The Secret War: Spies, Codes and Guerrillas 1939–1945. Max Hastings

derived from eavesdropping on the Italians, but few Enigma messages were broken quickly enough to assist decision-making on the battlefield. At sea, by contrast, there was an early golden prize for GC&CS’s labours.

      The impetus towards success was provided by a series of captures far out on the ocean, which dramatically increased Bletchley’s knowledge of the enemy’s naval communications. On 23 February 1941, British commandos raiding the Lofoten Islands seized the German armed trawler Krebs, from which spare rotors for a naval Enigma were recovered, though the machine itself had been thrown overboard. This ‘pinch’ prompted the Royal Navy to launch an operation explicitly designed to capture more Enigma material, targeting German weather-reporting trawlers between Iceland and Jan Mayen Island. On 7 May, a sweep by three cruisers located and seized the München – but too late to save its Enigma and associated coding data from the Arctic deep. On 25 June the navy caught its sister vessel the Lauenburg, again minus its Enigma, but with a useful haul of cipher material.

      Hut 8 now had enough information to read some U-boat signals, but the seizure which opened the traffic to fluent decryption was the fruit of chance and high courage, rather than of design. On 9 May 1941 a convoy escort group attacked and forced to the surface Julius Lempe’s U-110. A boarding party from HMS Bulldog commanded by Sub-Lieutenant David Balme secured the submarine, prevented its sinking, and brought back to his destroyer pearls beyond price: documentation for current Enigma. Though U-110 later sank under tow – fortunately so, from a security viewpoint – the short signal book, officer ciphering instructions and other material reached Bletchley safely, and the secret of the submarine’s capture was preserved beyond the war’s end. An Enigma machine was also recovered, but perversely this was the least useful element of the booty, because Bletchley had one already, together with assorted rotors seized in other ‘pinches’. Within days, Hut 8 was reading a steady stream of German naval messages. Ralph Erskine, one of the foremost experts on codebreaking at Bletchley, believes that the Park was already close to reading the Kriegsmarine traffic, even without the U-110 haul. What is for certain, however, is that it was impossible to break the U-boat ciphers without the assistance of captured material, which would again become a vital issue later in the war.

      The breakthrough into the Kriegsmarine ciphers came just too late to influence the pursuit of the Bismarck in the latter days of May 1941. Conventional direction-finding on the behemoth’s wireless transmissions, supported by air reconnaissance, were the key factors in enabling the Royal Navy to intercept and sink it on the morning of the 27th, though assisted in the last stage by decryption of a Luftwaffe signal revealing Brest as Bismarck’s destination. Thereafter, Bletchley produced a steady stream of messages that revealed U-boat positions and intended courses. The so-called Hydra cipher was laid bare, and other keys were progressively broken: the more the Park knew, the more it was able to discover. The flow of decrypts was never assured, however, and disturbing delays sometimes took place. ‘Huff-Duff’ – High-Frequency Direction-Finding – played an important secondary role in the location of U-boats. The outcome was a relentless shift in the balance of advantage in the Battle of the Atlantic through 1941 and into 1942. Here was a case where intelligence indisputably and importantly influenced events.

      Bletchley was also reading a significant portion of Italian naval traffic. On 25 March 1941, one of the small number of women decrypters, nineteen-year-old mathematician Mavis Lever in Dillwyn Knox’s team – he was famously supportive of talented girls in a male-dominated institution – played a critical role in breaking a message which revealed that the Italian fleet would shortly put to sea to attack British convoys. The warning enabled the Mediterranean C-in-C Admiral Sir Andrew Cunningham to contrive an encounter off Cape Matapan during the afternoon and night of 28 March which ended in a striking victory for the Royal Navy. By dawn on the 29th, three cruisers and two destroyers had been sunk, while the battleship Vittorio Veneto was damaged, an outcome that deterred the Italian surface fleet from making any further attempt to interdict British troop movements to Greece.

