could be achieved. Unfortunately the inherent complexities of the naval key and the naval operators' superior security long stood in the way. The first breaks, in the spring of 1941, into the key then used by surface vessels and U-boats in common (once mastered, it was read regularly from August until the end of the war, though only used by surface vessels from February 1942 onwards) made it possible to plot the U-boats' present and likely future positions and to re-route Atlantic
convoys accordingly, thus restricting the successes of the newly-introduced wolf-packs in the
battle of the Atlantic. However, decryption was still too intermittent and often too late to give much help in the hunt for the
Bismarck when she broke out into the north Atlantic in May 1941; most of the intelligence which led to her destruction came from air reconnaissance.
Reliance on ULTRA to maintain a continuous U-boat plot became so routine and so vital a feature of the Admiralty's control of the battle of the Atlantic that the sudden separation of surface and U-boat communications, and the introduction of a new and still more complicated key for the latter at the beginning of February 1942, was a shattering blow. So greatly were the cryptographers' difficulties increased that the new U-boat key could not be broken until December, with the result that the convoys suffered badly during the intervening months: when 700,000 tons of shipping were sunk in November, it began to look as if the UK might starve and American armies be unable to cross the Atlantic for the liberation of Europe. An added handicap to the defence against the U-boats was that at this time the Germans were reading the British convoy code (see
B-Dienst). This was changed in mid-December, just as the new U-boat key was broken; the Admiralty's sight improved as the Germans grew blind. (By curious coincidence, the same pattern had just occurred in the desert. Rommel's advance in the summer of 1942 was assisted by reading low-grade British codes as well as the American military attaché's telegrams to Washington: see
Italy, 6. Both leaks were stopped before the summer was over.)
The improved situation for the British brought relief to the hard-pressed Atlantic supply route, though the extent of the relief was not fully felt until May 1943, when so many U-boats were being sunk that
Dönitz withdrew the remainder; they never returned in such numbers.
A major part in this ocean victory was played by new methods of finding and destroying U-boats which had recently been developed: high-frequency direction-finding techniques (see
huff-duff) and centrimetric
radar, very-long-range reconnaissance aircraft, reinforced escort groups with escort carriers, and so on. The respective shares of responsibility cannot profitably be assessed, but the reading of the U-boat key may well have been the chief.
Having thus played so significant a role in winning the war at sea, from which final victory flowed, naval ULTRA's essential work was done. It never exercised so decisive an influence on events again, though later on it gave welcome assurance that the production of U-boats of a new type, able to move much faster under water and to breathe through a pipe (see
Schnorchel)—boats which might have renewed the Atlantic danger in still more alarming terms—was being retarded by bombing more even than had been hoped.
Critical turning-points, then, in the exploitation of both naval and army/air ULTRA were reached in the course of 1942, and great dangers were overcome by the end of the year. In the former case, cryptographic skill, assisted by the capture of several of the monthly lists of settings which Berlin issued to naval vessels, had found a way out of an apparent impasse, prevented the loss of a valuable source and gained what was, perhaps, a war-winning victory. In the latter, respect for ULTRA and the acceptance of intelligence as a leading instrument of command had been secured at last.
On land, however, the winter of 1942–3 saw much that seemed to contradict this. Montgomery flatly disregarded ULTRA's indications that if he moved smartly he could probably trap Rommel's army and annihilate it as it retreated across Libya; notably, he hesitated too long at Mersa el Brega in December instead of pushing on. In Tunisia, during the
North African campaign, their lack of experience of fighting Germans afflicted some of both the British and the American forces engaged. Intelligence was not accorded the place it had now won with the Eighth Army, and this delayed the Allied advance as much as military mistakes, although it must be admitted that the serious set-back which the Allies suffered at
Kasserine Pass in February 1943 was partly the result of unusual problems of interpretation posed by current ULTRA, so that accurate forecasting was abnormally difficult.
