Attack on Pearl Harbor participant Isoroku Yamamoto
Military planning
Preliminary planning for an attack on Pearl Harbor to protect the move into the "Southern Resource Area" (the Japanese term for the Dutch East Indies and Southeast Asia generally) had begun very early in 1941 under the auspices of Admiral Isoroku Yamamoto, then commanding Japan's Combined Fleet. He won assent to formal planning and training for an attack from the Imperial Japanese Navy General Staff only after much contention with Naval Headquarters, including a threat to resign his command. Full-scale planning was underway by early spring 1941, primarily by Rear Admiral Ryūnosuke Kusaka, with assistance from Captain Minoru Genda and Yamamoto's Deputy Chief of Staff, Captain Kameto Kuroshima. The planners studied the 1940 British air attack on the Italian fleet at Taranto intensively.Over the next several months, pilots were trained, equipment was adapted, and intelligence was collected. Despite these preparations, Emperor Hirohito did not approve the attack plan until November 5, after the third of four Imperial Conferences called to consider the matter. Final authorization was not given by the emperor until December 1, after a majority of Japanese leaders advised him the "Hull Note" would "destroy the fruits of the China incident, endanger Manchukuo and undermine Japanese control of Korea".By late 1941, many observers believed that hostilities between the US and Japan were imminent. A Gallup poll just before the attack on Pearl Harbor found that 52% of Americans expected war with Japan, 27% did not, and 21% had no opinion. While US Pacific bases and facilities had been placed on alert on many occasions, US officials doubted Pearl Harbor would be the first target; instead, they expected the Philippines would be attacked first. This presumption was due to the threat that the air bases throughout the country and the naval base at Manila posed to sea lanes, as well as to the shipment of supplies to Japan from territory to the south. They also incorrectly believed that Japan was not capable of mounting more than one major naval operation at a time.