What were the causes and effects of Pearl Harbor?

It is highly probable that many in the US, including the President, knew of an emminent attack on Pearl Harbor prior to Dec. 7th.

On January 27, 1941, the Peruvian envoy in Tokyo told the third secretary in the US embassy that he had learnt from intelligence sources that the Japanese had a war plan which involved an attack on Pearl Harbor. On 10 July, the US military attaché in Tokyo reported that the Japanese Navy were secretly practicing airborne torpedo attacks on targets moored in Ariake Bay-a bay that resembles Pearl Harbor. The US military attaché in Mexico also reported that the Japanese were building midget submarines which would be towed to Hawaii for an attack on Pearl Harbor.

A top British agent, codenamed 'Tricycle,' told the FBI that the Japanese planned to attack Pearl Harbor, but his information was dismissed. And a Korean agent told American broadcaster Eric Severeid that the Japanese were going to attack Pearl Harbor. The agent repeated his story to a US Senator who alerted the State Department, US Army and Navy intelligence, and President Roosevelt personally.

American intelligence had broken all the Japanese codes. On 24 September 1941, a message from Japanese Naval Intelligence headquarters in Tokyo to the Japanese consul general in Honolulu was deciphered. It requested the exact locations of all US Navy ships in Pearl Harbor. Such detailed information would only be required if the Japanese were planning an attack on the ships at their moorings. In November, another message was intercepted ordering more drills involving attacks on capital ships at anchor in preparation to 'ambush and completely destroy the US enemy.' The only American fleet within reach was at Pearl Harbor.

On 25 November, a radio message from Admiral Yamamoto ordering the Japanese task force to attack the US fleet in Hawaii was intercepted. US Intelligence was understaffed and it is not known whether this message was decoded at the time. However, that same day, the US Secretary of War, Henry Stimson, noted in his diary:

'FDR stated that we were likely to be attacked perhaps as soon as next Monday. FDR asked: 'The question was how we should maneuver them into the position of firing the first shot without too much danger to ourselves. In spite of the risk involved, however, in letting the Japanese fire the first shot, we realized that in order to have the full support of the American people it was desirable to make sure that the Japanese be the ones to do this so that there should remain no doubt in anyone's mind as to who were the aggressors.''

On 29 November, US Secretary of State Cordell Hull showed a reporter from The New York Times a message saying that Pearl Harbor was going to be attacked on 7 December. As the attack approached, the American government received information from numerous sources that 7 December would be the day. On 1 December, Naval Intelligence in San Francisco worked out from news reports and signals picked up by shipping companies that the Japanese fleet that had disappeared from home waters was then to the west of Hawaii. Those who believe that Roosevelt knew about the attack all along maintain that a number of other reports say that the Japanese would strike at Pearl Harbor, but they have yet to be declassified.

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