The Intelligence Failure of Pearl Harbor

On a late summer morning in 1940, Frank B. Rowlett, a 32-year-old civilian employee of the U.S. Army, climbed into his Ford sedan in Arlington, Virginia, and drove to his job in Washington, D.C. Though his work was all but obsessive and tempted him to revert to it during his commute, Rowlett was disciplined and kept his mind on the traffic. He parked in a lot behind the Munitions Building, the army's offices on Constitution Avenue, arriving at 7 a.m., an hour early, as was his custom. He walked down one of the wings that stretched out the back of the building like teeth on a comb. A steel gate and an armed guard blocked the entrance to Rooms 3416 and 3418. They were among the most secure in the entire structure, and the work that went on in them among the most secret in the U.S. government.

Rowlett was a codebreaker; he had charge of the team trying to crack the most secret diplomatic cipher of the Empire of Japan, a machine that American cryptanalysts called PURPLE, and within hours on that day, Friday, September 20, he would be celebrating one of the greatest moments in American cryptology.

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Tension with Japan had begun when the United States seized the Philippines in 1898. Within the Imperial Japanese Navy a vocal faction saw the westward march of the United States as squeezing and poisoning Japan. Friction intensified at the Washington disarmament conference of 1922, when the United States forced Japan to accept a lower warship ratio than it would have liked. This American diplomatic victory was achieved with the help of the charismatic cryptanalyst Herbert O. Yardley and his assistants, whose solution of coded Japanese diplomatic messages told American negotiators just how far they could push the Japanese.

But in 1929 Secretary of State Henry L. Stimson, believing that mutual trust worked best in international affairs and that therefore "gentlemen do not read each other's mail," refused to expend State Department funds for cryptanalysis. When Yardley, out of work in the Depression, wrote an indiscreet book in 1931 revealing the inside story, Japanese officials lost face, the Japanese press fulminated, relations with the United States deteriorated-and Japan improved its diplomatic cryptosystems. Tokyo adopted machine ciphers more complex than the system employing simultaneous use of multiple codebooks that Yardley and his team had cracked.