Did Navy Codebreakers Know Japan Was Going To Strike Pearl Harbor?

by Warfare History Network
9-12 minutes

The Japanese strike on Pearl Harbor on December 7, 1941—a “Day of Infamy,” as President Franklin D. Roosevelt described it—left the American Pacific Fleet in almost total ruin, plunged the United States into World War II, and set off a controversy regarding the events that led up to the attack that is still being hotly debated.

One of the most troublesome incidents in the pre-Pearl Harbor planning by the Japanese is the so-called “Winds Code” incident and what significance, if any, it had for the American code breakers who were monitoring Japanese diplomatic and military communications in the months leading up to the surprise attack.

Did the Navy cover up by not allowing the people who handled the Winds communication to testify before congressional committees after the war? And what happened to the Winds communications itself that was supposed to have been seen by different naval intelligence personnel in the days prior to the Pearl Harbor attack?

To understand the significance of the Winds message, we must trace the role of the U.S. military’s efforts in breaking the Japanese codes before Pearl Harbor.

Decrypting Magic

The Japanese used what they called a “Purple” machine to encode top-secret intelligence sent to their embassies around the world. The code word for American intercepts of Japanese diplomatic and military messages coming into the United States was “Magic.” The United States designated all the information collected from Purple as “Magic”—the highest-classified intelligence collected by the United States during the war.

The success of Magic allowed the United States to follow Japan’s route to war, keeping a detailed record of their every move. During the summer of 1940, the United States began sharing intelligence with the British who had their own secret communications vis a vis Germany called “Enigma.” In a move that would later prove disastrous in the pre-Pearl Harbor scenario, one of the Purple machines that went to the British was originally supposed to be given to the U.S. Navy at Pearl Harbor.

Another U.S. military organization doing cryptographic work that involved both Magic and the Winds communication was the Navy’s code-breaking group called OP-20G, led by Commander Laurance Safford.

The Magic information collected by the Navy was sent to various top military and civilian leaders in the American government. Unfortunately, Magic was not shared with all of the top military commanders including, most importantly, Navy Admiral Husband Kimmel and Army General Walter Short, the two commanding officers at Pearl Harbor.

Another unfortunate side of Magic was that the men who were apprised of its content could not agree among themselves as to which information was relevant or not. It was this lack of communication that led to the controversy over what the Winds message really meant.

Uncovering Japan’s Intentions

By the fall of 1941, U.S. code breakers had a pretty good idea as to what the Japanese government was thinking and planning regarding a potential conflict with the United States. Japan was still committed to its participation in the Tripartite Pact with Italy and Germany and refused to remove its troops from China. From the intercepts of Japanese communications that were picked up by U.S. code breakers, it was obvious that Japan was disinclined to tone down its harsh rhetoric regarding a possible war with either the United States or Great Britain.

More importantly, as far as the United States was concerned, a November 5, 1941, message from Tokyo to Washington setting a date of November 25, 1941, as a deadline for the completion of diplomatic negotiations with the United States, should have been a warning sign that trouble lay ahead.

Other intercepted communications from Tokyo gave instructions for the destruction of its code machines; a November 20 message from Tokyo stated that the current conditions would not “permit any further conciliation by us [Japan];” a November 22 note said if a diplomatic agreement was not reached by November 29 “that things are automatically going to happen.” Also important to the prewar scenario was a November 27 war warning message broadcast from Tokyo, along with a November 19 message from Tokyo giving details of the “Winds Execute” message that would be added to the end of the Japanese news broadcasts in case war between the United States, England, and Russia was imminent, and a November 19 note giving further instructions for Japanese diplomats in Washington to listen for Winds messages that would be read five times at the beginning and end of each transmission.

Uncovering the Winds Code Words

On December 4, 1941, American listening posts in various parts of the world decoded two communications sent from Tokyo to its Washington embassy on November 19 that carried information on the so-called Winds message to which naval intelligence officials had been alerted.

The first message, Circular No. 2353, said: “Regarding the broadcast of a special message in an emergency. In case of emergency (danger of cutting off our diplomatic relations), and the cutting off of international communications, the following will be added in the middle of the daily Japanese language short wave news broadcast:

In case of a Japan-US relations in danger HIGASHI NO KAZEAME––East Wind Rain.

