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rights in Europe that were not far short of Hitler’s. Even had the Führer perished, there would have been nothing plausible for Germany’s enemies to discuss with his domestic foes. At the very least, however, British paranoia about suffering a repeat of the Venlo humiliation permanently excluded MI6 from some useful sources, which the Russians and later the Americans were left to exploit. Moreover, for the rest of the war Broadway’s chiefs maintained an exaggerated respect for their German adversaries, derived from the memory of having been fooled by them in November 1939.

      Through the icy winter months of the ‘Phoney War’, the GC&CS at Bletchley struggled with the intractable Enigma problem, while Broadway’s spies produced little or no useful information about the enemy and his intentions. Kenneth Strong of War Office Intelligence wrote: ‘We had a continuous stream of callers from the Services with an extraordinary variety of queries and requests. What were the most profitable targets for air attacks in this or that area, and what effect would these attacks have on the German Army? Was our information about these targets adequate and accurate? How was the German Army reacting to our propaganda campaigns? I found some quite fantastic optimism regarding the effects from propaganda. The dropping of leaflets was considered almost a major military victory.’

      Some MI6 officers went to elaborate lengths to conceal their lack of agent networks. Reg Jones cited the example of Wilfred ‘Biffy’ Dunderdale, who was responsible for France, and fed to Jones’s branch a succession of tasty titbits on the German Ju-88 bomber, allegedly collected by spies. First there was information about its engines; then its electrics; and somewhat later its armament. Jones teased Dunderdale that he must have secured a copy of the aircraft’s operating handbook, then fed extracts to Broadway, to create an impression of multiple sources. The hapless officer admitted that Jones was right, but begged him to keep his mouth shut. He could keep his bosses much more interested, he said, by drip-feeding the data. This was not the only occasion when Dunderdale – like officers of all intelligence services – sought to ‘sex up’ the means by which his material had been acquired. He also produced details of German troop movements supposedly secured by agent networks, which in reality derived from French intercepts.

      Much could be learned from an enemy’s wireless transmissions, even without breaking his codes, through ‘traffic analysis’ – the study of signal origins, volume and callsigns to pinpoint units, ships, squadrons. Useful information was also gleaned by the ‘Y Service’, eavesdropping on voice transmissions, and by breaking simple enemy codes used for passing low-grade messages. The French forward cryptographical unit was based at ‘Station Bruno’, in the Château de Vignobles located at Gretz-Armainvilliers, fifteen miles east of Paris. Bruno received an important reinforcement following the fall of Poland. Guy Liddell of MI5 recorded on 10 October 1939 that seventeen Polish cryptanalysts were seeking asylum in Britain. Bletchley Park shrugged dismissively that it had no use for them, even though its chief Alastair Denniston had met some of the same men in Warsaw a few months earlier, and knew that their claims to have penetrated Russian and German ciphers ‘can to some extent be maintained’.

      Denniston suggested that they would be more useful at the Château de Vignobles, working with Gustave Bertrand, which was where they were sent – though Bletchley later changed its mind and tried in vain to get them back. It was at Bruno, on 17 January 1940, that the ex-Warsaw group broke its first wartime Enigma signal. By 11 March Col. Louis Rivet, head of the French secret service, was writing in his diary: ‘The decrypts of the Enigma machine are becoming interesting and numerous.’ During the months that followed, however, material was read far too slowly – out of ‘real time’ – to influence events on the battlefield. Instead, Allied intelligence officers strove to make sense of a jumble of humint warnings, of varying degrees of plausibility, about when Hitler intended to strike in the West.

