The Japanese surprise attack on Pearl Harbor in December 1941 was a complex operation that required precise information about the movement and disposition of the US Pacific Fleet, based at Hawaii. This information was gathered on the ground by a spy named Takeo Yoshikawa, who cabled regular reports to Tokyo. These reports proved to be invaluable to the success of the Japanese attack. However, this constant supply of information almost gave the game away.
The US Pacific Fleet moved from its bases on the West Coast of America to Pearl Harbor in 1940, so the Japanese consequently had very little information about it. The sailing of their fleet over 3,500 miles from home waters to Hawaii was a massive undertaking that required their ships to be refuelled numerous times en route. Before taking such a risk they therefore needed to know what was waiting for them when they arrived and the best time to attack.
Bureau 3 (Intelligence) of the Imperial Japanese Navy’s General Staff attached Takeo Yoshikawa to the Japanese Consulate-General in Honolulu. His mission was to gather news about Pearl Harbor. His orders were vague and were constantly refined by further instructions from his superiors in Tokyo.
Yoshikawa meticulously gathered vital intelligence on the movements of the US Pacific Fleet and Hawaii’s defences. He spent his days travelling between various observation points around Hawaii, reconnoitring airfields and the US Fleet. Changing his clothes several times through the course of the day, he would blend in as anyone from a tourist to a Filipino-American labourer. Yoshikawa even took geisha girls as cover on sightseeing flights over Pearl Harbor. Postcards he supplied were found in the cockpits of Japanese aircraft shot down over Pearl Harbor. Crucially, Yoshikawa was said to have discovered that Sunday mornings were the best time to attack, when the Fleet was home from manoeuvres.
Yoshikawa communicated this information regularly to Tokyo through commercial American telegraph companies. In the run up to the attack Tokyo’s need for information increased steadily, to the extent that Yoshikawa was reporting the US Fleet’s movements on a daily basis, leaving a large paper trail of his activities.
The number of cables sent by the Japanese Consulate hadn’t gone unnoticed and Robert L. Shivers, the FBI’s Special Agent In Charge in Hawaii tried to persuade the cable companies to share the coded messages with him but they refused, not wishing to break Federal Law.
American Intelligence had broken the Japanese diplomatic codes and were regularly intercepting and decrypting Japanese diplomatic traffic, as part of a program code named Magic. This used a machine code named Purple to decode these messages, which were then translated manually. The Americans were therefore aware that war was coming in the Pacific, but only knew as much as Japan’s diplomats, who were not informed in any detail of their Military’s plans. The US Army and Navy Departments in Washington did issue war warnings to their Pacific commanders, but the warnings were consequently vague and did not suggest that Hawaii would be a target.
Faced with an overwhelming amount of decrypted information, American Intelligence focused their efforts on translating the high level communications between Tokyo and the Japanese Embassy in Washington, which was conducting peace talks with the American government. This did eventually provide a warning of a coming attack, but it did not say where it would be.
Alan Bardos is a novelist and the author of Rising Tide. You can read a short story written by Alan in Issue 22 of the magazine.
