Marines used Navajo language as unbreakable code

By early 1942 the US military was losing the codebreaking race in the Pacific. The Marines needed fast, secure radio between units and headquarters, but the fixed codes and cipher machines of the day were slow to encrypt, slow to decrypt, and increasingly being read by the enemy. Any code that stayed in use long enough to train people on it was on the verge of being compromised.

Philip Johnston, a WWI veteran who had grown up on the Navajo reservation, proposed a radically different fix: stop trying to build a stronger machine code and instead transmit in the Navajo (Dine) language itself. Navajo was a mostly unwritten, tonally complex language with no European roots, spoken fluently by almost no outsiders — only about 28 Americans were thought to have more than a superficial knowledge of it. An intercepted Navajo transmission could not even be parsed, let alone decrypted.

The Marines tested the idea on February 28, 1942, and adopted it. The first class of 29 Navajo recruits built a military dictionary — starting at 211 terms and later expanded to 411 — plus a phonetic alphabet that grew from 26 to 44 terms for names and words with no Navajo equivalent. Everything was designed to be memorized, so there was no written codebook that could fall into enemy hands.

Used Navajo as the code itself: messages were spoken in a language few outsiders knew, with no written codebook to capture, so enemy codebreakers had nothing to crack.

Navajo was unwritten and tonally complex, so intercepted messages couldn't be parsed.

Only about 28 outsiders knew Navajo, so the enemy had no analysts who could understand it.

No written codebook existed, so there was nothing to capture and reverse-engineer.

The enemy's codebreaking machinery was built for ciphers, not for parsing unknown languages.

When rivals crack your shared channels, switch to a medium they can't parse—a niche language or private protocol—so their interception tools become useless.

The Navajo codes were never broken in the war. About 375–420 Navajos served; survivors received Congressional Gold Medals in 2001.