Philip Johnston proposed the use of Navajo to the United States Marine Corps at the beginning of World War II. Johnston, a World War I veteran, was raised on the Navajo reservation as the son of a missionary to the Navajos, and was one of the few non-Navajos who spoke their language fluently. Because Navajo has a complex grammar, is not nearly mutually intelligible enough with even its closest relatives within the Na-Dene family to provide meaningful information, and was an unwritten language, Johnston saw Navajo as answering the military requirement for an undecipherable code. Navajo was spoken only on the Navajo lands of the American Southwest, and its syntax and tonal qualities, not to mention dialects, make it unintelligible to anyone without extensive exposure and training. One estimate indicates that at the outbreak of World War II fewer than 30 non-Navajos, none of them Japanese, could understand the language.
Early in 1942, Johnston met with Major General Clayton B. Vogel, the commanding general of Amphibious Corps, Pacific Fleet, and his staff. Johnston staged tests under simulated combat conditions which demonstrated that Navajos could encode, transmit, and decode a three-line English message in 20 seconds, versus the 30 minutes required by machines at that time. The idea was accepted, with Vogel recommending that the Marines recruit 200 Navajos. The first 29 Navajo recruits attended boot camp in May 1942. This first group then created the Navajo code at Camp Pendleton, Oceanside, California. The Navajo code was formally developed and modeled on the Joint Army/Navy Phonetic Alphabet that uses agreed-upon English words to represent letters. As it was determined that phonetically spelling out all military terms letter by letter into words—while in combat—would be too time consuming, some terms, concepts, tactics and instruments of modern warfare were given uniquely formal descriptive nomenclatures in Navajo (the word for "potato" being used to refer to a hand grenade, or "tortoise" to a tank, for example). Several of these portmanteaus (such as gofasters referring to running shoes, ink sticks for pens) entered Marine corps vocabulary and are commonly used today to refer to the appropriate objects.
A codebook was developed to teach the many relevant words and concepts to new initiates. The text was for classroom purposes only, and was never to be taken into the field. The code talkers memorized all these variations and practiced their rapid use under stressful conditions during training. Uninitiated Navajo speakers would have no idea what the code talkers' messages meant; they would hear only truncated and disjointed strings of individual, unrelated nouns and verbs.
The Navajo code talkers were commended for their skill, speed and accuracy accrued throughout the war. At Iwo Jima, Major Howard Connor, 5th Marine Division signal officer, had six Navajo code talkers working around the clock during the first two days of the battle. These six sent and received over 800 messages, all without error. Connor later stated, "Were it not for the Navajos, the Marines would never have taken Iwo Jima.
As the war progressed, additional code words were added on and incorporated program-wide. In other instances, informal short-cut code words were devised for a particular campaign and not disseminated beyond the area of operation. To ensure a consistent use of code terminologies throughout the Pacific Theater, representative code talkers of each of the U.S. Marine divisions met in Hawaii to discuss shortcomings in the code, incorporate new terms into the system, and update their codebooks. These representatives in turn trained other code talkers who could not attend the meeting.
The deployment of the Navajo code talkers continued through the Korean War and after, until it was ended early in the Vietnam War.