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March 1944 : The Invasion Postponed

General Eisenhower had now obtained an increased supply of landing craft for D-day from shipyards in the USA, the British shipyards being fully loaded building the Mulberry Harbour components that were to be towed across the Channel following D-day. But to build up enough landing craft the date of D-day had to be postponed to early June; the first day when moon and tide were suitable turned out to be 5th June. Major-General Sir Percy Hobart, commander of the 79th Armoured Division, was pleased to get the postponement as it enabled him to find a solution to a new problem. His experimental teams had developed variants of the two main types of battle tanks, the Churchill (British) and Sherman (US), to be landed in Normandy to provide close support for the infantry during the landings; Montgomery, adopting a suggestion from Hobart, intended to land tanks with the first wave, an innovative approach that was to prove vital on D-day. Hobart devised tanks that carried searchlights for turning night into day, fascines and bridges for spanning ditches and bomb craters, flame throwers, flails and ploughs for clearing mines, armoured bulldozers, and waterproofing that would enable tanks to ‘swim’ ashore. Now commandos who had landed on the actual invasion beaches had found patches of sand into which the tanks would sink; not to be beaten, Hobart devised a ‘Bobine’ Churchill tank that carried a carpet of strong fabric that it would lay before it as it moved over the sand. But the month’s delay enabled Rommel to further strengthen the defences, and reduced the campaigning time before the weather would deteriorate in the autumn.

In Italy the threat to the Anzio beachhead is now past, as the Allies knew from decrypts, but the fight in the mountains dragged on with a further third attack on Monte Cassino on 8th March, mounted by the New Zealand Division after a huge bombardment. On 22nd March General Alexander calls off the attack to avoid further heavy losses. The news from Russia remains good, with the attacks in the south now rolling forward to threaten the Rumanian oil-fields. Hitler moves into Hungary on 19th March, Sigint providing no warning because the Germans used Enigma to carry false information to deceive their own forces. This month, the best news of all for those in the UK, dreading the approach of D-day, is the daylight defeat of the Luftwaffe over Germany, although the full degree of this victory is not recognised for the moment.

The Defeat of the Luftwaffe. In February 1944 it became clear that the USAAF daylight deep-penetration air raids were causing rising casualties to the Luftwaffe fighter force over Europe. Though the kill-rate claimed by the US crews continued to be greatly exaggerated, it was clear from Enigma decrypts that they were indeed imposing an unacceptably high loss rate on the German single-engined fighters, and the Germans were having to use these in an attempt to protect their quite effective, cannon and rocket firing, twin-engined fighters. There were already indications that deficiencies in the supply and training of pilots, and the beginnings of fuel shortages, were handicapping the Luftwaffe by day. (This was not yet apparent for their night-fighter force, which was almost getting the upper hand in the battle with the RAF heavy night bombers). So the USAAF takes the brave decision to cease routing their raids with a view to avoiding the defences; instead they determine to choose targets and routes with the aim of forcing the enemy fighters to do battle. On 4th March they mount a major attack on Berlin, and continue similar daylight attacks over Germany on many days during the month. The German fighter losses are typically at least 10% of the fighters they put up, and sometimes their losses rise as high as 40 %. The US long-range fighters, such as the P51 Mustang, prove superior to the German aircraft, most of which are new production of relatively old, now outclassed designs, such as the Bf 109. Before the end of the month it becomes clear that the Luftwaffe fighters are now often abstaining from all-out combat; Enigma confirms that this is due to a critical shortage of fighter pilots. Between January and April 1944 the Germans have lost more than 1,000 fighter pilots. Japanese Naval Attaché Coral decrypts, newly broken by BP & US OP-20-G, report that the Luftwaffe is ‘being crushed by overwhelming superiority’; moreover the Attaché reports that Allied claims that 20% of German fighter production had been destroyed are ‘reasonably accurate’. By the end of March the USAAF has established the daytime air superiority over all Western Europe that the Allies were to retain for the remainder of the war. Though the US losses were heavy they could replace their lost planes and aircrews; the Germans could still provide the aircraft but not the trained pilots. This victory has been accounted ‘one of the most decisive in the war in the air’.

