Prior Knowledge
One of the most important elements in America's foreknowledge of Japan's intentions was our government's success in cracking Japan's secret diplomatic code known as "Purple." Tokyo used it to communicate to its embassies and consulates, including those in Washington and Hawaii. The code was so complex that it was enciphered and deciphered by machine. A talented group of American cryptoanalysts broke the code in 1940 and devised a facsimile of the Japanese machine. These, utilized by the intelligence sections of both the War and Navy departments, swiftly revealed Japan's diplomatic messages. The deciphered texts were nicknamed "Magic."
Copies of Magic were always promptly delivered in locked pouches to President Roosevelt, and the secretaries of State, War, and Navy. They also went to Army Chief of Staff General George Marshall and to the Chief of Naval Operations, Admiral Harold Stark. However, although three Purple decoding machines were allotted to Britain, none was sent to Pearl Harbor. Intercepts of ciphered messages radioed between Tokyo and its Honolulu consulate had to be forwarded to Washington for decrypting. Thus Kimmel and Short, the Hawaiian commanders, were at the mercy of Washington for feedback. A request for their own decoding machine was rebuffed on the grounds that diplomatic traffic was of insufficient interest to soldiers.
How untrue that was! On October 9, 1941, the War Department decoded a Tokyo-to-Honolulu dispatch instructing the Consul General to divide Pearl Harbor into five specified areas and to report the exact locations of American ships therein.
There is nothing unusual about spies watching ship movements — but reporting precise whereabouts of ships in dock has only one implication. Charles Willoughby, Douglas MacArthur's chief of intelligence, later wrote that the "reports were on a grid system of the inner harbor with coordinate locations of American men of war ... coordinate grid is the classical method for pinpoint target designation; our battleships had suddenly become targets." This information was never sent to Kimmel or Short.
Additional intercepts were decoded by Washington, all within one day of their original transmission:
• November 5th: Tokyo notified its Washington ambassadors that November 25th was the deadline for an agreement with the U.S.
• November 11th: They were warned, "The situation is nearing a climax, and the time is getting short."
• November 16th: The deadline was pushed up to November 29th. "The deadline absolutely cannot be changed," the dispatch said. "After that, things are automatically going to happen."
• November 29th (the U.S. ultimatum had now been received): The ambassadors were told a rupture in negotiations was "inevitable," but that Japan's leaders "do not wish you to give the impression that negotiations are broken off."
• November 30th: Tokyo ordered its Berlin embassy to inform the Germans that "the breaking out of war may come quicker than anyone dreams."
• December 1st: The deadline was again moved ahead. "[T]o prevent the United States from becoming unduly suspicious, we have been advising the press and others that ... the negotiations are continuing."
• December 1st-2nd: The Japanese embassies in non-Axis nations around the world were directed to dispose of their secret documents and all but one copy of their codes. (This was for a reason easy to fathom — when war breaks out, the diplomatic offices of a hostile state lose their immunity and are normally overtaken. One copy of code was retained so that final instructions could be received, after which the last code copy would be destroyed.)
An additional warning came via the so-called "winds" message. A November 18th intercept indicated that, if a break in U.S. relations were forthcoming, Tokyo would issue a special radio warning. This would not be in the Purple code, as it was intended to reach consulates and lesser agencies of Japan not equipped with the code or one of its machines. The message, to be repeated three times during a weather report, was "Higashi no kaze ame," meaning "East wind, rain." "East wind" signified the United States; "rain" signified diplomatic split — in effect, war.
This prospective message was deemed so significant that U.S. radio monitors were constantly watching for it, and the Navy Department typed it up on special reminder cards. On December 4th, "Higashi no kaze ame" was indeed broadcast and picked up by Washington intelligence.
On three different occasions since 1894, Japan had made surprise attacks coinciding with breaks in diplomatic relations. This history was not lost on President Roosevelt. Secretary Stimson, describing FDR's White House conference of November 25th, noted: "The President said the Japanese were notorious for making an attack without warning and stated that we might be attacked, say next Monday, for example." Nor was it lost on Washington's senior military officers, all of them War College graduates.
As Robert Stinnett has revealed, Washington was not only deciphering Japanese diplomatic messages, but naval dispatches as well. President Roosevelt had access to these intercepts via his routing officer, Lieutenant Commander McCollum, who had authored the original eight-point plan of provocation to Japan. So much secrecy has surrounded these naval dispatches that their existence was not revealed during any of the ten Pearl Harbor investigations, even the mini-probe Congress conducted in 1995. Most of Stinnett's requests for documents concerning Pearl Harbor have been denied as still classified, even under the Freedom of Information Act.
It was long presumed that as the Japanese fleet approached Pearl Harbor, it maintained complete radio silence. This is untrue. The fleet barely observed discretion, let alone silence. Naval intelligence intercepted and translated numerous dispatches, some clearly revealing that Pearl Harbor had been targeted. The most significant was the following, sent by Admiral Yamamoto to the Japanese First Air Fleet on November 26, 1941:
The task force, keeping its movement strictly secret and maintaining close guard against submarines and aircraft, shall advance into Hawaiian waters, and upon the very opening of hostilities shall attack the main force of the United States fleet and deal it a mortal blow. The first air raid is planned for the dawn of x-day. Exact date to be given by later order.
So much official secrecy continues to surround the translations of the intercepted Japanese naval dispatches that it is not known if the foregoing message was sent to McCollum or seen by FDR. It is not even known who originally translated the intercept. One thing, however, is certain: The message's significance could not have been lost on the translator.
1941 also witnessed the following:
On January 27th, our ambassador to Japan, Joseph Grew, sent a message to Washington stating: "The Peruvian Minister has informed a member of my staff that he has heard from many sources, including a Japanese source, that in the event of trouble breaking out between the United States and Japan, the Japanese intended to make a surprise attack against Pearl Harbor with all their strength...."
On November 3rd, still relying on informants, Grew notified Secretary of State Cordell Hull: "War with the United States may come with dramatic and dangerous suddenness." He sent an even stronger warning on November 17th.
Congressman Martin Dies would write:
Early in 1941 the Dies Committee came into possession of a strategic map which gave clear proof of the intentions of the Japanese to make an assault on Pearl Harbor. The strategic map was prepared by the Japanese Imperial Military Intelligence Department. As soon as I received the document I telephoned Secretary of State Cordell Hull and told him what I had. Secretary Hull directed me not to let anyone know about the map and stated that he would call me as soon as he talked to President Roosevelt. In about an hour he telephoned to say that he had talked to Roosevelt and they agreed that it would be very serious if any information concerning this map reached the news services.... I told him it was a grave responsibility to withhold such vital information from the public. The Secretary assured me that he and Roosevelt considered it essential to national defense.
Dusko Popov was a Yugoslav who worked as a double agent for both Germany and Britain. His true allegiance was to the Allies. In the summer of 1941, the Nazis ordered Popov to Hawaii to make a detailed study of Pearl Harbor and its nearby airfields. The agent deduced that the mission betokened a surprise attack by the Japanese. In August, he fully reported this to the FBI in New York. J. Edgar Hoover later bitterly recalled that he had provided warnings to FDR about Pearl Harbor, but that Roosevelt told him not to pass the information any further and to just leave it in his (the president's) hands