[an error occurred while processing this directive]
The Choctaw Indians used for encoding messages during World War I weren't the only Native Americans ever used. During World War II, people remembered their effectiveness and enlisted the Navajo Indians to speak important messages in their native language over insecure channels.
The Navajo language is unwritten, and its syntax makes it sound unintelligible to anyone who hasn't had extensive exposure to it. It has no alphabet and no symbols. The language was used only in the Navajo lands of America's southwest. It was estimated at the time that there were only 30 non-Navajos who could understand the language. None of them were Japanese.
Philip Johnson, who was brought up on a Navajo reservation, was one those few non-Navajos who could speak the language fluently. He also happened to have served during World War I, where he learned how the Choctaw Indians were used to secure communication. He believed that the complexity and obscurity of the Navajo language qualified it as a useful encryption mechanism for the allied use.
What convinced the Marine Corps to use Navajos, though, was the speed with which they could convey secure communications. Traditionally, with machine encryption, it would take 30 minutes to encode, transmit, and decode a three-line English message. The Navajos could perform the same task in 20 seconds. Navajos began Codetalking in May of 1942.
In total, about 400 (of 50,000) Navajos were utilized as Codetalkers. Throughout the war they were lauded for their skill, accuracy, and speed. After the Battle of Iwo Jima, Major Howard Connor said that "were it not for the Navajos, the Marines would never have taken Iwo Jima." The Battle of Iwo Jima was an extremely important skirmish in the Pacific Ocean during World War II.
The Japanese were never able to break the Navajo code. The chief of Japanese Intelligence said that, although they could break the codes used by the Army, the Marine code remained a mystery. This was despite the abduction by the Japanese of a Navajo Indian who was not a codetalker. Although the Japanese forced him to translate the radio traffic, he "never figured out what you guys [were saying]" because the codetalkers used a special dictionary (reproduced here) with Navajo words and their special military equivalents. Consequently, to an untrained Navajo the radio traffic appeared to be a garbled mess of half sentences and fragmented phrases.
Because the ability to Codetalk remained valuable after World War II, the program remained classified. In 1992, an exhibit was opened in the Pentagon near Washington DC to commemorate the Navajo's contributions to the allied victory in World War II.
There is also a memorial to the Navajo Codetalkers in the Phoenix area. Here is a picture of it, used with permission from Mario Profaca.