The USAAF works closely with BP in setting up these operations. The BP BMPs provide daily Intelligence, much of it originating in Luftwaffe Enigma that, along with excellent photo-recce material, helps in the selection of targets and in target recognition. (‘BMP’ derives from the names of the three Air Intelligence officers in Hut 3 who first developed these daily newsletters back in the spring of 1942:- Bonsall, Moyes, and Prior. Arthur Bonsall went on to become of Director of GCHQ in the 1970s). Decrypt outputs from Hut 3 come daily direct to the US Eighth Air Force HQ in Bushy Park, providing Intelligence on the Luftwaffe order-of-battle, dispositions, production, wastage, casualties, serviceability and reserves. The Enigma weather reports are vital in planning the raids. BP is particularly valuable in the part of the offensive that is directed at aircraft on the ground, at airfields, depots and training stations. The US fighters become adept at arriving over the Luftwaffe forward bases just as the German fighters are about to become airborne. Up-to the minute reports on the take-off times of enemy fighters, the routes they are to follow to their assembly points, their intentions and tactics in combat are provided direct to the US fighters using a radio link, known as the Hook-Up provided by RAF Kingsdown. (Much of this material at first came from radio voice intercepts at this RAF Y station. The head of BP’s day-fighter section in Hut 3 was transferred to Kingsdown to become the Chief Intelligence Officer of the Hook-Up.). During March this well-informed assault forces the Luftwaffe to abandon radio when assembling their fighters. By the end of March the US Eighth Air Force has driven the Luftwaffe from their bases within voice-radio range of the UK, but the USAAF react by flying German speaking airborne radio interception operators to provide material for analysis direct to BP and Kingsdown. The US Strategic Air Force commands said that their most important source of Intelligence in this great victory came from the Luftwaffe Enigma decrypts provided by BP. Lewis Powell, an Operational Intelligence Officer, who became a member of the US Supreme Court, wrote ‘The achievements of air operations were immeasurably aided by the genius of the British Intelligence service that made Ultra available’. But now the US Air Force has to turn to concentrate against Overlord targets in France. Meanwhile on the night of 30/31st March Bomber Command suffers the loss of 108 bombers in an ill-prepared raid on Nuremberg, an unacceptable loss rate of almost 14% of the 795 British aircraft involved, the highest loss-rate for the RAF during the war. The Germans lose just 5 night-fighters that night and there is little industrial damage. In the year from April 1943 until the end of March 1944, 2,703 RAF bombers had failed to return from missions over Europe.

Imphal & Kohima. On the India/Burma boarder the 14th Army under General Slim is preparing to launch an attack, Operation Capital, aimed at clearing the Japanese out of Burma, with the Chindits already established in strength around Indaw, behind the Japanese lines. But then Intelligence is received that warns that the Japanese 15th Army has been reinforced by four divisions and is about to make its own attack into India, what they call the ‘March on India’, heading for the frontier garrison towns of Kohima and Imphal. This Intelligence is invaluable because it shows that the Japanese are short of supplies and are aiming to capture the British supply depots at these centres. It also discloses that the Japanese air force in Burma has dwindled to the point where it would be unable to prevent the British from re-supplying these bases by air. So General Slim decides on the dangerous strategy of forming defensive boxes round the two centres and allowing the Japanese Army to exhaust itself in attacks on them. But Bill Slim knows the formidable nature of his reformed army, who had just held the ‘Admin Box’ in the Arakan for three weeks, until the Japanese fall back defeated. They are now to fight ferocious battles for Kohima and then Imphal, being cut off for weeks but re-supplied by air. When the Japanese finally abandon the siege and withdraw on 22nd June, less than a quarter of their 85,000 strong invasion force are fit to fight.