The
Sicilian campaign, which opened in July 1943, saw a new and profitable aspect of ULTRA. Because the island was so obvious a target after Tunisia, efforts were made to persuade the Axis that the landings would be elsewhere. ULTRA soon showed beyond doubt that Sicily was not being reinforced but that a panzer division, newly re-equipped after bruising battles in the
German–Soviet war, was being hastily transferred from France to southern Greece. No other source could have proved the efficacy of the
deception planners' rumour-mongering so conclusively, thus relieving the operational commanders' minds as they prepared an amphibious undertaking on an unprecedented scale.
The Italian surrender and the Allied landing in Italy in September 1943 (see
Reggio di Calabria and
Salerno) gave prominence to another new aspect of ULTRA's capabilities. What would Hitler's reaction be? Would he protect Rome and contest every inch of ground, as he had told Rommel to do at El Alamein, or would he prudently retire to the Alps to economize on men and
matériel? He hesitated for several weeks, and ULTRA showed him doing so. Finally he decided to stand on successive defence lines across the peninsula (see
Winter Line, for example), and ULTRA reported the decision as soon as he made it in early October. This was a strategic prize of the greatest moment; it enabled the Allies to design the
Italian campaign to draw maximum advantage from the willingness Hitler thus displayed to allow Italy to drain away his resources. Nothing subsequently during the war in Italy could bear comparison with it, though the steady flow of decrypts was maintained surprisingly well, given the availability of landlines. Among the most important were those which showed
Kesselring had decided to make no further attempt to wipe out the
Anzio bridgehead after the failure of his big attack in February 1944.
In spite of the Germans' natural preference for landlines, which restricted the amount of ENIGMA traffic in France before the Normandy landings of June 1944 (see
OVERLORD), enough intercepts were collected to identify all the divisions manning the coastal defences and to locate all but two of them with sufficient precision. Directly the Allies landed the volume of traffic grew enormously. The most pressing question now was whether the three panzer divisions in the Pas de Calais would move to Normandy at once and counter-attack the beachhead before it was properly consolidated. Would the FORTITUDE deception plan which had held them away until now, still carry conviction? ULTRA was able to keep watch on all three divisions closely enough to show that they were all remaining in place for the first crucial ten days (the first to move was not ordered to Caen until 19 June, D+13), a service of prime importance in complementing the deception plan, and again one which no other source could have rendered as reliably.
After usefully demonstrating that the German troops east of Avranches, where the First US Army was to attack at the end of July, were weak and exhausted, ULTRA could claim the initiative in what was to become the German disaster of the Falaise pocket (see
Normandy campaign).
Bradley had already directed the Third US Army up the Loire valley towards Paris when on 6 August ULTRA gave notice that armour was being assembled for an attack near Mortain; in consequence, the attack was stopped in its tracks. Then in the early hours of 10 August came the
coup de grâce. On the previous evening Hitler ordered a renewal of this attack, with the object of driving through to the sea at Avranches and cutting the American supply line. News of the move reached Allied headquarters less than twelve hours later. The certainty that the Germans were facing west, when they should have been extricating themselves from the risk of encirclement by looking to their rear in the east, lent extra force to the Allied onslaught.
Elements retreating from the pocket in August and September filled the air with reports of their movements and strength, and among much else these showed that 1st SS Panzer Corps was to refit in the general area of Arnhem where Montgomery was planning to make a bridgehead across the lower Rhine. So firmly entrenched, however, was the conviction that German resistance was nearing its end that this knowledge was not enough to cast doubt on the wisdom of launching operation
MARKET-GARDEN.
The myth, mistakenly put about in the late 1970s, that German radio silence prevented ULTRA from giving warning of the
Ardennes campaign in mid-December is still current in some quarters. It must be flatly contradicted. For the previous three months ULTRA evidence had been accumulating that a large armoured force was being assembled in north Germany for an undisclosed purpose. From early November the movement of its component divisions westwards across the Rhine by rail was steadily reported, and at the end of the month there began a stream of urgent requests for air reconnaissance of just those parts of the Ardennes into which the German spearheads thrust on 16 December. None of this evidence was wholly unambiguous, and no intention to attack was ever expressed. Nevertheless, it is regrettably plain that neither SHAEF nor Twenty-First Army Group ever analysed it impartially enough to allow the deduction that it might as easily represent preparations for an attack, as for a counter-attack when the Allies next moved forward.
