The Navajo code talker program, proposed by Philip Johnston (a civil engineer who had grown up on the Navajo reservation), exploited the extreme complexity and obscurity of the Navajo language. Navajo is tonal, has no written form (at that time), and its syntax and grammar are completely unrelated to any European or Asian language. Fewer than 30 non-Navajo people could speak it in 1942.
The approximately 400 Navajo Code Talkers developed a double-encrypted system: they first created a special military vocabulary (since Navajo had no words for modern military equipment), then used a letter-substitution code where each English letter could be represented by multiple Navajo words, making frequency analysis impossible.
At Iwo Jima, six Code Talkers worked around the clock for the first two days, sending and receiving over 800 messages without a single error. Major Howard Connor later said, "Were it not for the Navajos, the Marines would never have taken Iwo Jima." The Japanese, who had successfully broken every other American code, never came close to cracking Navajo. The program remained classified until 1968.