The Origin of U.S.-British
Communications Intelligence Cooperation (1940--1941)
by Robert L. Benson
In this article I have excluded a discussion of certain Comint cooperation in the pre-war period between the
British and the FBI and U.S. Coast Guard. The role of the BSC remains to he told.
By mid-June 1940 Great
Britain was virtually alone in the war against Nazi Germany. France had just been defeated and most of Western
Europe was occupied by German forces. A German invasion of the British Isles seemed quite possible. The danger was
compounded by the tremendous losses in equipment that Britain had suffered in the Battle of France. At this point
Britain turned to the neutral U.S. for a broad range of assistance. It was in this context that the highly secret
U.S.-British relationship began in communications intelligence.
During that June the U.S. Naval Attaché in London received a proposal from British sources for exchange of
material and general cooperation in communications intelligence. especially in regard to the Japanese
The Japanese threat to Malaya and other British possessions in Asia was extremely grave. especially during a
period when reinforcement and resupply of empire garrisons was so difficult. In any event the British proposal was
rejected by the U.S. Navy.
On 8 July 1940, Lord Lothian. the British ambassador to the U.S. addressed a
memorandum to President Roosevelt suggesting, among other things, that the British government would appreciate a
broad exchange of secret technical information especially in the .radio field."
favorably viewed this suggestion and the War and Navy Departments were directed to consider the details of
technical exchanges. On 22 July Genera, Sherman Miles, the Assistant Chief of Staff, G-2 (Intelligence). War
Department General Staff. was designated as the Army's coordinator for technical exchange with the British.
Two weeks later a high-level U.S. military mission went to Great Britain to hold technical discussions. learn
British war plans and generally to assess the British capability to hold out against Germany. The Army was
represented by Generals George V. Strong of the General Staff and Delos C. Emmons of the Air Corps. The naval
representative was Admiral Robert Ghormley, the Assistant Chief of Naval Operations. Admiral Ghormley, at least,
had received his instructions personally from President Roosevelt. Concurrently, a British technical mission
headed by Sir Henry Tizard visited the U.S.
At this point the Comint issue was raised by the U.S. Army. On
1 September 1940 Colonel Spencer B. Akin, Chief of the Army's Signal Intelligence Service (SIS). and his principal
assistant. Mr. William F. Friedman, prepared an informal position paper for the Army's Chief Signal Officer.
General Joseph o. Mauborgne. Akin and Friedman suggested that the Army should fully exchange with the British. on
a reciprocal basis. all information related to cryptanalysis. Likewise there should be an exchange of intercept
traffic especially. so that the U.S. could obtain Japanese and German tactical traffic. But. wrote Akin and
Friedman, the Navy would have to agree to all this. Commander Lawrence Safford, the head of Op-20-G. the Navy's
Comint organization. posed no .objection to an exchange of traffic but would not agree to a sharing of
The matter was then dramatically escalated by a message to the War Department from General Strong in London.
London No. 401, September 5,1940
Are you prepared to exchange full information on
German, Italian and Japanese code and cryptographic information therewith? Are you prepared to agree to a
continuous exchange of important intercept in connection with the above? Please expedite the reply. This message
for the Chief of Staff from Strong.
The Navy would later claim that General Strong had acted abruptly and unilaterally. According to
Captain Alan Kirk, the U.S. naval attaché in London, General Strong. while addressing a British staff group,
suddenly offered the British all U.S. information on cryptanalysis of Japanese diplomatic systems. The British,
said Kirk, were astounded at Strong's offer but readily accepted it. That Strong acted without the Navy's
agreement is certain, but he was following a policy, as we have seen, that had been advanced within the War
General Strong's message was urgently couriered to General Mauborgne, who was visiting the Army Signal Corps
center at Fort Monmouth, New Jersey. Mauborgne was advised that the Navy had already rejected Strong's proposal.
General Mauborgne telegraphed his reply to the War Department:
Col. Spencer B.
Akin, U.S. Army, Chief of the Signal Intelligence Service during 1940-41 (a later photograph as a Major
N.J.. September 7,1940
As matter of utmost importance to Narional Defense
strongly urge concurrence Chief of Staff in proposa I General Strong that this governmenr exchange complete
rechnical information re Japanese. German and Italian codes and cipher systems but believe constant exchange
inrercepred traffic unnecessary. Each government should reiy upon own intercept services for collection material
General Mauborgne's position was endorsed by General Miles and was
forwarded to the Chief of Staff, General George C. Marshall, General Marshall approved the exchange. The matter
was put before Secretary of War Henry L. Stimson in early October. In a memo to Mr. Stimson's military aide,
General Miles stated that it was absolutely essential to undertake an immediate exchange with Great Britain of all
"... information concerning military, military attaché, and diplomatic codes. ciphers, cipher devices and
apparatus and code and cipher systems employed by Germany. Italy and Japan together with all information
concerning the methods employed to solve messages in codes and ciphers of the classes mentioned." General Miles
emphasized that the U.S. could significantly assist Great Britain by providing Comint material, while the U.S. in
turn would (hopefully) receive hitherto unavailable Axis military traffic and solution data." What is especially
interesting is the timing of General Miles' memo. Only days before, on 27 September 1940. the SIS had made their
first solution to Japanese diplomatic messages enciphered in the Purple Code. This astounding breakthrough,
accomplished because the SIS was able to construct a duplicate of the Japanese Purple machine, was now to be
shared with the British.