The excellent high level Intelligence comes partly from the growing teams at BP who are working on the major Japanese codes, and partly from the Wireless Experimental Centre at Andand Parbat outside Delhi. It is thought that there were 1000 staff there at peak. The staff included members of the Intelligence Corps, British and Indian Army and Air Forces. Many of the codebreakers had served an apprenticeship at BP before coming out. Unlike at BP and at the Naval Intelligence Centre at Colombo where there were strong contingents of Wrens, there were relatively few British WAAFs or ATS there. It was commanded by Colonel Peter Marr-Johnson who did not have the light touch of a Denniston or the understanding authority of a Travis; Marr-Johnson was a man who believed in military discipline, so WEC was not an entirely happy environment for codebreakers. WEC controlled two major outstations at Bangalore and Barrackpore and had some 88 radio listening sets round India. As well there were three mobile Y stations with Slim’s army on the frontier, one of which was actually inside the box at Imphal. WEC had direct links to Arlington Hall in Washington, who had control of the breaking of the high grade Japanese General Army Administrative code, 7890. But WEC had a considerable team working on exploiting this code. The BP teams working on Japanese codes in Hut 10 (now housed in Block F) had recently broken an Army air force code called BULBUL, which enabled WEC to predict Japanese air raids, so helping to secure air superiority on the frontier. By the end of 1943 BP had also almost entirely built up the codebooks of the Japanese Army Air Force 6633 code used by their squadrons or above. This code was valuable in March 1944 when it carried the Japanese reactions to the Chindit landings. (Elsie Hart, a very bright ATS staff sergeant at BP, devised an index to the Japanese material; ‘Elsie’s Index’ became the hub round which the whole, ever-growing, section working on these Japanese Army codes revolved). The Australian & US Army Central Bureau, on the racecourse at Brisbane, had broken the 2468 Japanese Army ‘Water Transport Code’ in April 1943, and this had led to the breaking of other Japanese Army high-level codes. They receive a windfall in January 1944 when the Australians capture the entire cryptography library of the Japanese in New Guinea. The major codebooks of the high-command keys were found in a water-filled pit, but lacking their covers as these had been sent in to their HQ as evidence of their destruction. The Japanese did not change the codebooks because they are unaware of the capture. There are now many isolated Japanese garrisons, so some the books remained in use until the end of the war.

BP Developments. Hut 6 is alarmed to read a message that suggests the use of the variable reflector, UKD, is going to be extended to all the Luftwaffe Enigma keys. This would slow the rate of decryption dramatically. The Enigma teams at BP are trying to remove bottlenecks to speed up message handling in anticipation of the expansion in traffic and keys after D-day. This month they break three Army keys, Bantam (Western front operational) & Nightjar (Military occupation forces in France) being in use in the invasion area. They also broke two new Luftwaffe keys, an exceptionally good month.The success of Colossus in tackling the Fish codes is now well established so a preliminary order for four further Colossi is placed on the 14th March. [This order was increased to 12 at the end of April]. It is said that when Max Newman went to beg Tommy Flowers for the first of these to be ready by 1st of June he replied ‘I knew you would be back and have already called up the parts’. Just how dramatic the sight of the first computer at work was is recorded in the end-of-war ‘Tunny Report’: ‘It is regretted that it is not possible to give an adequate account of the fascination of a Colossus at work: its sheer bulk and apparent complexity; the fantastic speed of thin paper tape round the glittering pulleys; the childish pleasure of not-not, span, print main heading and other gadgets; the wizardry of purely mechanical decoding letter by letter, (one novice thought that she was being hoaxed); the uncanny action of the typewriter in printing the correct scores without and beyond human aid; the stepping of display; periods of eager expectation culminating in the sudden appearance of the longed-for score; and the strange rhythms characterising every type of run: the stately break-in, the erratic short run, the regularity of wheel-breaking, the stolid rectangle interrupted by the wild leaps of carriage-return, the frantic chatter of a motor run, even the ludicrous frenzy of hosts of bogus scores. Perhaps some Tunny-breaking poet could do justice to this theme; but although an ode to Colossus and various fragments appeared, all seemed to have been composed in time of distress and despondency, and consist almost wholly of imprecation or commination’. (Should the modern reader be unfamiliar with the word ‘commination’ it means the threatening of divine vengeance).

The Bletchley Park Trust welcomes the preparation of these notes, but the authors are responsible for the statements and the views expressed.

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'Does anyone know the author of the brilliant passage about the observing the first computer, Colossus, operating? It comes from the end-of-war, 500 page 'Report on Tunny' of which the main authors were Donald Michie and Jack Good. But it is known that Jack Good shared digs with Henry Reed, the poet ( author of the famous 'Naming of the Parts') and post-war playwright. He worked in Hut 3 as a translator. Is it possible that he was allowed to watch Colossus working after the end of the war, and inspired this purple patch - which seems out of place in an highly mathematical account of the work of the Newmanry'

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