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Killing Yamamoto: How the U.S. assassinated the Japanese admiral who planned Pearl Harbor: Operation Vengeance

On December 7th, 1941, the Imperial Japanese Navy Air Service launched their surprise military strike on Pearl Harbor. The United States President Franklin Delano Roosevelt proclaimed that this date “will live in infamy,” mainly due to there not being a formal declaration of war and the attack happening without an explicit warning from the Japanese. In fact, negotiations with Japanese diplomats in Washington were still ongoing. The United States was, quite literally, caught by surprise. 

Although Japan reasoned the attack was preemptive, the entire attack was classified as a war crime by the end of the war during the Tokyo Trials. And even today, any American will know what you mean when you mention Pearl Harbor. Now, the commander-in-chief of the Imperial Japanese Combined Fleet, Isoroku Yamamoto, is generally seen as the primary person responsible for the attack. He was integral to the planning and execution of the surprise military strike. You could say Yamamoto was the mastermind behind the Pearl Harbor Attack. 

Admiral Isoroku Yamamoto 1884-1943

The Americans certainly saw it that way. Nearly a year and a half after the Pearl Harbor attack, the United States Office for Naval Intelligence intercepted and deciphered a coded message from the Japanese. The Americans realised they had struck gold. The message contained the detailed travel schedule of Yamamoto, who was planning to visit troops on the Solomon Islands in an attempt to boost morale. What followed was preparing an incredibly daring and risky secret operation: Operation Vengeance, the mission to assassinate the mastermind behind the Pearl Harbor Attack.

Deciphering the Itinerary

By February 1943 the tide of the war in the Pacific was decisively shifting in favour of the United States. The Japanese had retreated from Guadalcanal, lost many warships, aircraft carriers and aircraft, and the morale of Imperial troops was plummeting. From his base in Rabaul, Papua New Guinea, Yamamoto decided to visit troops on the frontlines on Bougainville, part of the Solomon Islands Archipelago. The visit’s goal was to increase soldiers’ dwindling morale. They often complained about the lack of senior commanders ascertaining the frontlines’ situation. 

Now, over the years American, British, French and Dutch codebreakers cooperated in order to break the Japanese naval codes and cyphers. Japan’s main, and most secure communication scheme used by the Imperial Japanese Navy was referred to as JN-25. Intercepting dozens of coded Japanese diplomatic and military messages, slowly but surely the grasp on JN-25 strengthened. One of the critical methods was the so-called known-plaintext attack, abbreviated to KPA, and commonly known as exploiting “Cribs.” Basically, the process of cribbing meant cryptographers inferred coded messages with the partial knowledge of plaintext they expected. Japanese military orders often contained sentences such as “I have the honor to inform your excellency…”.  Knowing this helped cryptographers to decipher intercepted coded messages.

And although the Japanese Navy adopted improved variants, namely JN-25b, c and eventually d, Allied codebreakers managed to decipher large parts of the messages that were transmitted by the Japanese, albeit without their knowledge. 

When on April 13, a coded message from Yamamoto’s command bunker in Rabaul was sent to several command posts in the area, the Allied codebreaker gears began grinding. Although it used the newly adopted JN-25d cypher, they deciphered it within a day. Much to the codebreakers’ surprise, the message not only contained Yamamoto’s intention to visit troops on the frontline in the Solomon Archipelago but in fact included the time and date of his planned travel, the number of fighter planes as part of his escorting squadron, a detailed route and his destination: the airfield at Balalae Island. 

According to sources describing Operation Vengeance from the American perspective, upon learning of the contents of the message, U.S. Commander in the Pacific, Admiral Chester Nimitz is said to have sent his own message to the Pacific Fleet commander William Halsey. It read no less than: “TALLY HO X LET’S GET THE BASTARD.”

Operation Vengeance

Because Yamamoto planned on visiting Balalae Island five days after the message was intercepted, time was of the essence. Yet having all this information did not necessarily mean an operation to take out one of Japan’s most senior commanders was a cut and dry case. The closest American airbase near Balalae was on the recently conquered Guadalcanal. That was over four hundred miles away, and the Navy and Marine fighter planes such as the F4F Wildcat and F4U Corsairs did not have enough fuel, and thus not enough range, to be able to reach Yamamoto’s squadron on their way. 

The only aircraft that would be able to reach the squadron was the single-seated, twin-engined Lockheed P-38 Lightning fighter aircraft, outfitted with additional fuel tanks. Eighteen P-38s were assigned to the mission. In order to avoid Japanese radar detection, the aircraft had to fly at an altitude of no more than 15 metres, at least 80 kilometres offshore of islands, for a distance over 600 kilometres. Of the eighteen, four P-38s were designated the so-called ‘Killer Group.’ These were tasked with taking out Yamamoto’s G4M, while the other fourteen P-38s covered the group against potential counter-attacks. After all, the operation took place close to Japanese airbases.

