More than 60 years later, Americans still wonder how Japan’s surprise attack on the Pacific Fleet could have succeeded. The joint congressional committee that investigated the attack in 1945 and 1946 put the question sharply: ‘Why with some of the finest intelligence available in our history, with the almost certain knowledge that war was at hand, with plans that contemplated the precise type of attack that was executed by Japan on the morning of December 7–Why was it possible for a Pearl Harbor to occur?’

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The best intelligence came from breaking Japanese codes. Solving the secret messages of a hostile power is like putting a mirror behind the cards a player is holding, like eavesdropping on the huddles of a football team. It is nearly always the best form of intelligence. It is faster and more trustworthy than spies, who have to write up and transmit their reports and who are always suspected of setting up or falling for a deception. It sees farther into the future than aerial reconnaissance, which detects only what is present. It is broader in scope than the interrogations of prisoners, who know little more than what they have experienced. And it is usually cheaper and less obtrusive, hence more secret, than all of these. But it has a serious double-barreled failing: It cannot provide information that a nation has not put onto the airwaves, and its apparent omniscience and its immediacy seduce its recipients into thinking they are getting all the other nation’s secrets.

This is one of the lessons of Pearl Harbor. American code-breakers performed prodigies, giving remarkable insight into Japanese thinking. But that insight was not total, and so even the extraordinary U.S. cryptanalysis could not warn policymakers of Japan’s secret intentions.

The nations of the world learned the value of code-breaking during World War I. Radio–used extensively in that conflict for the first time–gave them their opportunity. Messages were easily intercepted, so armies and navies sheathed them in codes and ciphers. But linguists and mathematicians on both sides learned to crack them, and the information thereby obtained provided victory after victory to generals, admirals and political leaders. Cryptanalysis substantially helped France to block the supreme German offensive in 1918, Germany to defeat Russia, Britain to bring the United States into the war, the United States to convict a German spy. When hostilities ended, the powers refounded these agencies to retain in peace the benefits won in war.

The United States was one of these nations, and its main target was Japan. Before World War I, Japan had defeated China and then Russia to become mistress of the western Pacific. Now it was building a fleet to match that of the United States and, under a League of Nations mandate, had occupied islands that enabled it to menace the ocean routes to the Philippines. It was generally felt that Japan constituted the greatest danger to the United States.

The State and War departments jointly set up the Cipher Bureau in 1919 under the inspiring leadership of Herbert O. Yardley, a 30-year-old who had created and run a code-breaking unit for military intelligence in World War I. The Cipher Bureau scored the first great achievement of American code-breaking while working out of a narrow brownstone at 141 East 37th Street in Manhattan. Despite only a rudimentary knowledge of Japanese, Yardley and his associates cracked Japanese diplomatic codes. A bewhiskered missionary then turned the messages into English. Sent to the State Department, the translated messages informed American negotiators at the Washington naval disarmament conference of 1921-22 about Japan’s fallback position on capital ships. Armed with this knowledge, the negotiators pushed Japan to promise to build such ships in a U.S.­Japan tonnage ratio not of 10-to-7, as Japan had wanted, but to 10-to-6–the equivalent of three fewer battleships.

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