While it would be foolish to attempt to apportion responsibility for the eventual Allied victory between the various strands (including other sources of intelligence) which made up that victory, there can hardly be any doubt that the regular provision of absolutely reliable—if sometimes incomplete—intelligence about German actions, resources, and intentions was certainly among the foremost.
Two further aspects of Allied co-operation call for mention. The US armed forces came into the ULTRA picture remarkably late. Hitler's declaration of war on the USA on 11 December 1941 was quickly followed by the first Washington conference (see
ARCADIA), which confirmed the policy that the new allies should concentrate their main effort on Germany first. Yet it was not until more than a year later, on 25 April 1943, that an American cryptological and intelligence reconnaissance party arrived at Bletchley Park. A formal agreement to share ULTRA followed in May. But, no doubt because of the natural reluctance of the British to risk their priceless secret without fully satisfactory safeguards, for many months to come the authorities of the two countries seemed to be, in the words of the official US historian, ‘walking round each and eyeing each other like two mongrels who have just met’.
The first American recruits reached Hut 3 only in January 1944, and the same historian (who was one of them) reckoned that the party did not come of age until
D-Day. The American contingent never numbered more than 68, as against several hundred British; a dozen were absorbed into the British staff on a completely equal footing, and the remainder were attached, after training, to US field commands as the channels though which ULTRA was delivered in accordance with regulations laid down by
General Marshall, the US Army's Chief of Staff, on 15 March 1944. The experiences of these officers, recorded directly after the end of the war, shed interesting light on the degree to which they managed to ‘educate the generals to the value of intelligence’ and provide illuminating examples of the way in which ULTRA assisted operations.
Secondly, how much did Stalin and the USSR learn, directly or indirectly, about the intelligence content of ENIGMA? The short answer is: directly, nil, indirectly a good deal in 1941–2, but not thereafter.
Churchill's declaration of common cause with the Soviet Union immediately upon the German invasion in June 1941 had been preceded the previous month by an ENIGMA-based warning (which he ascribed to ‘a trusted agent’) of troop movements towards the Soviet frontier. Stalin did not reply. Churchill's motive was clearly not so much altruism or an appeal to left-wing sentiment at home (Clementine Churchill's ‘Aid to Russia Appeal’ took care of this) as national self-interest—the more the Germans faced east and spent their strength in the USSR, the less possible a new SEALION and the more probable eventual victory.
Only a month after BARBAROSSA he persuaded ‘C’ the head of MI6, and the
Chiefs of Staff to agree to send suitably camouflaged information on a regular basis ‘provided no risks are run’. The service continued at a steady rate while air and army ULTRA provided advance information about German tactics and intentions during the crisis in front of
Moscow in the late autumn, but fell off about midsummer 1942; nevertheless information about German plans probably gave some slight assistance towards preventing a breakthrough in the Caucasus and in the defence of
Stalingrad. The USSR showed scant gratitude and no reciprocity, and by the winter of 1942–3 was cold-shouldering the Allies in intelligence matters. The Red Army rejected any idea of cryptographic partnership in December 1942, for instance, while the painfully slow progress towards a similar partnership with the USA reflected the fears of many on the British side that by sending so much Churchill was wearing the disguise thin and endangering the precious ULTRA secret. Reluctance on both sides reduced the service to a trickle in 1943 and brought it to an end in 1944.
There is no evidence that the Soviets ever broke ENIGMA themselves, though they could read German army and airforce hand ciphers. Gordievsky, the high-level KGB defector, considers that ‘the odds are that Soviet cryptanalysts were unable to read ENIGMA on any regular basis, though they sometimes captured key-tables.
Ralph Bennett
2. USA
In the
Pacific war the Allies used the same designator as they did in Europe for the special intelligence derived from the interception and decryption of enciphered Axis army and navy
radio communications. Although the Americans also often used
MAGIC to cover all Japanese communications broken into by cryptanalysis, the two sources were distinct: ULTRA was military, MAGIC diplomatic.
ULTRA was a secret weapon of enormous importance. Without it, the war against Japan would have been far more perilous and difficult than it was. Logically the Americans and British would make the best use of their interception and breaking of Japanese codes (as well as German) by pooling their interception, cryptanalysis, and finished intelligence, and they approached that point by the end of the war. Nevertheless, the sharing was very slowly and painfully achieved and integration was never complete.
