the Middle East, Britain had a considerable incentive to continue to work on American commercial traffic, much of which was in commercial code or plain text. Indeed, a close reading of Churchill’s assurance to Roosevelt suggests that it might have related to diplomatic traffic only.44 Some GC&CS staff recall work on the traffic of American commercial attachés throughout the war, although as yet no documents have been released.45 Predictably, clear traffic from American oil companies was being intercepted in 1944 as they began to look for new markets in Europe.46
For this very reason, the US Army and Navy were agreed that nothing should be passed to the British about American code-making procedures, such as the Sigaba cypher machine. General George Marshall, the US Army Chief of Staff, specifically forbade any such exchange in September 1940.47 Anxiety about protecting national cypher systems persisted through the war on both sides of the Atlantic. In February 1945 Britain’s newly formed Cypher Policy Board debated a proposal by its Secretary, Captain Edmund Wilson, for ‘free and complete interchange’ with the Americans on cypher machine development, together with scrambler phones and secure speech.48 This horrified both Edward Bridges, the Cabinet Secretary, and Sir Stewart Menzies, Chief of SIS, and the idea was rejected. Cooperation on communications security would focus on machines specially designed for combined use.49
The gradual collapse of the British monopoly over Ultra intelligence paved the way for closer Anglo-American sigint cooperation. As we have seen, Bletchley’s initial idea for wartime cooperation was that the Americans would continue their pre-war focus on Japanese traffic; meanwhile the British would handle the work on Enigma, dispensing its product to the Americans as they saw fit. Although they had informed the Americans about Enigma in 1941, some precise details of processing had been withheld. Bletchley was determined to prevent the Americans working on Enigma in parallel, even though the Battle of the Atlantic gave Washington a legitimate need for Ultra intelligence. However, once the German Navy introduced an improved Enigma machine with four rotors, the British could not produce enough ‘bombes’ to deal with the increased number of tests required to break it.50 In September 1942, Joseph Wenger, who led the US Navy code-breakers, proposed spending $2 million to acquire no fewer than 230 four-wheel ‘bombes’. This was ten times the number available to Bletchley. John Tiltman, Britain’s Soviet code specialist, realised that American sigint was beginning to operate on an industrial scale, and that for Bletchley Park the game of ‘Ultra monopoly’ was surely up.51
In September 1942, Edward Travis and the head of Bletchley Park’s Naval Section, Frank Birch, travelled to Washington and concluded the ‘Holden Agreement’, which established full and integrated collaboration on German naval traffic, including Enigma. This was a key part of the emerging Anglo-American sigint relationship, and a constituent part of the secret alliance which still exists to this day.52 Travis’s hand was strengthened by the remarkable fact that the US Navy breathed not a word about the Holden Agreement to the US Army. The British therefore persisted in their hopes of keeping control over the processing of Ultra material derived from Luftwaffe and German Army traffic. Nigel de Grey, the Deputy Director of Bletchley Park, was apoplectic at the possibility of the Americans being allowed to duplicate further British work on Enigma. However, a US Army code-breaker based at Bletchley, Colonel Telford Taylor, suggested a tactful way forward. He advised his superiors in Washington that all they needed for the time being was a small ‘foothold’ in the work on Enigma, which would allow them to gain experience. More level-headed organisational types at Bletchley Park, such as Gordon Welchman, could see that the ability of the Americans to procure unlimited numbers of bombes was crucial, adding, ‘We certainly need help.’53 The result was the BRUSA agreement, a further crucial landmark in the construction of the Anglo-American sigint relationship. On 17 May 1943, Bletchley agreed to American participation in work on German Army and Air Force traffic. A second Holden Agreement on naval sigint followed in 1944. These treaties were of enormous importance, and paved the way for more ambitious post-war sigint alliances.
The exigencies of war had broken Britain’s cryptographic monopoly on Ultra. However, Ultra was a military system, representing the core work of Bletchley Park. There is no evidence that Britain and the United States concluded an overarching treaty on diplomatic or commercial sigint, the material that GC&CS worked on at Berkeley Street. In 1942, Alastair Denniston, who had been moved sideways to manage diplomatic sigint, arranged for cooperation on a number of specific countries such as Italy, France, Spain, Portugal, Japan and, of course, Germany. However, this was done on an ad hoc basis. There was no diplomatic BRUSA agreement. It seems that the Americans were not intercepting and working on a range of materials that would have prompted a wider deal. Typically, Denniston told John Tiltman, with evident relief, ‘They do no work on any of the Near Eastern governments.’54
Denniston’s main point of contact in the United States was William Friedman and the US Army code-breakers, who dominated American work on diplomatic systems. The Americans were keen to cooperate, since up until 1941 the US Army had been intensely focused on the diplomatic cyphers of Japan. In 1940 the Americans lost access to Japan’s diplomatic cypher, and it was only recovered as the result of a prodigious effort by a team under Frank Rowlett. By contrast, British code-breakers were working on the diplomatic cyphers of some twenty-six different countries.55 Therefore, when the Americans offered access to ‘Magic’, the British reciprocated with a wide range of diplomatic material, including high-grade Italian systems. Then, in March 1942, John Tiltman visited Washington and brought with him Spanish and Vichy French cyphers. Given the arrival of American forces in the Mediterranean, this was valuable material. By 1944 the Americans had received more material from the British on diplomatic cyphers used by the Greeks, Hungarians, Iranians and Iraqis. However, the processing went on behind a curtain. Denniston asked at one point, ‘Do they actually work on the stuff which we send them, or do they simply put it in their library?’ Diplomatic cyphers from countries that the British considered to be client states, such as Egypt, were withheld.56
Sharing diplomatic product caused some embarrassing problems. Foreign diplomats in London or Washington often reported their conversations with British officials in the messages they sent home. The British sometimes did not want the Americans to ‘listen in’ on these conversations, since they might involve ‘disparaging remarks about American policy or officials’. Therefore, they developed a special reserved series called ‘Res’,
that contained material that was not to be given to the Americans. This was not an effective solution, because, as Alexander Cadogan, the senior official at the Foreign Office, explained to Stewart Menzies, the Americans would often obtain and break some of the same traffic themselves, and so would ‘become suspicious’. By the spring of 1944 the Americans clearly knew about ‘Res’, and pressed the British to abandon the practice. However, Cadogan refused, since the war was drawing to an end, and the antagonistic politics of post-war settlements were looming.57
The Americans nurtured their own anxieties. Would Anglo-American sigint cooperation continue after the war? As early as 1942, Colonel Alfred McCormack, one of the more important visitors to Bletchley, warned his superiors in Washington that the British were ‘very realistic people’, and so would ‘certainly at some time – possibly while the war is still on – resume work on United States communications’.58 However, continued convergence of Anglo-American sigint was ensured by early fears of the Soviet Union, which were visible as early as 1942. Senior officers on both sides