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All quotes come from r/askhistorians comment by User druidofdarrowdelf, but it doesn't answer my own questions.

The Japanese captured a Navajo POW, Joe Kieyoomia, who wasn't part of the Code Talker program. He was forced to listen to the transmissions by his captors. The Japanese had studied the Navajo transmissions and had narrowed it down through a lengthy process (I can't understand Japanese so can't read the primary sources. Sorry for the vagueness), to figure out it was Navajo. Kieyoomia listened into these transmissions and heard phrases like "Red soil ahead" among all of the organizational information that was also coded. He thought it was complete gibberish, and told the Japanese that it made no sense. The Japanese thought he was lying, and tortured him regularly to extract more information about the Navajo language and code out of him. I don't feel as though Kieyoomia's resistance to Japanese efforts led the Japanese to believe that it really was just the language, and such limited their code breaking efforts there.

The code itself was simple, a glorified slang with a weird syntax, with applications for the military alphabet. The CIA provides an example of ship names. In English or any other language this could be cracked with time. Imagine a modern US infantry squad calling for "Apache support". To someone without knowledge of US weapon systems, this would appear a strange request. However after a few times hearing that and having helicopter gunships show up, the link could be drawn and part of the code could be cracked.

Then why couldn't the Japanese use Joe Kieyoomia's translation to infer the causation between Navajo words and their "Message or True Reading", and thus crack the code? For example, it feels straight forward and common sense that "the Navajo did not have a word for submarine so they translated it to iron fish".

I don't know if Japanese Intelligence Community was shambles. The Army/Navy rivalry was replicated between the different organizations, resulting in no real clear unified efforts and quite a few efforts to steal glory from each other. There were more focused on American operation codes that were transmitted in text, far more regularly and often and contained more information. But this all feels irrelevant, when the Japanese appeared to have gained headway into the Navajo Code already.

Research on the Navajo language

Navajo is one of the most difficult languages to learn. Its language family is the Athabaskan language. FEW non-Navajo have been nearly fluent. Navajo heavily relies on inflections of tone and use of nasal noises on vowels, so simple changes in pitch of a vowel mean a completely different word.

Navajo is being preserved better now due to many programs and educational outreach. But in 1942 Navajo was a dying language and there were FEW texts on it. You couldn't just pick books on Navajo off the shelf.

Research on the verbal use and speed of the Navajo Code Talkers

The Navajo Code was a tactical, and very rarely strategic code that was only submitted verbally. The Navajo Code was never used in a written form. If it had been, it could have been subjected to the same methods of code breaking that the world was using on operational and strategic codes like Enigma. The Navajo code was far less complex than Enigma and would not have held up well to such attacks. Audio recordings on wax cylinders were expensive and difficult to maintain in the Pacific theater, so even recording the radio messages sent by Code Talkers was incredibly difficult.

The Navajo Code was not only used on a tactical level, but on a much smaller scale than US operational codes. When the Code Talkers program was under development, time was its major selling point, not its heavy encryption. The current way for US forces to communicate on a open level was either "in the clear", i.e. just over the radio, or to use an encoding machine that usually took about an hour to encode and decode messages. The former the Japanese heavily exploited with their high percentage of fluent English speakers who gathered intelligence on the US and "sent out bogus messages in American code to lure marines into ambushes". The latter wasn't great when you needed fire support.

On the other hand Code Talkers could communicate messages to other Code Talkers nearly instantly. Fire support, movements, positions, could all be transmitted nearly "in the clear" in terms of speed. Also, Code Talkers worked like a sort of passcode, or key. With Navajo being so complex and the Code Talkers being such a small group, they recognized and knew each other during transmissions. And once attached units also recognized this, Code Talkers messages were treated as critically important, the Japanese couldn't falsely transmit them. Code Talkers were able to let everyone know when the Japanese sent out false messages being as if they were American. This uniqueness kept the code in the military arsenal (and the Navajo language was classified) until 1965, when frequency hopping radios started to become available and replaced the Code Talkers concept.

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    good question, but you've semi-answered it yourself. if Navajo was only used in a verbal, tactical, context, then by the time they got Joe K. to listen to it, which would be very difficult if he was not "on the spot", the information would already be largely irrelevant. consider also the distances involved in the Pacific campaigns and "on the spot" would be quite unlikely to happen. – Italian Philosophers 4 Monica May 10 at 21:21
  • @ItalianPhilosophers4Monica But the relationship between the Code Word (plain text) and True Meaning (cipher text) would've stayed the same. – Swansea May 10 at 22:45
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    If I can find my copy of Code Talker, the memoirs of one of the original code talkers, I will create an answer. The gist is that the US military had the initial group of code talkers create a code based on their language. It was never just about talking plan Navajo speach. – Gort the Robot May 11 at 3:51
  • I speak Japanese and a bit of Navajo, as both have subject-object-verb formats. I going to make the unsubstantiated claim that Japanese would have had a difficult time hearing the tones as their language is temporal, not tonal. Chinese, which only has 4 tones and a temporal elongation is very difficult for native Japanese to learn. The brain's ability to reject noise is a harsh filter to other languages, but it does let you do things like understand someone with no teeth. – b degnan May 11 at 14:20
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    Navajo is one of the most difficult languages to learn. There is no absolute measure of difficulty; this statement needs to be qualified with respect to the native languages in question. – Azor Ahai -- he him May 11 at 14:40
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Question: Why couldn't Japan crack the Navajo Code in WW2, when they captured a Navajo native speaker?