The sharing of precious national assets such as the techniques and successes of cryptanalysis was shocking to the military services. Furthermore, American intelligence was rigidly compartmentalized, army separate from navy and cryptanalysis from evaluation, so that the more centralized British had to deal individually with a set of American rivals. At one point, the US Navy refused to provide the US Army with ULTRA material it was already passing to the British. The same pattern of parochialism,
rivalries, and compartmentalization obstructed the flow of intelligence among and within the South-East Asia, South-West Pacific, and Central Pacific combat theatres.
As Allied power mounted and wider engagement with the enemy proved the value of ULTRA, the demand for it grew and the need for pooling resources, integrating management, and widening access became apparent. Facing the enlarging role of the USA in the war and its dominant position against Japan, the British were the more inclined to tie their efforts to those of the Americans. Enhancing the US advantage was the advanced state of American communications technology, especially in high-speed calculators, permitting grand scale attacks on the most difficult codes and ciphers.
Most important in the forming of a British–American partnership in special intelligence (ULTRA and MAGIC) was the BRUSA (Britain and the United States of America) agreement of 17 May 1943. According to Bradley F. Smith (see below), this provided for the sharing of ULTRA between the British cryptanalytic centre at Bletchley Park and its US Army counterparts in Washington. The US Army took responsibility for Japanese military and air codes and ciphers, the British for those of Germany and Italy. The US Navy made its own arrangements with the British, so that for the war against Japan two special intelligence networks evolved, navy and army, each connected to Bletchley Park. More limited sharing occurred between networks. ULTRA was never shared with the Chinese and Free French, and only on rare occasions, and then in disguised form, with the Soviets (see above). The rising demand for ULTRA, the multiplicity of Japanese codes and changes therein, and the growing complexity of cryptanalytic methods and devices, created a voracious demand for personnel whose numbers at Arlington Hall, the US Army's cryptanalytic centre in Washington, reached 7,000 during the course of the war.
In the months of Allied weakness, and Japanese ascendancy and triumph following
Pearl Harbor, it was vital to learn where the next blow would fall. The Japanese admirals' code had proved impervious to attack, but the Japanese Navy's operational code, JN-25, had yielded some ground. This was an enciphered code. To encrypt, the sender would substitute five-digit numbers from a code ‘dictionary’ for the words and phrases of a message, then add to each (without carrying) another five-digit number from a second book of 100,000 randomly selected groups, using an enciphered key which indicated where to start. Place names had their own two or three-letter designators.
The breaking of JN-25 was never as final and complete as that of MAGIC. The code was first introduced in 1939, and the eve of war with Japan, the British
Far East Combined Bureau at Singapore had penetrated JN-25 to the point of monitoring Japan's main fleet movements, according to F. H. Hinsley, the authority on British intelligence. The smaller Dutch unit at Bandung, Java, it is claimed, was also reading some fleet communications, as was the cryptanalytic unit of the Office of Naval Intelligence in Washington (Op-20-G). However, JN-25 became unreadable again when the Japanese Navy replaced its book of random additives on the eve of Pearl Harbor. No evidence has yet been found that the American–British–Dutch–Australian coalition had foreknowledge of the Japanese attacks of 7/8 December 1941.
After Pearl Harbor the US Navy made an all-out effort to break the new version of the JN-25 code. Working on the task were the codebreaking units at Washington, Pearl Harbor, and Corregidor. With the fall of
the Philippines the unit in Malinta Tunnel on Corregidor was evacuated to Melbourne where it joined forces with Australian navy cryptographers and continued to work on JN-25. In addition the British unit evacuated from Singapore continued its work at Colombo. Leading the way was the navy radio intelligence unit at Pearl Harbor under Commander Joseph Rochefort, who possessed a powerful intuitive faculty, command of Japanese, extensive experience in cryptanalysis, and an extraordinarily retentive memory.