Now, the Airborne Early Warning and Control System, or AWACS for short, is an airborne radar picket system that detected aircraft, ships and vehicles. Yet, the P-38s were not outfitted with it. They weren’t even equipped with a land-based radar to guide them or detect the squadron escorting Yamamoto. This was a problem for two reasons. Firstly, if Yamamoto changed his schedule at the last minute, the squadron of P-38s would be flying around, not knowing what to do or where he was. So they had to count on him not diverting a single minute from the travel schedule, betting on Yamamoto being punctual, something he was known for. Because they were aware of the average speed and probable route of the squadron escort, consisting of Mitsubishi G4M “Betty” and Mitsubishi Zero’s, together with the assumed wind speed, they could more or less calculate the position of the squadron at all times. 

P-38 Lightning

That brings us to the second problem, which to be fair builds forth upon the first problem: there was no wiggling room for getting lost or being early. If they beat Yamamoto’s squadron by ten minutes, they could hardly fly in circles in territory that was crowded with Japanese airbases and most likely had swarms of patrols both in the sky and on the water. 

Planning the mission in detail, the Office calculated that the P-38s would intercept the squadron at 9:35 AM. On the morning of April 18 it was go-time. The squadron flew at an altitude of 15 meters at most for hundreds of kilometers. They reached the point where they would intercept Yamamoto’s squadron one minute early, at 9:34 AM. And the Japanese arrived right on time. The Americans had calculated it correctly, and Yamamoto, unwittingly, honoured his reputation for punctuality. 

Flying at around 1.4 kilometres height were the two Mitshubsihi G4M “Betty”. Onboard of one was Admiral Yamamoto. The other one carried his right-hand-man, Vice Admiral Matome Ugaki. Six Mitsubishi A6M Zeros escorted them. When the Americans realised the Japanese were on time, the four P-38s part of the Killer group shed their additional fuel tanks and began climbing to attack the Bettys. Due to technical difficulties, one of the P-38s had to abandon its climb early on. The other twelve of the protective squadron climbed even further to prevent any reinforcements from Japanese airbases interfering. 

The sudden appearance of P-38s must have been a surprise to the Japanese pilots. After all, they were merely 15 minutes away from the Balalae landing strip. As soon as pilots of the Zeros saw them, they engaged in a dogfight. One P-38, piloted by Thomas Lanphier, fought the Zeros while the other two chased the G4M, one of them containing Yamamoto. Lieutenant Rex T. Barber shot down one of the G4Ms and narrowly avoided collision mid-air. The G4M crashed in the Bougainville jungle. 

Yamamoto hours before his death

Lieutenant Besby F. Holmes damaged the other G4M, but the job was eventually finished by Barber who shot the aircraft out of the sky. This one crash-landed in the water. One of the P-38s was shot down by a Zero. Now, both G4Ms crashed, but the commander of one of them survived. Aboard the G4M that crashed in the water was Vice-Admiral Matome Ugaki, who in fact survived the crash and was picked up by the Japanese Navy. Yamamoto wasn’t as fortunate, however. 

Yamamoto’s ashes return to Japan

The wreckage of his plane still lies amidst the Solomon jungle. It is accessible, but only by trekking through thick vegetation and swampy grounds. A Japanese search-and-rescue party recovered Yamamoto’s remains the next day. His remains were cremated, and he was given a state funeral on June 5, 1943, over a month after his death. It is said over a million mourners attended the funeral.

The Controversy

Up until May 21st, so nearly a month, the Japanese kept Yamamoto’s death a secret. When the broadcast finally aired in Japan, it stated Yamamoto was “killed in aerial combat and met his gallant death in his war plane.” When news of the success of the mission, and Yamamoto’s demise reached the U.S. military, it was an incredible morale boost. 

But the story had an unfortunate twist thanks to the pilots that flew the P-38s as part of the Killer Group. They became embroiled about the question of who actually shot down Yamamoto’s G4M. This rather public fight overshadowed the success of the mission.

Yamamoto’s aircraft in the jungle

In October that same year, Time magazine published a detailed article. In it, Captain Thomas Lanphier received credit for downing the G4M. This was disputed by other pilots, among whom Barber. He claimed he was responsible for shooting down the G4M. But that wasn’t the only problem: the Time article contained many sensitive details. Navy command considered Major John Mitchell responsible for his pilots, and because he didn’t keep them in check, he’d suffer the consequences. Instead of the Medal of Honor, which he would most likely have received for the mission, he was awarded the Navy Cross. This military decoration was considered less prestigious than the Medal of Honor.

Until his death in 1987, Lanphier kept up the claim he downed the G4M, something Barber contested until his death in 2001. Now, as for Vice Admiral Matome Ugaki who survived the crash. He is fascinating in his own right for he survived the end of the war, only to become Japan’s final kamikaze pilot. I’ve created a video about his final kamikaze attack, it should appear on-screen shortly. I’ve also created a video about Admiral Isoroku Yamamoto’s life, which was incredibly fascinating to research. The Admiral studied at Harvard University and knew the United States from the inside, actually opposing the war initially. He was even put under 24/7 protection during the 1930s because the army feared he would be assassinated for being deemed too “Pro-American.” If you’d like to know that story, consider checking out that article.

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