Because it wasn't enough to just speak Navajo, they still had to know the code.

So let's say in the code a turtle signified a company of soldiers and up the hill signified heading to the south coast of the island. It would take a Navajo speaker to tell you 3 turtles were headed up the hill. But if you didn't know what that meant, you had no hope in understanding the code.

The code was spoken over wired telephone lines which the Japanese tapped into. The messages might have been broken if the given Navajo speakers were situationally relevant to the message. But being POW's, they were often given the messages to translate removed from the battlefield both by time and distance. This meant they couldn't compare the messages to actual events on the battlefield to try to decipher them. They could not just read, 3 turtles crawled up the hill, and see three companies of men began to head down to the south coast, and associate the two events.

From Comments:

@Oleg Lobachev How hard is it to learn the needed amount of Navaho anyway? An intelligence officer could do this and then observe the field... –

It's basically impossible to learn in a few months in order to be helpful to the Japanese, especially in a wartime Japan, removed from any resources. What resources would a Japanese code breaker have to learn an unwritten language if the population who speaks that language is behind enemy lines and no written resources exist.

PBS
Although Navajo is the most-spoken Native American language in the U.S., it is rarely spoken outside of the Navajo reservation. An unwritten language without a traditional alphabet or symbols, Navajo's extreme complexity of syntax makes it unintelligible to anyone without extensive exposure and training.

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from Kentaro
Sorry I reversed my upvote. The reason is your above claim without any proof. – (This claim without any proof)

@JMS My understanding is that the Japanese knew the Marines were using a Navaho Indians as code talkers.

My apology, the comments are too tight a space to give sources, so often I don't even try. I say the Japanese knew the Marines were using Navajo Indians as code talkers, because they sought out a Navajo POW and enlisted him to help break the code. Thus the Japanese knew they were dealing with not only an American Indian code, but specifically one based on the Navajo language.

Joe Kieyoomia was a Navajo soldier from New Mexico, who was not a code talker. He was captured by the Japanese when the Philippines fell at the beginning of the War. The Japanese tried unsuccessfully to put him to work trying to crack the Navajo code.

Joe Kieyoomia
The Japanese tried unsuccessfully to have him decode messages in the "Navajo Code" used by the United States Marine Corps, but although Kieyoomia understood Navajo, the messages sounded like nonsense to him because even though the code was based on the Navajo language, it was decipherable only by individuals specifically trained in its usage.

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@Swansea
@JMS Why weren't the Japanese deliver intercepts to the Navajo POWs and back to Japanese intelligence in a timely manner, like you wrote? To use the example in my quote, why didn't the Japanese report, after the US called "Apache support", everything that followed including helicopter gunships to the Navajo POWs?

    1. These battlefields on Pacific Islands weren't small localized affairs, they involved tens of thousands of men, Battle of Okinawa utilized 180,000 American combat troops. There were no "apache helicopters" in WWII, but how could the japanese know what any given message would refer to when their enemy was operating across such a scale?
    1. The code conveyed by the Navajo was professionally done. It wasn't as easy as just listening for key words and trying to match them up to what was happening. Think of a code done in English. It wouldn't be much of a code if once hearing it a few times the other fellow knew what you were speaking of.
    1. I don't believe the Navajo POW in question Joe Kieyoomia ever left his POW camp in Nagasaki. It wasn't practical to send him to Islands under invasion. The US had literally hundreds of Navajo code talkers, The japanese weren't just looking for one guy, they needed the ability to train hundreds of intelligence officers in this code in order to capitalize on breaking it. Of coarse though they never broke it.
    1. Japanese intercepts were from radio and hard wired messages. Both had challenges in utilizing POW's proximal to the battle field. and both necessitated a clandestined component which would prohibit also having those sources being entirely aware of occurrences on a battle front which could be many miles across. Wire taps had to be conducted behind American lines, it would have been difficult to bring a navajo fluent POW on such a mission; much less transport him to remote locations under siege.

from Kentaro @So, I need the source! You don't have to hurry, your answer is not bad, so I'm waiting for your effort. Anyway, thanks