Rochefort and his team worked relentlessly for months in their cluttered, windowless ‘dungeon’ in the basement of Fourteenth Naval District Headquarters. At the height of activity in 1942 the Rochefort unit was using three million IBM punch cards a month for storage and retrieval of every five-digit code group received. Gradually a small but usable portion of the messages became clear.
One advantage was that American carrier raids on Japanese-controlled islands in the central Pacific and on Tokyo (see
Doolittle raid) stirred up intensive signalling in the Japanese fleet, allowing more intercepts and fodder for cryptanalysis. A second advantage was that while additives had changed the code itself had not, so equivalents discovered before Pearl Harbor were still usable once the cipher was stripped away. A third advantage was that the Japanese Navy was so widely deployed, with so many ships and air groups to command by radio, that it proved impossible to distribute new code books in time for a scheduled revision on 1 April 1942. Because of this, Rochefort's team gained nearly two more months' use of the old code, a crucial benefit as it turned out.
In March 1942 evidence from several sources began to accumulate of a forthcoming Japanese drive towards Australia with JN-25 decrypts showing that ships and air groups were being staged southwards. A British JN-25 decrypt revealed dispatch of two carriers from the Indian Ocean to the great Japanese anchorage and base at
Truk in the Caroline Islands. Estimating Japanese intentions required inspired guesswork, for at this time Pearl Harbor was decrypting on average only about 12–15% of a message. Nevertheless, Rochefort correctly concluded that the Japanese build-up was aimed at Port Moresby, on the south-eastern coast of New Guinea, and at the Solomon Islands. That estimate was repeatedly confirmed by decrypts in April to the point where
Admiral Nimitz, C-in-C of the Pacific Fleet, was prepared to take the risk of sending a second precious carrier, the
Lexington, to join the
Yorktown in the south-west Pacific.
The result was the battle of the
Coral Sea of May 1942. The Americans suffered heavily, with the sinking of the
Lexington and damage to the
Yorktown as compared with the Japanese loss of a light carrier and damage to the attack carrier
Shōkaku. However, both Japanese warships suffered plane losses that kept them out of the forthcoming
battle of Midway, while the
Yorktown was repaired in time for it.
Above all, the Japanese purpose was foiled; the invasion forces withdrew. Japan had sought New Guinea, from which air attacks could be launched against northern Australia, as a barrier for defence of the southern rim of its new empire. New Guinea and the Solomons would also open its way across the Coral Sea to the New Hebrides, New Caledonia, and Fiji Islands, severing the American supply route to Australia. ULTRA therefore played a key role in securing this vital flank for the Allies.
Even as they were predicting a southward thrust, the Rochefort group detected signs that Japan was preparing a larger naval offensive against some other unknown objective. They were correct: the objectives were Midway, some 1,816 km. (1,135 mi.) west of Pearl Harbor, and, to create a diversion, Kiska and Attu in the western Aleutians (see
Aleutian Islands campaigns). In the view of
Admiral Yamamoto, commander of the Combined Fleet, seizure of Midway would so threaten Hawaii that it would force the Pacific fleet to sail out and counter-attack, whereupon a far-flung net of Japanese carrier and battleship groups would close in and destroy it. Next would come invasion of Hawaii and then the Americans would sue for peace.
American intelligence was groping to learn what sort of operation to expect, and where and when it would occur. By now a substantial portion of JN-25 intercepts were being read and scraps of information from these were suggestive: frequent mention of a ‘forthcoming campaign’, requests for the type of hose used in mid-ocean refuelling and for maps of the Aleutians and Hawaiian waters, summonses to staff conferences aboard flagships, and plans for reconnaissance of a place intelligence identified as Hawaii. Decrypts and traffic analysis indicated that warships were gravitating towards Saipan, the logical assembly base for a central Pacific campaign. A ‘hot’ decrypt linked the proven equivalent for ‘invasion force’ with ‘AF’, a geographic designator. Rochefort guessed AF was Midway, while another designator linked the invasion force with Saipan.
By mid-May Nimitz was convinced that Midway was the target. Nevertheless Washington remained sceptical, fearing a Japanese deception plan. To lock Midway firmly in place as the object of all these Japanese preparations, Rochefort hit upon a trick, which Nimitz approved. By undersea cable—and therefore secure from Japanese interception—Midway was ordered to radio that its fresh-water supply equipment had broken down. This information was of course important to the Japanese, intent on occupation of the islet, and it was soon rebroadcast by them which confirmed that the AF indicator meant Midway.
