U.S. Had Clues to Attack, but Missed the Big Picture

If the Americans were reading Japan’s Purple code, how come Pearl Harbor?

Barbara Wohlestetter, in her detailed book about events preceding the attack on Dec. 7, 1941, gives perhaps the most succinct answer:

“The signals lay scattered in a number of different agencies. . . .”

Purple, the highest Japanese diplomatic code, warned that war was imminent but did not disclose where it would begin.

The U.S. Navy determined, using radio direction finders, that Japanese ships were heading toward Indochina and Malaya. Since there was no traffic to the Japanese aircraft carriers, it was assumed that they were in home waters. It was a deadly error.

A few Americans suggested that it could mean the carriers were steaming toward Pearl Harbor or the West Coast under radio silence. One was Col. Rufus Bratton, chief of the Far Eastern Section of the Army Military Intelligence Division.

His Navy counterpart, Cmdr. Arthur H. McCollum, spoke for himself and many others when he told Bratton: “They (the Japanese) know as well as you and I that the fleet would not just be sitting there (in Pearl Harbor) waiting to be attacked.”

Yet between Nov. 15 and Dec. 6, 1941, 49 espionage messages between Honolulu and Tokyo had given the precise locations of ships at the naval base and whether they were guarded by torpedo nets. Although 18 of those messages were intercepted, only three had been processed by the time of the attack.

Navy code breakers assigned low priority to messages at the consulate, where the Honolulu spies were based. Diplomats, it was assumed, were inferior spy material.

Washington code breakers, for instance, did not relay to Honolulu a decoded message from the consulate there that divided the harbor into a map-like grid. That, said Rear Adm. Edwin T. Layton, Pacific Fleet intelligence officer at the time, was “blind stupidity at least, gross neglect at best.”