( on how the JIA associated the Navajo Language with the unbreakable code. )

@Kentaro, It seems like interrogations of the many Marine POW's the Japanese captured would be the obvious source. I'm not stating that as a fact, although it probable is how they found out. I did find this source which claims a Japanese interrogator Goon pieced it together upon interrogating Joe Kieyoomia. That doesn't seem reasonable that this Translator thousands of miles removed from the battle field in Nagasaki would piece this together. I think captured POW's who would certainly have been questioned vigorously on knowledge of this code would be the more reasonable source. Then Goon got involved when the Japanese searched their camps for a native Navajo speaker. That's just my hypothesis. The Japanese knew about Joe Kieyoomia because when they captured him in the philippines they mistook him for an ethnically Japanese person. It took some time for him to convince them he was Navajo. Here is that source.

Code Talker: The First and Only Memoir By One of the Original Navajo Code Talkers
Despite the United States' insistence upon secrecy, the Japanese somehow learned that the unbreakable code being utilized by the Americans had something to do with the Navajo language. No one knows exactly how or when this information was obtained, but it has been hypothesized that a japanese translator with the surname Goon first associated the Navajo language with the unbreakable code while participating in the interrogation of Joe Kieyoomia. Kieyoomia, a Navajo man who has survived the Bataan Death March, was questioned by Goon and tortured by his Japanese captors in their attempt to force him to crack the code. ....

Several Navajo prisoners reported, postwar that the Japanese had tried to get them to figure out the marine's code. None of these captives were code talkers, and none shed any light on the complicated secret language.

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The Native Americans had code words for things that were not in their language. For example, "iron fish" could mean "submarine". So not only did the Japanese have to translate the language, but they also had to decode the phrases and put them together. Combine this answer with JMS's answer and you have a good picture of what was going on with the code talkers.

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  • What you're saying is some Navajos were only there to pass on the messages without knowing how to decode them, right? That would keep some loose ends tied up. Even if a code-talker defected to the Japanese, they might not know the code and then get harmed by the exact people they had defected to! Brilliant! – Someone May 11 at 13:40
  • So they didn't even pass it on? They were just there? On a (somewhat) unrelated note, you would think that the US would expand the reservations for their help. – Someone May 11 at 13:50
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The Japanese captured a Navajo POW, Joe Kieyoomia, who wasn't part of the Code Talker program. He was forced to listen to the transmissions by his captors. The Japanese had studied the Navajo transmissions and had narrowed it down through a lengthy process (I can't understand Japanese so can't read the primary sources. Sorry for the vagueness), to figure out it was Navajo. Kieyoomia listened into these transmissions and heard phrases like "Red soil ahead" among all of the organizational information that was also coded.

Please provide me with a source that Japanese forces dealt Joe Kieyoomia in such a way.


2 irrefutable sources that JIA did not know the U.S code was based on Navaho's throughout the pacific war.

After I googled at Google Japan as "Pacific war, Navaho", this page showed up.

It says,

これらの暗号はその後も再び使用される可能性があったため、1980年代まで米軍の機密情報として扱われていた。

These codes (including other Indians' such as Choctaw, Comanche, Seminole) had a potential use again, the U.S forces kept secret up until 1980's.

After knowing such an information, I googled in English and found this site.

There is a line,

It took a lot longer than expected but Adolph Nagurski, a late Code Talker from the Navajo Nation, has finally been recognized for his service to the United States. As a member of the U.S. Marine Corps, Nagurski was among hundreds of Navajo citizens who used their language to transmit, receive and translate codes during World War II. Military officials credited their system -- which was never broken by foreign enemies -- with helping the U.S. and its allies achieve victory in the Pacific Theater. But like his fellow recruits, Nagurski kept his wartime service a secret. Even though the Code Talker project was declassified in 1968, their story didn't become more well known until Congress recognized the heroes with gold and silver medals.

So even the U.S site says the code was "classified" up until 1968.

How could the JIA forces know such a secret code was Navajo's which was declassified after 23 years later from the end of the war?

Now it turns out it was quite highly likely that the unfortunate Joe Kieyoomia was lynched (according to the Japanese site) only probably by the JIA who tried to seek any information from one of the "ordinary soldiers".

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    How does the program being classified mean it's impossible for Japan to have figured out which language was being used? I'm not making any claims either way about whether they did manage to identify the language, I just don't understand why you think the program remaining classified is "irrefutable" evidence that they can't have. – Chris H May 12 at 7:32
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    The premise of this answer's logic (as mentioned explicitly and repeatedly by @ChrisH), is incredibly flawed. a) "Classified" is the default for government projects.with military applications. b) There are other countries in the world besides the USA and Japan. c) The irrefutable (?) sources you quote make the same point: "had a potential use again" – Cireo May 12 at 8:58

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