The United States had been warned in advance that there would be a surprise attack on Pearl Harbor. They had also broken all Japanese codes and were aware of all the details needed to prevent the attack on Pearl Harbor. Roosevelt had made a campaign promise to not get involved in a war with Germany unless the United States was attacked first; so, he made strategic use of Japan’s plans as a way to go to war with Germany. The FBI was warned by one of Britain’s top agents, Duskov Popov, about a planned attack on Pearl Harbor and that it would be soon. The FBI told him that the information was too precise and too complete to be true.
They assumed it was a trap since the information detailed exactly where, when, how, and by whom the United States was going to be attacked. Both Churchill’s secret envoy and head of secret service failed to convince the US government to take Popov seriously. In the fall of 1941, Kilsoo Haan, an agent for the Sino-Korean People’s League, told Eric Severeid of CBS that the Korean underground had proof that the Japanese were going to attack Pearl Harbor before Christmas. In addition, one Korean had even seen the plans. In late October, Haan finally convinced US Senator Guy Gillette that the Japanese were planning to attack in December or January, and he alerted the State Department and Army and Navy Intelligence, but it was soon discarded. Haan called Maxwell Hamilton at the State Department in December, and told him that the Korean underground had information that the Japanese would attack Pearl Harbor the coming weekend.
He also forwarded a written report pointing to the publication of Hawaiian Air Corps schedules for November and December by the Japanese paper Nippu JiJi. On November 19th, the Americans intercepted a word code that established a series of messages to be inserted into the daily weather forecasts of Tokyo Radio. The British decrypted “Higashi No Kaze Ame” on November 25th. The phrase, which meant “East Wind Rain,” reached Roosevelt on November 26th and was decoded as “United States War” on the 28th. Ralph Briggs, at the Navy’s East Coast Intercept station, received the “East Wind Rain” message in the early hours of December 4th. He put it on the TWX circuit immediately and called his commander.
The log sheet between 0500 hours and 1300 hours has been deleted from the files. JN-25, the Japanese Navy’s Cryptographic System, was used by Yamamototo to give the order that the Striking Force will move out of Hitokappu Wan on the morning of November 26th and advance to the standby position on the afternoon of December 4th. This was a critical message because it revealed that a strike force had assembled in the Kurile islands, it was huge comprising of the First Fleet, the Second Fleet, and a number of aircraft carriers, and the voyage was so far that it required fueling in 8 days. The only target that fit this voyage length, mid ocean refueling, and the inclusion of aircraft carriers was Pearl Harbor. Churchill sent a secret message to Roosevelt telling him of the coded message. At the Army Board inquiry it was stated, “On November 26 there was received specific evidence of the Japanese intention to wage offensive war against Great Britain and the United States.
” A few weeks earlier, The Hawaiian commanders had already begun a search for an alleged fleet of aircraft carriers, without notifying Hawaii, and on December 1st, the Twelfth Naval District in San Francisco found the Japanese fleet. This fleet received a message via JN-25 that under the necessity of self-preservation and self-defense, Japan was now in a position to declare war with the United States. On December 2nd, the order was given to wait for the code “Climb Mt. Nitaka” before attacking, and on December 3rd, a list of ships at Pearl Harbor was sent to Tokyo. By December 5th, the first 13 parts of Japan’s declaration of war had been completely decoded and a Cabinet meeting was held to inform Roosevelt of all the evidence gathered.