The Army proposals were approved by Secretary Stimson and ultimately the President, although a detailed account of
how the decision was made does not seem to exist. Implementation was delayed by practical matters. Extra Purple
machines to be delivered to the British had to be constructed. This was done by the Navy which, reluctantly, now
agreed to join the Army in a general Comint exchange with the British. In mid-January 1941 a British joint-service
staff group arrived in the U.S. aboard the new battleship George V. This group was the nucleus of the permanent
British joint Staff Mission in Washington. On 17 January Captain Abraham Sinkov and Lieutenant Leo Rosen of the
SIS and Lieutenant Robert Weeks and Ensign Prescott Currier of Op-20-G departed for England aboard the George V.
They took with them two Purple machines and other cryptologic materials. They delivered this material to the
British Comint organization, the Government Code and Cipher (GCCS), at Bletchley Park. As Mr. Sinkov would recall,
the planning and execution of this mission had been so secret that he (Sinkov) never knew if the British expected.
to receive the Purple machines or indeed if they even knew that the U.S. had solved the system.
Col. Raymond E. Lee,
U.S. Army, U.S. Military Attaché, London during 1940-41.
mission remained at Bletchley Park over two months. From GCCS they received information on German, Italian,
Japanese, Russian and Latin American cryptographic systems, both civil and military/naval, and learned the status
of various British Cornint operations. The naval members of the U.S commission studied British intercept and
direction-finding techniques and received British equipment to take back to Op-20-G. The British, now in
possession of Purple, and with the approval of Prime Minister Churchill, shared their greatest secret: that the
German Enigma cipher machine, used by all the German armed forces, had been solved and was being exploited.
Captain Sinkov and his associates were sworn to secrecy and gave their assurances that they would reveal the
Enigma secret to only a limited (and specifically named) number of American officers.4
The Sinkov group was not allowed to make written notes concerning Enigma and the British would not furnish any
documents relating to solution or exploitation techniques. in -fact, the British technical briefing on Enigma was
not adequate to allow the U.S. to duplicate the British success.
The Sinkov group returned to the U.S. on a British destroyer in April. While this group had been at Bletchley
Park, related discussions were taking place in the Far East.
Sinkov, U.S. Army, Signal Intelligence Service. Captain Sinkov's mission to Bletchley Park belped establish the
U.S.-U.K. Comint relationship.
In February 1941 the British Admiralty
authorized the British naval command in the Far East to undertake an immediate exchange of Comint with U.S. naval
forces in [he Philippines. Soon thereafter a Purple machine was delivered from Bletchley Park to the British
Comint organization in Singapore, The U.S. Naval Comint unit in the Philippines. known as the Cast unit, received
a Purple machine from 0p-20-G at about the same time. At the end of February Commander Jefferson Dennis of the
Cast unit and Captain Archer Allen, the U.S. Naval Observer in Singapore met in Singapore with British Comint
personnel. A significant amount of cryptanalytic information on Japanese naval and merchant marine systems was
exchanged. A British representative visited the Cast unit on Corregidor in April 1941 and detailed arrangements
were made for regular exchange of Japanese traffic and cryptanalytic data. !his exchange of Comint material,
either by radio or 10 bulk via Pan American Clipper, continued until the U.S. entered the war."5
With wide-ranging Comint cooperation with the British now a fact of U.S. policy, additional specific arrangements
continued to be made. In the late Spring of 1941, Captain Edward G. Hasting~, Royal Navy, a veteran Comint
officer, came to Washington to direct the working committee of the U.S.-based adjunct of the British Joint
Intelligence Committee. Hastings was, in fact, a representative of the Chief of the Secret Service (CSS) and was
mainly concerned with British- U.S. Comint activities.6
A regular exchange of Comint between
the Navy's Op-20-G in Washington and GCCS at Bletchley Park began in June 1941. Material was passed by radio and
cable or, most often during that period, by air pouch. Ultimately the medium for exchange was the office of the
British Security Coordination (BSC) in New York City. The BSC. headed by William Stephenson, was the Western
Hemisphere arm of the British Secret Service.
In August 1941 Commander A. G. Denniston, head of GCCS,
visited the SIS and Op-20-G in Washington. He was apprised of U.S. Comint efforts and was shown the
machine-processing techniques being used by the SIS.7 Denniston in turn advised his hosts of the status
of various British operations. As a result of the Denniston visit a British officer, Major Geoffrey Stevens, was
permanently assigned to SIS as the GCCS liaison officer.
So, by the time of the Pearl Harbor attack, the
U.S. and Great Britain had been limited partners in communications intelligence for a year. Unfortunately, the
full development of this relationship did not automatically begin when the U.S. and British openly became allies
in December 1941. This will be described in a future article.
1. Earlier in
1940 the U.S. Naval Attaché seems to have been approached regarding possible Comint cooperation. However, this
earlier request was narrower and less urgent. During World War I, the U.S. and Britain had had some
association in Comint.
2. Colonel Raymond V. Lee was the U.S. Military attaché in Great Britain.
3. In 1940 the
Army's SIS had no means of intercepting German or Italian military traffic and extremely limited access to
Japanese military traffic. This situation did not greatly change until late 1942. Throughout the pre- war period
concentrated on diplomatic traffic.
4. Captain Sinkov agreed to reveal the Enigma secret only to
General Miles, General Mauborgne and Mr. Friedman.
5. The author has not seen any U.S. documents that describe
the decision-making process that preceded these Comint
exchanges in the Far East.
6. Captain Hastings remained in this position until Spring 1943. His relations with the U.S. Comint
organizations of the Army,
Navy, FBI and Coast Guard were often stormy and marked by not inconsiderable
confusion on all sides. The British Secret
Service was also known as the Secret Intelligence Service (SIS)
7. One issue disturbed Commander Denniston. He observed that the Navy's 0p-20-G was proceeding on its
own in an attack on
the German Naval Enigma. Denniston had hoped. for reasons of security and efficiency.
that the U.S. would concentrate on