Closing the book on Pearl Harbor

Cryptologia, Apr 2000 by Budiansky, Stephen

ABSTRACT: Recent attempts to resuscitate well-worn conspiracy theories concerning the Pearl Harbor attack are based on selective reading of documentary evidence and ignore conclusive, recently declassified materials which show that JN-25 and other Japanese naval codes were not being read by U. S. Navy codebreakers prior to the Japanese attack.

KEYWORDS: JN-25, Pearl Harbor, Crane Files, OP-20-G, Japanese naval codes.

The Japanese attack on Pearl Harbor, it is a safe bet, is the most studied event in American history. Even before the war had ended it had been the subject of eight boards of inquiry and investigation which sought to discover how the United States had been so unprepared and how a chain of almost unbelievable gaffes had left the U. S. Pacific Fleet sitting like a row of ducks at anchor on the bright morning of December 7, 1941. After the war, Republicans in Congress, convinced that a man who could defeat their party four times running was capable of anything, helped to launch a massive probe of Pearl Harbor with the all but stated aim of proving that Franklin Roosevelt or other high political and military officials of the government knew of the planned Japanese attack in advance and did nothing to avert it, so desperate was FDR's circle to get America into the war. To the fury of America's intelligence community, the Congressional probe and the other official investigations (whose complete transcripts were published in a massive, 39-volume series in 1946) thoroughly compromised the fact that the United States had broken the major Japanese diplomatic cipher machine, which the United states called PURPLE, well before the war. Such codebreaking successes were with much justification considered to rank among the nation's most closely guarded secrets, even after the defeat of Nazi Germany and Japan. Bitter experience had shown that revelations of success in reading another nation's codes often brought about swift improvements in cryptologic security. This was not just a theoretical concern in 1946; at that time the United States and Britain were beginning to break the Soviet diplomatic and KGB codes (the projects known as VENONA in the United States and ISCOT in Britain), and these were revealing the existence of a sizable espionage operation within the United States as well as Soviet control and manipulation of Communist groups in post-war Europe. Any leaks about Allied codebreaking prowess was thus an extremely touchy matter: Both Britain and America had kept extremely tight wraps on their codebreaking establishments even after the war's end. In that pre-whistle-blower age when faith in government still ran strong, employees at the codebreaking establishments signed a pledge that they would never at any time betray any information about their work, and they didn't.

Aside from the fear that disclosures would make their work more difficult in the future, the codebreakers had another justifiable fear about the investigations: it was one thing to acknowledge that we had been spying on the Germans and Japanese, quite another that we had, even before our entry into the war, been spying on the French, the Russians, and the Mexicans and other Latin American nations. The fact that the American codebreakers had engaged in a full sharing of ideas, methods, intercepted messages, and even some decrypts with Britain a full year before America's entry into the war was also a source of great potential embarrassment. This cooperation was a definite part of Churchill's efforts to get the United States and Britain "somewhat mixed up together," in his apt phrase, and Roosevelt heartily approved the suggestion for these and other close military-to-military contacts beginning in the summer of 1940 as Britain fought alone against Hitler.1

The decrypted Japanese diplomatic messages, which went under the U. S. codename MAGIC, were minutely scrutinized during the Congressional investigations, and have been ever more minutely scrutinized in the years since, for any hint of warnings ignored, and it is well established that on the night of December 6, FDR was handed a signal which made it abundantly clear that Japan was preparing to break off diplomatic relations, which could mean nothing but war. But, to the eternal disappointment of the conspiracy theorists, the messages contain not a hint of where Japan might strike. Conventional wisdom suggested the Philippines, and Secretary of the Navy Frank Knox was hardly alone in being flabbergasted when the news came in from Pearl Harbor: "My God, this can't be true, this must mean the Philippines!" he exclaimed. All that the MAGIC decrypts prove is the decidedly nondastardly fact that America had no choice but to wait for Japan to strike the first blow; it was inconceivable that America might launch military action preemptively, even with a clear warning that Japan was about to break off diplomatic relations.

This, of course, has not deterred a number of "revisionist" historians who have been determined at all costs to prove that FDR had to have known in advance that Pearl Harbor was the target and, knowing it, deliberately hid the information. But these narratives, however exciting, have always been rather unsatisfactory, for they depend on all sorts of convoluted tales about Japanese spies, and jargon messages slipped into Radio Tokyo broadcasts, and somebodysaid-that-somebody-said testimony from dubious sources.