      The first of these had come in the previous November when Major Gijsbert Sas, Dutch military attaché in Berlin, received a dramatic tip-off from his friend Colonel Hans Oster of the Abwehr: the Wehrmacht, said Oster, would launch a full-scale offensive against the British and French armies on the 12th of that month. This coincided with several other identical or similar warnings – including an important one from Col. Moravec’s Czechs in London, relayed by their man in Switzerland from Agent A-54, the Abwehr’s Paul Thummel. When nothing happened on 12 November, the British and French chiefs of staff assumed that they were the victims of Nazi disinformation. The Dutch already suspected Sas of being a double agent, and the credibility of the other sources, including A-54, suffered accordingly. Yet the warnings were correct. Hitler had indeed intended to strike in November. He was enraged that his generals insisted upon a last-minute postponement until spring, because the army was unready to move. Here was a vivid illustration of a precept later advanced by a British Army intelligence officer: ‘Perfect intelligence in war must of necessity be out-of-date and therefore ceases to be perfect … We deal not with the true, but with the likely.’

      The next excitement took place one day in January 1940: thick fog caused a German courier aircraft flown by Major Erich Hönmanns to forced-land in neutral Belgium. Local police arrested the pilot and his passenger, an officer named Reinberger, interrupting them as they attempted to burn papers they carried, and retrieved the charred sheets from a stove. Within forty-eight hours the French and British high commands were reading the Wehrmacht’s plan for its intended invasion of France and the Low Countries, focused on a thrust through Holland and Belgium. Here was a textbook example of a genuine intelligence coup, with wholly unhelpful consequences. The French were confirmed in their conviction that the Germans would attack through Belgium as they had done in 1914, and as all France’s deployments anticipated. The British suspected an enemy deception: the material seemed too good to be true. Guy Liddell of MI5 wrote wearily on 14 January: ‘A German aeroplane came down in Belgium … with certain papers found on the pilot indicating projected attack by the Germans on Belgium and Holland. It looks rather as if this may have been part of the scheme for the war of nerves.’ Cadogan at the Foreign Office described receiving ‘complete plan of German invasion of the Low Countries. Very odd. But one can’t ignore these things, and all precautions taken.’

      Kenneth Strong wrote ruefully afterwards: ‘So often I have heard it said that if we only had the plans of the other side things would be simple: when they actually came our way we found great difficulty in persuading ourselves that they were genuine.’ Most important, however, the capture immediately forfeited all virtue, because the German proprietors of the plan knew that the Allies had it. Thus, Hitler insisted on changing the invasion concept, to thrust instead through the Ardennes, which proved the one authentic strategic inspiration of his life. Here was another critical lesson about intelligence, especially important for codebreakers: captured material became worthless if its originators discovered that it was in enemy hands.

      Alexander Cadogan noted in his diary for 19 January 1940 that Stewart Menzies now seemed to expect the Germans to attack soon after 25 January, and added dismissively, ‘but he’s rather mercurial, and rather hasty and superficial (like myself!)’. If this remark somewhat short-changed the diarist, it was scarcely a ringing endorsement of ‘C’. There was one further strand: low-grade Abwehr messages decrypted by MI5’s Radio Intelligence Service offered indications about the looming onslaught. At that time, however, machinery was lacking to analyse such material, to feed it into the military command system and ensure that notice was taken by commanders. In that pre-Ultra universe, politicians, diplomats and generals were chronically sceptical about intelligence of all kinds. When a new warning reached MI6 via Moravec’s ‘London Czechs’ – that Abwehr officer Paul Thummel expected a great Wehrmacht thrust on 10 May, it vanished in the welter of ‘noise’ that spring.

      The 9 April German invasion of Norway caught the Western Allies totally by surprise. Though no decrypts were available, the Admiralty ignored or misread plentiful clues about Hitler’s intentions. When the Wehrmacht’s amphibious forces began to land on the Norwegian coast, the Royal Navy’s major units were far away, awaiting an anticipated breakout into the Atlantic by German battleships. Through the weeks that followed, Wehrmacht eavesdroppers easily tracked the British brigades struggling to aid the little Norwegian army, while intelligence learned little or nothing about the invaders’ lightning movements.

      On 10 May 1940, Hitler launched his Blitzkrieg in the West. The