Japanese gather preliminary data on Pearl Harbor

On this day in 1941, the Japanese consul in Hawaii is instructed to divide Pearl Harbor into five zones and calculate the number of battleships in each zone—and report the findings back to Japan.

Relations between the United States and Japan had been deteriorating quickly since Japan’s occupation of Indo-China and the implicit menacing of the Philippines, an American protectorate. American retaliation included the seizing of all Japanese assets in the States and the closing of the Panama Canal to Japanese shipping. In September 1941, Roosevelt issued a statement, drafted by British Prime Minister Winston Churchill, that threatened war between the United States and Japan should the Japanese encroach any farther on territory in Southeast Asia or the South Pacific.

The Japanese military had long dominated Japanese foreign affairs. So, although official negotiations between the U.S. secretary of state and his Japanese counterpart to ease tensions were ongoing, Hideki Tojo, the minister of war who would soon be prime minister, had no intention of withdrawing from captured territories. He also construed the American “threat” of war as an ultimatum and prepared to deliver the first blow in a Japanese-American confrontation: the bombing of Pearl Harbor.

In September 1941, Nagai Kita, the Japanese consul in Hawaii, was told to begin carving up Pearl Harbor into five distinct zones and to determine the number of warships moored in each zone. Little did Japan know that the United States had intercepted the message; unfortunately, it had to be sent back to Washington for decrypting. Flights east were infrequent, so the message was sent via sea, a more time-consuming process. When it finally arrived at the capital, staff shortages and other priorities further delayed the decryption. When the message was finally unscrambled in mid-October—it was dismissed as being of no great consequence.