Japan-USSR relations: KITANOKAZE KUMORI––North Wind Cloudy.

Japan-British relations: NISHI NO KAZE HARE––West Wind Clear.”

The second circular, No. 2354, came later:

“If it is Japan-US relations: HIGASHI.

Japan-Russia relations: KITA.

Japanese-British relations (including Thai, Malaya, and Netherlands East Indies): NISHI.

The above will be repeated five times and included at beginning and end. Relay to Rio de Janeiro, Mexico City, San Francisco.”

The Winds message was also picked up by a variety of Allied listening posts across the globe. The British Singapore station seized the message on November 28 and transmitted it to the U.S. Asiatic Fleet headquarters where Admiral Thomas Hart, the commander in chief, Asiatic Fleet, sent it to the headquarters commanders of both the 14th Naval District and the 16th Naval District. On December 4, the message was sent by Consul General Walter Foote at Batavia to the State Department in Washington. In his message regarding the broadcast, Consul General Foote said, “I attach little or no importance and view it with some suspicion. Such have been common since 1936.”

The same low-key reaction to the Winds message came on December 3, when a top U.S. Army officer stationed in Java cabled the message to Brig. Gen. Sherman Miles, the acting ACS/Intelligence head, War Department. It was broadcast in a low-grade designation called “deferred,” and was subsequently not decoded until 1:45 am on December 5.

These two messages were sent from Tokyo on their J-19 diplomatic code, not the more significant Purple code that U.S. naval intelligence had been privy to for months. On the Navy’s part, they alerted all their stations to be on the lookout for the next phase of the Winds code––the so called “Execute” stage of the plan.

Station “M” Finds the Smoking Gun

Subsequently, a full-court press inside the United States was ordered to listen for the “Execute” phase. Naval code-breaking stations in San Francisco and Fort Hunt in Virginia had Japanese-language translators sent in on an emergency basis. The Federal Communications Commission, one of whose jobs was the monitoring of Japanese weather broadcasts, was put on alert. If they picked up an “Execute” broadcast, they were to call Colonel Rufus Bratton, commander of the G-2 (Army Intelligence) Far Eastern Section.

In Hawaii, the Navy’s top intelligence code breaker, Joseph Rochefort—head of the Combat Intelligence Unit of the 14th Naval District in Pearl Harbor and Station Hypo, a U.S. monitoring unit that watched Japanese naval movements—was alerted to the Winds message.

During this tense time, the FCC picked up a false message from Japan at 10 pm on December 4 which said, “Tokyo today north wind slightly stronger may become cloudy tonight. Tomorrow slightly cloudy and fine weather. Kanagawa Prefecture today north wind cloudy from afternoon more clouds. Chiba Prefecture today north wind clear, may become slightly cloudy. Ocean surface calm.”

One of the U.S. listening posts that played a huge role in the Winds Affair was Station “M,” located at Cheltenham, Maryland. Early on December 4, 1941, 27-year-old senior radio operator Ralph Briggs picked up a cryptic message in a weather forecast being broadcast from Japan. Warned to listen for any unusual weather broadcasts attached to messages from Japan, Briggs heard the words he’d been alerted to. It was “East Wind Rain––HIGASHI NO KAZEAME [a possible disruption of Japan-U.S. relations].” It now seemed that the “smoking gun” from Tokyo had just been received.

Briggs began the process of transmitting his find to the other intelligence agencies and government officials. He sent one copy to the Army Signals Intelligence Unit and another to the White House. The Navy’s OP-20G got their own copy by 9 am on December 4.

The Winds message was then translated by Lt. Cmdr. Alvin Kramer, who was in command of the Translation Section of the Navy Department Communications Unit. According to extemporaneous accounts, Kramer, upon seeing the Winds message, said, “This is it.” By noon on December 4, multiple copies of the Winds message had been circulated among the Army and Navy’s intelligence divisions, their senior officers, the State Department, and the White House. As some conspiracy theorists believe regarding the significance of the Winds message, the Roosevelt administration had three days to read and digest its contents and prepare the country for war with Japan. Yet, nothing was done to alert the fleet at Pearl Harbor or any other branch of the military.