During the World War 2, the US military knew that the enemy was successfully breaking some of its codes. Furthermore, cipher machines were too big and complex to be used by infantry in combat conditions.
Philip Johnston, one of just a handful of non-Navajos who spoke the language of the native-American Navajo, knew that their unwritten language was extremely complex and spoken only in the Navajo region. He met with Major General Clayton B. Vogel of the Marines and suggested to use Navajos for secure military communications.
Tests proved that in simulated battle conditions, and with no equipment other than the radio itself, Navajos could encode, transmit, and decode a short message in about 20 seconds. Cipher machines required half an hour to do the same job, and their transmission could be deciphered by the enemy.
The General was convinced and recommended the recruitment of 200 Navajo code talkers. In May 1942 the first group of 29 Navajo code talkers was recruited. They developed a complex code dictionary, which assigned Navajo words to all modern military terms which did not exist in the Navajo language ("tank" and "fighter aircraft" for example, were replaced with "tortoise" and "hummingbird"). The code talkers dictionary also included codes for many terms ("battalion" was "red soil"), and multiple alternative phonetic codes for English letters. The result was such that even a Navajo was unable to understand their messages without knowing their code. The entire code had to be completely memorized by the code talker, nothing was written.
Navajo code talkers were then assigned to all marine battalions, where they quickly gained appreciation for their skill, speed, and accuracy. They were the ultimate "cipher machine" of World War 2. During the first two days of the battle of Iwo Jima, six Navajo code talkers, working in shifts, sent and received almost a thousand coded messages without an error. It was said that without them Iwo Jima could not be taken.
Japanese military intelligence, which listened to American transmissions and was able to decipher other American codes, was unable to break this one. Since it was a spoken code, they guessed right that it was a native-American language. They even forced a captured Navajo army soldier to try to break the code, but since the code talkers used a code, not plain Navajo language, he was also unable to understand what they were saying.
By the end of World War 2, there were about 400 Navajo code talkers. Because of their great potential value for future wars, the existence of Navajo code talkers remained for decades one of the most guarded secrets of World War 2. In 1981 president Reagan awarded them with a certificate of appreciation, and in 2001 they received congressional medals. The Navajo code talkers dictionary is now available in the US Navy's website.