It was tonal — the meaning of a word changed depending on the pitch at which it was pronounced, and those tonal distinctions were not marked in writing and not intuitive to speakers of non-tonal languages.

Its phonological system included sounds that did not exist in English, Japanese, or most European languages.

Its grammar was complex and structured in ways that were fundamentally different from the Indo-European family of languages.

And it had, as of 1942, almost no presence in Japanese academia or intelligence — there were virtually no Japanese speakers who had studied Diné Bizaad, and there were no significant written resources that would have allowed rapid self-education in the language.

Johnston proposed to the Marine Corps that Navajo men could be recruited to serve as communications specialists — not simply speaking their language over the radio, which would have provided only limited security, but using a specially developed code built on top of the language that would add an additional layer of complexity.

He demonstrated the concept in January 1942, appearing at Camp Elliott near San Diego with four Navajo men who transmitted and decoded simulated military messages in a matter of minutes — messages that Marine communications officers with access to standard encryption equipment could not have processed in anything close to the same time.