There remained the question of when. Instructions for a Combined Fleet departure from Saipan on 27 May suggested 2 or 3 June, but so much hinged on timing that an unquestionable source was needed. The date was in a separately enciphered code group of a Combined Fleet operational order and on this Rochefort's group concentrated their most intense efforts for three days and nights, finally unlocking the secret that the occupation force transports were to arrive at Midway on 6 June. Since a decrypt revealed that the air strike on the island was to commence two days earlier than the landings, the date for the beginning of the Midway operation could be set at 4 June. The fleet intelligence officer was then able to predict almost exactly the spot and time where the carriers would launch their attack. Only days later the JN-25 code was changed and again became undecipherable.
As a result of these intelligence estimates, Nimitz had time to withdraw his carriers from the south-west Pacific, hastily repair the
Yorktown, and position his forces to surprise the Japanese. At the battle of Midway four Japanese aircraft carriers, the heart of the Imperial Japanese Navy's striking power, were sunk. The invasion force retreated and the tide of Japanese conquest turned.
Revisions of JN-25 after Midway and again in August 1942 made the code unreadable until the end of that year. Traffic analysis and the Australian
Coast Watcher system supplied vital information during the ferocious and costly naval
battles of Guadalcanal, but the fleet code was sorely missed. By 1943, with more and more codebreakers and translators at work, combined with the capture of Japanese code books, JN-25 again became readable and, with temporary lapses, remained so until the end of the war. ULTRA struck a deadly blow in April 1943 when the itinerary of Yama moto's inspection trip to the Solomons was decrypted, giving precise times and places. P38 Lightning fighters intercepted the admiral's plane and shot it down, killing him.
Most difficult to penetrate were the Japanese Army codes, on which no progress was made until the spring of 1943. The British Wireless Experimental Centre (WEC), in New Delhi, an outstation of Bletchley Park, led the way, but opinion is not unanimous as to how much
Slim was helped by ULTRA intelligence during the
Burma campaign. Also working on the army codes were
MacArthur's Australian–American cryptology unit at Brisbane, called Central Bureau, and the US Army's establishment at Arlington Hall in Washington. Successively, the water transport, or shipping, code, the code disguising addressees of messages (which WEC probably penetrated first), the army's general administrative code, and the Army Air Force code, were broken. But this took time and it was the US Navy's success in breaking the Japanese naval code which greatly aided MacArthur's early air and land operations (see the defence of Milne Bay in
New Guinea campaign, for example, and the
Bismarck Sea battle).
On the mistaken assumption that lower-level codes would be easier to decrypt, MacArthur's Central Bureau first tried to break the low-level three-digit, regimental one used by the Japanese Army. But because each regiment had its own code and communicated in it only to its divisional HQ and not laterally, to other regiments—and because the close proximity of a regiment to its divisional HQ meant only low power was used to transmit signals—the Bureau was never able to intercept sufficient messages to work on. However, this was not the case with the army's mainline four-digit code which was used between the higher commands (divisions, armies, area armies). Successes with this were achieved from 1943 onwards because, as the Japanese spread into MacArthur's vast
South-West Pacific Area, the numbers of signals in the four-digit code proliferated; also, because of the distances involved, these took more power to transmit and were, therefore, easier to intercept.
In mid- 1942 the stresses and strains of holding such a huge perimeter forced the Japanese Army to reorganize its shipping administration and in December of that year an Army Water Transport Code was introduced to enable the large number of independent commands to communicate with each other and with their superior HQ. So pressed were the Japanese that they made the Water Transport Code a simplified version of their mainline one but failed to make it as secure. This was discovered by an astute cryptanalyst at Central Bureau and in April 1943 that code was broken.
