When Admiral Yamamoto was shot down Japan didn’t suspect their radio codes were compromised. Here’s why.

When Admiral Yamamoto was shot down Japan didn’t suspect their radio codes were compromised. Here’s why.

By Dario Leone
Sep 4 2021
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Admiral Chester W. Nimitz knew that if the Japanese thought Admiral Yamamoto had been ambushed, they could suspect their code had been broken and change it. He decided the risk was worth it.

Admiral Yamamoto was Japan’s mastermind behind Pearl Harbor, and he was a top target in the war that followed. In 1943, US codebreakers cracked the secret of Yamamoto’s flightplan for a forthcoming visit to Rabaul. Nevertheless after Admiral Yamamoto was shot down Japan didn’t suspect their radio codes were compromised.


‘The US used a cover story that Yamamoto’s plane had been spotted by Australian coast watchers. They also after the assassination sent P-38’s in the same area to give the impression that it was a regular route for the USAAF,’ says Brent Cooper, an aviation expert, on Quora.

‘Yamamoto, then 59, was one of the most hated men in America. Not only had he planned the attack on Pearl Harbor; almost as galling, he had reportedly bragged that he was “looking forward to dictating peace to the United States in the White House.” In reality, he had never made this boast. It was a product of Japanese propaganda, but Americans took it as the gospel truth.

The Japanese navy, widely deployed throughout the Pacific, heavily relied on coded radio transmissions to send many of its most secret messages—and the U.S. Navy was listening. American cryptanalysts had broken the latest version of the JN-25 code just in time for the Battle of Midway in June 1942. With advance knowledge of Japanese plans, the outgunned U.S. Navy inflicted a stunning defeat on a superior enemy force.

‘The cryptanalysts were about to score again.

We Killed Yamamoto: a detailed description of the P-38 assassination of the man behind Pearl Harbor
Admiral Isoruku Yamamoto

‘In early April 1943, Yamamoto planned a one-day inspection trip from Rabaul to bases around the southern tip of Bougainville. In preparation, his staff sent the itinerary to local commanders. Although the staff wanted Yamamoto’s schedule hand-delivered to Bougainville, Japan’s Eighth Fleet naval headquarters was so confident in the security of the JN-25 code that it sent the message by radio.’

Cooper continues;

‘The Japanese had modified parts of their JN-25 code on April 1, as they periodically did, but for U.S. Navy code-breakers it was only a temporary setback—the basic code system remained unchanged. Therefore, American cryptanalysts could soon read large parts of new messages. On April 14, they intercepted and decoded Yamamoto’s travel schedule. It was a code-breaker’s dream. As he read it, Marine Lieutenant Colonel Alva Lasswell, one of the top cryptanalysts, exclaimed, “We’ve hit the jackpot.”

‘The decoded itinerary not only included the date and precise times for Yamamoto’s upcoming visits to the bases on Bougainville, but also revealed that he would be flying in a twin-engine bomber escorted by only six fighter planes. Ironically, his inspection tour was set for April 18, 1943, exactly one year after the Doolittle Raid on Tokyo.

‘Admiral Chester W. Nimitz, commander of the U.S. Pacific Fleet, conferred with Commander Edwin T. Layton, his chief intelligence officer. They understood that this could be their only chance to get Yamamoto because it might be the closest he would ever venture to the front. They calculated that American P-38 Lightning fighters based on Guadalcanal could fly the more than 800-mile round trip distance to Balalae airfield and back.

‘Nimitz knew that if the Japanese thought Yamamoto had been ambushed, they could suspect their code had been broken and change it. He decided the risk was worth it, because the Japanese had no one of comparable stature to replace Yamamoto. To be safe, he and Layton concocted a cover story: that Australian coastwatchers hiding in the jungles of Rabaul had tipped them off.

We Killed Yamamoto: a detailed description of the P-38 assassination of the man behind Pearl Harbor

‘Nimitz ordered Admiral William F. Halsey, commanding the area of operations that included Guadalcanal, to get Yamamoto. Like Nimitz, Halsey was concerned the mission would endanger their code-breaking secrets. Nimitz said he would assume responsibility for the risk and suggested that every effort “be made to make the operation appear fortuitous. Best of luck and good hunting.” Halsey’s headquarters transmitted the order: “Tally Ho. Let’s get the bastard.”

‘Meanwhile, U.S. officials were trying to make it appear as if the attack on Yamamoto had been sheer happenstance. Over the next few weeks, they repeatedly sent P-38s to Balalae to give the impression that the long journey was a regular mission for American fighter patrols. Additionally, American officials made no public statements to suggest they knew that Yamamoto had been killed.’

Cooper concludes;

‘FDR’s response:

‘Behind the scenes, President Franklin D. Roosevelt reacted with glee, writing a mock letter of condolence to Yamamoto’s widow that circulated around the White House but was never sent:

‘Dear Widow Yamamoto: ‘Time is a great leveler and somehow I never expected to see the old boy at the White House anyway. Sorry I can’t attend the funeral because I approve of it.

‘Hoping he is where we know he ain’t.

‘Very sincerely yours,

‘/s/ Franklin D. Roosevelt’

Photo credit: Photo credit: Adam Tooby via Osprey, U.S. Navy

P-38 print
This print is available in multiple sizes from AircraftProfilePrints.com – CLICK HERE TO GET YOURS. P-38J Lightning “SCAT III” – 43-28341, 479th FG, 434rd FS – 1944

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Dario Leone

Dario Leone

Dario Leone is an aviation, defense and military writer. He is the Founder and Editor of “The Aviation Geek Club” one of the world’s most read military aviation blogs. His writing has appeared in The National Interest and other news media. He has reported from Europe and flown Super Puma and Cougar helicopters with the Swiss Air Force.

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