Attack on Pearl Harbor

battle_name=Attack on Pearl Harbor

United States preparedness

U.S. civilian and military intelligence forces had, between them, sufficient information to anticipate Japanese aggression weeks, or even months, before the attack. The armed forces at Pearl Harbor had a number of warnings on the day of the attack. Both of these information sources could have brought Pearl to a higher level of alert and made the attack less damaging.

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U.S. signals intelligence, through the Army Signal Intelligence Service and the Office of Naval Intelligence's OP-20-G codebreaking unit, intercepted Japanese diplomatic traffic and had broken many Japanese ciphers. Distribution of this intelligence was poor and did not include material from the Japanese military. Often the information was incomplete, contradictory, or insufficiently distributed, as in the case of the Winds Code. Warnings were sent to the U.S. forces in the Pacific in November 1940. Despite the growing information pointing to a new phase of Japanese aggression, there was little information specific to Pearl Harbor.

Related Topics:
Signals intelligence - Signal Intelligence Service - Office of Naval Intelligence - OP-20-G - Cipher - Winds Code

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American commanders were warned that tests had shown that shallower torpedo launching was possible, but they did not fully appreciate the danger posed by the secret Japanese torpedo. Expecting that Pearl Harbor had natural defenses against torpedo attack, the U.S. Navy failed to add torpedo nets or baffles, which they judged cumbersome. Due to a shortage of planes, long reconnaissance patrols were not being made. At the time of the attack, the Army was training rather than on alert. Most of its portable anti-aircraft guns were stowed with the ammunition kept locked in separate armories. To avoid upsetting the property owners, the officers did not keep the guns dispersed onto private property.

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