The Japanese Navy's Water Transport Code had already been partly penetrated when code books had been salvaged from a Japanese submarine sunk off Darwin in January 1942. Now, between them, the two Water Transport codes yielded invaluable intelligence on the army's shipping movements and provided US submariners with copious information about convoys, including the number and names of ships, their cargoes, destinations, routes, and often the noon positions. Submarines were now in a position to intercept effectively and sinkings rose dramatically: by the end of 1944 the majority of Japan's merchant ships had been sunk and its supply of oil was cut off. Indeed, it was discovered after the war that US Navy information on
V-J Day about the number and location of Japanese merchant ships was more complete than the Japanese government's.
The next significant advance for codebreakers came when the Japanese military attachés' code was broken. Inroads into it had first been achieved in the summer of 1942 at Bletchley Park. By the following September the daily signals between the attachés and Tokyo were being read on a regular basis, and these provided high-level intelligence which included Tokyo's assessments of its operations in the Pacific war and in the
China incident. Between July 1943 and mid- August 1944 Arlington Hall decrypted over 5,000 of these signals, 2,100 of which the war department considered were of strategic significance. They included details of new U-boats, German jet aircraft, and the fortifications of certain French ports which were of invaluable help in planning the Normandy landings (see
OVERLORD). As Edward Drea remarks in his book it is little wonder that Arlington Hall described the Japanese military attachés as the most efficient
spies the USA had in Europe.
By mid-1943, therefore, some Japanese army codes had begun to yield up their secrets, but often the process was too slow to be of operational use. The big breakthrough in the Japanese Army's mainline four-digit code did not come until January 1944 when the code books of the retreating 20th Division on New Guinea were found. They had been buried in a steel trunk which was discovered when the area was swept by a mine detector. This piece of carelessness enabled the US Army for a time to decrypt army signals with all the speed and precision its naval counterparts had achieved with the Japanese Navy's signals. In January 1944 Arlington Hall decrypted under 2,000 army messages; in March it decrypted over 36,000. It was this breakthrough that allowed MacArthur to pinpoint Japanese forces on New Guinea and to bypass them by
landing at Hollandia. In the same manner, intelligence from the Japanese fleet code provided Nimitz with precise troop strengths in the
Marshall Islands, leading him to invade Kwajalein in January 1944 rather than more heavily defended islands.
Even when the Japanese Army changed the key to its mainline code after the Hollandia landings, MacArthur's luck held: material relating to the new key was found. When this, too, was discontinued by the Japanese for a new key the old one was used by Central Bureau to decode a backlog of signals which it had previously not been able to break. One of these revealed that a barge carrying the new code books had been sunk off Aitape and it was supposed they had been destroyed by the fire that had sunk the vessel. But though books turn to ash they do not disintegrate unless burnt page by page, and it is still possible to read their contents. A diver was sent to Aitape, the steel trunk containing the codebooks was recovered, and 85% of the key was eventually reconstructed.
But ULTRA intelligence could not, and did not, reveal every move the Japanese made in the Pacific. For example, soon after MacArthur invaded Luzon in January 1945 (see
Philippines campaigns) the Japanese Army again changed the key of its mainline code and it took some time for Central Bureau to accumulate sufficient signals for the new key to be broken. The invaluable Water Transport Code also failed to produce the necessary intelligence as the Japanese High Command had abandoned Luzon and so was not sending either reinforcements or supplies there. The Japanese Army Air Force had been destroyed, so this produced nothing either, and as the batteries of Japanese radios failed, and could not be replaced, internal and external radio links gradually petered out. Because of this dearth of signals intelligence, US intelligence for the Luzon campaign was particularly flawed and the numbers of Japanese on the island were constantly underestimated. However, more code books were captured on Luzon, and then on
Okinawa, and from 1 July 1945 all Japanese signals relating to the build-up of Japanese forces on their home islands were monitored. These revealed the determination with which the Japanese were preparing for the US invasion and the surprisingly (to the Americans) large forces at their disposal. To some extent this ULTRA intelligence also ran counter to MAGIC decrypts which seemed to indicate that a faction of the Japanese government wanted to surrender. Drea has suggested (p. 201) that ULTRA's revelations on Japan's military build-up, and the military's determination to resist, was a factor in influencing the Americans to employ the
atomic bomb.
Waldo Heinrichs
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Hinsley, F. H. , et al., British Intelligence in the Second World War, Vols. 1–3 (London, 1979–88).
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