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Japanese Katakana morse code
Japanese katakana morse code
Photograph courtesy U.S. National Security Agency


The first warning of a possible Japanese attack on Pearl Harbor came in a coded cablegram from the U.S. ambassador to Japan, Joseph C. Grew, to the U.S. State Department on January 27, 1941.

Grew’s cable told of a report that “the Japanese military forces planned to attempt a surprise mass attack on Pearl Harbor.” A State Department Japanese-language specialist believed the warning and urged his superiors to treat it as such. But both Grew and U.S. intelligence specialists dismissed the report as a wild rumor.

Throughout 1941 U.S. radio interception stations kept track of major Japanese ships by listening to messages. Although the messages could not be decoded, they usually could be traced to specific ships because U.S. eavesdroppers knew the Japanese “fists”--the characteristics of individual Japanese telegraph operators--and linked them to certain ships. But in December 1941 the messages stopped--the Japanese fleet had been ordered into radio silence.

The mystery of the “missing” ships worried Lt. Comdr. Edwin T. Layton, intelligence officer on the staff of the U.S. Pacific Fleet commander at Pearl Harbor, Adm. Husband E. Kimmel.

“Do you mean to say,” Kimmel asked Layton on December 2, “they could be rounding Diamond Head [an Oahu landmark] and you wouldn’t know it?”

“I hope they would have been spotted before now,” Layton said.

Kimmel and Layton had no idea that the Japanese Pearl Harbor strike force was at sea and sailing under radio silence so strict that Japanese officers later said that telegraph keys were sealed shut.

Although there was not much solid information about Japanese plans, intelligence officers in Washington did have some general indications of Japanese moves. But for perceived security reasons, the Washington officers declined to share that information with intelligence officers in Pearl Harbor.


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