      Spring brought an increasing flow of decrypts about Wehrmacht operations in the Eastern Mediterranean. Senior officers strove to streamline the transfer of information from Bletchley to battlefields, so that material reached commanders in real time. One of the most significant intercepts, detailing German plans for the May 1941 invasion of Crete, reported ‘probable date of ending preparations: 17/5. Proposed course of operation … Sharp attack against enemy air force, military camps and A/A positions … Troops of Fliegerkorps XI: parachute landing to occupy Maleme, Candia and Retiomo; transfer of dive-bombers and fighters to Maleme and Candia; air-landing operations by remainder of Fliegerkorps XI; sea-transport of flak units, further army elements and supplies.’ Churchill personally annotated the flimsy: ‘In view of the gt importance of this I shd like the actual text transmitted by MOST SECRET together with warnings about absolute secrecy.’ This information was passed to Wavell and Freyberg, the relevant commanders, at 2340 on 6 May. The loss of the subsequent Battle of Crete, following the German invasion which began on the morning of the 20th, emphasised a fundamental reality about Enigma decrypts: they could change outcomes only when British commanders and troops on the ground were sufficiently strong, competent and courageous effectively to exploit them. Stuart Milner-Barry of Hut 6 said that he and his colleagues looked back on Crete as ‘the greatest disappointment of the war. It seemed a near certainty that, with … every detail of the operation spelt out for us in advance … the attack would be ignominiously thrown back.’

      The Cretan signal, informing British generals of German intentions in time to respond, was an exception rather than a commonplace in 1941. Bletchley was able to provide an ever-growing flow of information about the deployments of the enemy, not least in Eastern Europe, most of it derived from Luftwaffe and army-Luftwaffe decrypts. Wehrmacht traffic stubbornly resisted penetration, but German railway codes provided information about – for instance – troop movements to Yugoslavia, Greece and Eastern Europe in the summer of 1941. Hitler’s looming invasion of the Soviet Union, the towering event of the war, was also the first great strategic development for which Ultra intelligence provided explicit warning. While Britain had no power to influence or impede Hitler’s Operation ‘Barbarossa’, it was clearly of the highest importance to Churchill and his generals to be able to monitor its unfolding.

      It became a source of increasing frustration to the prime minister that British troops in North Africa failed to frustrate or defeat Rommel when they had not only superiority in men, tanks and guns, but also an ever-growing stream of information about German deployments and movements, for instance at Halfaya Pass in May. Churchill pored intently over his own daily file of Ultra material. When he read a decrypt reporting petrol stocks at various Luftwaffe airfields in Libya, he scrawled on it in his red ink: ‘CAS [Chief of Air Staff] How many hours flying can their a/c do on this – about? WSC.’ Air Chief Marshal Sir Charles Portal responded testily: ‘Unfortunately it is not possible to make any general deduction since the figures only relate to the stock at Benghazi. We do not possess complete figures for the supply and consumption of oil and petrol throughout Libya. All we know is that there are indications of an overall shortage which is limiting operations in the forward area.’ This problem was endemic when decrypts were fragmentary. Stewart Menzies performed an important service by dissuading the prime minister from fulfilling his frequent desire to dispatch raw Ultra direct to commanders-in-chief in the field, as he had done in the case of Crete. ‘C’ was surely correct, on security grounds, and also because decrypts that lacked the context of other intelligence could be highly misleading to untrained eyes.

      On land, in 1941 Bletchley provided more guidance to strategy than to tactics: it gave Churchill’s high command an authoritative, though never comprehensive, picture of German deployments in every theatre of war. Ultra could do little to assist the RAF’s ongoing struggle with the Luftwaffe for mastery of the skies. Only the Royal Navy gained immediate advantage, both in the Mediterranean and the Atlantic. Nothing altered the fact that, until the worldwide balance of strength began to shift in the Allies’ favour in the latter part of 1942, the operational superiority of German and Japanese forces enabled them to keep winning victories. Bletchley was an increasingly important weapon, but it was not a magic sword.

      The practices and disciplines of GC&CS evolved progressively, with many wrangles and turf wars along the way. Deputy director