Intelligence and the Japanese Attack on Pearl Harbor
Lori S. Tagg, US Army Intelligence Center of Excellence Command Historian
In the months leading up to December 1941, with war raging in Europe, the President of the United States and the Japanese Emperor negotiated for peace in the Pacific. These efforts had been largely unsuccessful. On December 6, 1941, the Army’s Signal Intelligence Service (SIS) intercepted a communication from the Japanese government to its delegation in Washington, DC. The SIS decrypted the first 13 parts of the message spelling out Japanese claims of American transgressions in the Far East. At 5:00 am December 7, the 14th and final part of the message arrived, declaring “The Japanese Government regrets to have to notify the American Government that in view of the attitude of the American Government it cannot but consider that it is impossible to reach an agreement through further negotiations.” War was imminent.
The “Fourteen Part Message” had been transmitted using the Japanese diplomatic code referred to as the Purple system. Breaking this code had eluded the best efforts of SIS cryptographers until August 1940, when SIS was finally able to read Purple message traffic between the Japanese government and its official representatives in the US. The process of decoding and translating Purple messages and disseminating the resulting intelligence (known as Magic) was long and tedious due to volume of traffic, the difficulty of the code, the limited number of cryptographers and Japanese linguists, and the security surrounding Purple. Of paramount concern was ensuring the Japanese did not learn the US had broken the code, so access to the Magic material was granted to only a few top officials.
In the early morning of December 7, SIS immediately recognized the import of the Fourteen Part Message, and after informing the President, the Chief of Staff of the Army alerted the commanders of both the Hawaiian and Philippine departments that the potential for a Japanese attack was high, although the target was still unknown. Given the sensitivity of the message, it had to be sent by telegraph, a process hampered by Sunday office closures. The message reached Honolulu at 7:33 am Hawaii-time and was dispatched by bicycle messenger to Fort Shafter. Half way to his destination, the messenger sought cover in a roadside ditch when the Japanese began its aerial bombardment. He did not reach Fort Shafter until 11:45 and, by the time the message was decoded and delivered to the Adjutant General’s Office, the time was 2:58 pm and the attack was over. Eighteen US ships and 188 aircraft were damaged or lost; human casualties included 2,335 service members and 68 civilians with another 1,178 wounded. Additional losses were suffered during simultaneous attacks on Thailand, Malaya, Singapore, Guam, Hong Kong, Wake, and the Philippine Islands.
Through the benefits of hindsight, much has been written about the intelligence failures leading to the attack on Pearl Harbor. To be clear, none of the Magic decrypts precisely laid out Japan’s intent to attack Pearl Harbor. Nevertheless, US communications security certainly contributed to the failure to inform ground commanders of the potential for attack in a timely manner. Yet, it was not the only contributing factor. Both the Army and Navy intelligence organizations had been undermanned since World War I, and growth in 1941 came too late to reap the advantages that would have been available from a long established intelligence collection effort. When Japan restricted accessibility to foreign military observers in 1941, the US ambassador warned the State Department of its limited “ability to give substantial warning” of possible naval or military operations.
Additionally, the Army’s Military Intelligence Division (MID) concentrated on the Japanese Army, leaving Japanese naval operations to the US Navy. With all evidence indicating the Japanese Army would continue its aggression in the Southwest Pacific, MID intelligence estimates, myopically, focused on that region. Furthermore, dismissing its own recently approved doctrine of the period, MID admittedly overlooked Japanese capability to launch a carrier-borne air assault on Hawaii, instead evaluating the enemy’s intentions. Fixated on a most likely course of action in the Southwest Pacific, apparently neither MID nor the Office of Naval Intelligence presented an attack on Pearl Harbor as a real possibility. After the war, Gen. Sherman Miles, the Assistant Chief of Staff, G2, lamented, “We underestimated Japanese military power…judged largely on her past record…. We had a yardstick. We had no reason to doubt our yardstick’s approximate accuracy. Yet it was wholly false.”
The blame for the attack on Pearl Harbor cannot be laid solely on intelligence failures. The Pearl Harbor investigations affixed plenty of blame to faulty leadership, inflexible policies and procedures, and overall complacency after more than two decades of peace. These same investigations, however, called attention to the long overlooked concepts that intelligence work not only required expert personnel and continuity in time of peace, but that it also should be recognized as an essential function of command.
The most iconic photo of the Pearl Harbor attack showing the USS Arizona listing after being hit during the Japanese air assault. (National Archives)