Skipping a complete history of U.S. intelligence efforts, it is still helpful to get a feeling for where the U.S. Navy stood as to intelligence on 7 December 1941. At that time, the open intelligence agency responsible for providing intelligence to the U.S. fleet was the Office of Naval Intelligence (ONI). ONI basically consisted of a small administrative, analysis and reporting office in Washington D.C., Naval Attaché’s at many embassies and consulates, and District Intelligence Officers with very small staffs at all Naval Districts and some commands. Despite these efforts, the strict secrecy programs of the Japanese Navy contributed to the failure of ONI to fully appreciate the enormous war making capabilities of the Imperial Japanese Navy (IJN). Very little intelligence from prior decrypted IJN communications was distributed beyond the top ONI officers or was permitted in ONI reports
Beginning about January 1924 with limited funding, a naval capability to intercept, locate, analyze and decrypt communications of potential enemies was started in the offices of the Director of Naval Communications (DNC). This fledgling organization was eventually organized as OP-20-G and was located in the Navy Department offices in Washington D.C. It established intercept and High Frequency Direction Finder (HFDF) sites in the Pacific, Atlantic and continental U.S. as well as a Japanese telegraphic code school for radio operators in Washington D.C. Decryption and analysis units were established in the Pacific. In addition to intercepting naval communications from Japanese, German and Italian navies, the Navy also copied diplomatic messages of many foreign governments. The majority of effort was directed towards Japan and included breaking the early Japanese “Blue” book fleet code. Liaison was established with the Army’s counterpart, Signals Intelligence Service (SIS) through its Washington headquarters in the nearby Munitions Bldg. With the assistance provided by the Navy’s break into both the prior M-1 machine cipher used by Japanese naval attaches and other assistance, the Army was able to reconstruct a working cipher machine to produce immediate plain language texts of the new Purple diplomatic cipher used by the Japanese. Navy Lieutenant (j.g.) Francis A. Raven solved the Purple key system so that a key only had to be recovered every tenth day. The Navy constructed at least five of these machines. An interesting development was that the fifth Purple machine that was earmarked for the cryptanalysts at Station Hypo at Pearl Harbor was given to the British in addition to a machine previously provided to London. One wonders what the outcome of the Pearl Harbor attack would have been if this machine had not been diverted from Pearl Harbor and that unit was permitted to also decrypt the diplomatic messages leading up to the attack on Pearl Harbor.
One ONI program that greatly assisted OP-20-G’s Japanese decrypting and translation efforts was the establishment of a resident language program in Japan for naval officers. Many of these language officers eventually found there way into OP-20-G billets in Washington, Pearl Harbor or the Far East. One drawback of this program was that since most of these language officers were naval academy graduates, they had to alternate their radio intelligence work with tours of duty at sea in order to be promoted within the Navy’s current policies. In fact, some of these officers were passed over for promotion for lack of suitable sea experience despite the incomparable value their work proved to be up to and throughout WWII.
In 1941, SIS was primarily involved in intercepting and decrypting diplomatic traffic with an emphasis on Japan and Germany. When the volume of Tokyo-Washington diplomatic traffic increased with the Nomura talks beginning in mid-1941, the decrypting of high level Japanese messages with the Purple cipher machines was shared on alternating days between the Army and Navy. While the output of all readable Japanese diplomatic decrypts was considerable, there were not enough cryptographic personnel available in either service to timely decrypt all Japanese diplomatic traffic in readable systems, even for sensitive locations like Honolulu.
At this time, SIS had a total of about 331 officers, men and civilians but not all were engaged in cryptanalysis. As opposed to Navy practice, relatively little effort was expended on the interception and decryption of Japanese Army messages. OP-20-G had about 730 personnel throughout the world for all purposes. Additional funds and the general military increases had recently achieved even these small figures since the draft was instituted in 1940. To get a perspective on the numbers required by war’s end to adequately exploit Japanese and German messages over a fraction of their former geographical areas, SIS had 7,000 personnel plus those in Australia and OP-20-G exceeded 8,000. In addition to its contribution of decrypting Japanese diplomatic traffic, OP-20-G and Station Cast at Cavite/Corregidor had succeeded in establishing the basic encryption method of JN-25, the code used by major IJN fleet units and commands. Washington had recovered a number of stereotyped messages in the second version, JN-25B but not on a timely basis. A JN-25B key change on 1 December was a setback to this cryptanalytic effort. The exclusive assignment of a little used IJN Admiral’s code to the Pearl Harbor unit proved to be a terrible waste of some of the best cryptanalysts the Navy had.
It is important
to emphasize the lack of any formal distribution procedures to inform responsible
fleet commanders of the intelligence information being gleaned from decrypts
of Japanese communications. In the Navy, this was complicated by
the self appointed intelligence expert of then Captain Richmond K. Turner
known as “Terrible Turner”, the new head of the Navy’s War Plans department
of CNO. The weakness of Admiral Stark as CNO let Turner completely
usurp the functions of ONI and DNC to fulfill their responsibilities to
properly warn fleet commanders of the impending Japanese actions based
on the Purple diplomatic decrypts and other indicators. More serious
war warning messages and a more accurate picture of the current situation
as indicated by Japanese decrypts that were advocated by Captain Laurence
Stafford as OP-20-G, Admiral Noyes DNC, and the acting Director of Intelligence
(DNI), Captain Kirk, were forestalled or greatly watered down by Turner.
One excuse Turner tried to give for such perfunctory warnings was that
Pearl Harbor had all the Japanese diplomatic decrypts, which was false.
Earlier, Captain Turner was convinced Japan would only attack Russia and
just before Pearl Harbor he convinced Stark that Japan was not ready to
attack the U.S. only the British. The new DNI Theodore S. Wilkinson
refused to challenge Turner’s rebuff of a further specific war warning
drafted by Captain Arthur H. McCollum on 5 December. Again on 6 December,
Stafford tried again but was dismissed by Noyes so as not to antagonize
Turner. On the Army side, General George G. Marshall and intermediaries
vetoed similar requests made by Colonels Rufus S. Bratton and Otis K. Sadtler.
Later, Marshall denied receiving the related decrypts. As Washington
politics go, both Stafford, Bratton and Sadtler were relegated to rather
minor posts and discredited, while Noyes and Turner were given prime advancement
billets and promotions. Although General Marshall was held to have
been derelict in his duties by the first Army board of inquiry on the Pearl
Harbor attack, the subsequent congressional investigation only found Admiral
Kimmel and General Short at fault for the Pearl Harbor disaster.
Marshall had the backing of both Secretary of War Stimson and President
Roosevelt. Stimson instigated a fierce campaign to reverse Marshall’s
prior dereliction finding. During the latter hearings, none of Turner’s
subordinates would break ranks and reveal Turner’s derelictions due to
his great wartime achievements and rank as Vice Admiral. Only subsequent
revelations have verified Turner’s and Marshall’s responsibility for impeding
more appropriate and timely warnings urged by intelligence professionals
based on Purple decrypts.