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war during the disastrous incident when in November 1939 two SIS officers, Captains Payne and Best, were kidnapped at Venlo on the Dutch–German border by Gestapo agents posing as Resistance figures; German generals in British captivity were eavesdropped upon when they discussed important subjects such as rocketry. Nonetheless, Ultra was by far the most important intelligence source and, because of its direct nature, the least corruptible in analysis. The Bletchley code-breakers were, as Churchill put it, ‘the geese who laid the golden eggs’ and who, just as importantly, ‘never cackled’. They were also almost all amateurs, recruited from civilian life, although their contribution was far to outweigh that of the career intelligence officers of the day.6

After the invasion of Poland in September 1939, several senior Polish cryptographers escaped with their replica Enigma machine and were installed by the Deuxième Bureau in a château near Paris, where they began – with British and French help – to decode messages, although at the time it took them two months to do so, meaning that the information they divulged had usually been long superseded by events. On 12 February 1940, however, the German submarine U-33 was attacked off the west coast of Scotland and two of the extra rotor wheels used by the naval Enigma were captured. Five weeks later, a brilliant, eccentric, accident-prone, homosexual Cambridge mathematics don called Alan Turing installed something known as a bombe machine, an electro-mechanical device which made hundreds of computations every minute, at the Government Code and Cypher School at Bletchley Park in Buckinghamshire, 40 miles north-west of London. Other heroes of Bletchley were to include the mathematicians Stewart Milner-Barry and Alfred Dilwyn (‘Dilly’) Knox. In modern computing parlance, while the Poles provided the Enigma hardware, the assorted civilian geniuses stationed at Bletchley provided the software that produced Ultra.

Far from being a school, Bletchley was a department of SIS, operating from a Victorian mansion that housed 150 workers in 1939, before expanding into huts in the grounds to fit 3,500 people by 1942 and no fewer than 10,000 by the end of the war. (Several of the huts can still be seen today, including the ones where the most important work was done, along with captured Enigma machines and the bombe predecessors of the computer.) Huts 6 and 3 deciphered, translated, annotated and passed on Wehrmacht and Luftwaffe signals, while Huts 4 and 8 (run by Turing and subsequently the chess champion Hugh Alexander) did much the same thing for the Kriegsmarine, sending reports to the Naval Intelligence Division of the Admiralty. Hut 4 also analysed sudden increases and decreases of signals traffic volume, which could suggest possible enemy intentions. On 4 April 1940, five weeks before Hitler unleashed Blitzkrieg on the West, same-day decoding of the German Army codes first became possible, but on 1 May the British at Bletchley and Poles in France were ‘blinded’ for three weeks when the Germans altered their indicating systems.7 Overall, however, Wehrmacht and Luftwaffe signals were decoded between three and six hours after they were sent, and naval signals during the battle of the Atlantic could be read as swiftly as one hour after transmission.8

Before May 1940 the cracking of the codes depended upon chance factors such as the transmission of flaws and errors, as with one German unit reporting every morning the same phrase, Verlauf ruhig (situation unchanged), thus giving the Cambridge mathematics don in Hut 6, Gordon Welchman, who had improved Turing’s bombe machine in 1940, a vital clue about several letters.9 The major expansion of the Luftwaffe before the war meant that its signallers were generally less well trained and disciplined, and more sloppy, than their Army and Navy counterparts. The fact that there were only twenty-six letters in the alphabet, the key flaw in the machine that no letter could represent itself in the code, and the absence of number keys meaning that every number