The Japanese felt that they were being pushed into a corner by President Roosevelt and felt that they must do something to protect the Japanese Empire.

January 1941

Sometime between January and March, 1940, Admiral Isoroku Yamamoto devised his plan to destroy the U.S. Navy in Hawaii and demoralize the American people. Because the U.S. was the primary military threat in the Pacific, Japan had prepared war plans to deal with this problem. The U.S. had similar war plans aimed at Japan.

The original Japanese plan had been to conduct one large naval battle against the American Navy, destroying it, resulting in the inability of the U.S. to interfere with Japanese expansion throughout Asia. However, when Admiral Isoroku Yamamoto assumed command of Japan's Combined Fleet in August of 1939, he had lived in America for several years and he knew Americans and the type of people we were. He knew that this war plan was impractical. He needed a new plan which would remove the threat of U.S. intervention from his flank.

March 1941

Nagao Kita, Honolulu's new Consul General, arrives on Oahu with Takeo Yoshikawa, a trained spy. As the military of both countries prepared for possible war, the planners needed information about the opponent.

The U.S. knew that Hawaii was full of Japanese intelligence officers but because of our constitutional rights, very little could be done. Untrained Japanese agents like Kohichi Seki, the Honolulu consulate's treasurer, traveled around the island noting all types of information about the movement of the fleet. When the attack occurred, the Japanese had a very clear picture of the Pearl Harbor setup and where individual ships were moored because of Seki's information.

April 1941

U.S. intelligence officers continued to monitor Japanese secret messages.

American scientists had developed a machine, code named "Magic," which gave the United States intelligence officers the ability to read Japanese secret messages. The machine "Magic" provided all types of high quality information.

Even though the U.S. had all the data needed to make a clear picture of Japanese intentions, the Navy had a struggle between the Office of Naval Intelligence and the War Plans Division about which department should be the primary collection office. When the War Plans Division was finally picked the for the data collection, all of the Navy's intelligence collection was degraded .

At times the Army and the Navy did not talk to each other, again reducing the ability to understand Japan's intentions. Finally, Washington did not communicate all the available information that was received to all commands, thinking that such a transmission would result in duplication. All in all the U.S. knew that Japan was going to expand its war, but the question remained, where? If U.S. intelligence officers had communicated all the available information, preparations for the attack could have been improved.