Guadalcanal, Tarawa, Peleliu, Iwo Jima: the Navajo code talkers took part in every assault the U.S. Marines conducted in the Pacific from 1942 to 1945. They served in all six Marine divisions, Marine Raider battalions and Marine parachute units, transmitting messages by telephone and radio in their native language — a code that the Japanese never broke.
The idea to use Navajo for secure communications came from Philip Johnston, the son of a missionary to the Navajos and one of the few non-Navajos who spoke their language fluently. Johnston, reared on the Navajo reservation, was a World War I veteran who knew of the military's search for a code that would withstand all attempts to decipher it. He also knew that Native American languages — notably Choctaw — had been used in World War I to encode messages.
Johnston believed Navajo answered the military requirement for an undecipherable code because Navajo is an unwritten language of extreme complexity. Its syntax and tonal qualities, not to mention dialects, make it unintelligible to anyone without extensive exposure and training. It has no alphabet or symbols, and is spoken only on the Navajo lands of the American Southwest. One estimate indicates that less than 30 non-Navajos, none of them Japanese, could understand the language at the outbreak of World War II.
For other information, check this out at :http://www.californiaindianeducation.org/native_american_veterans/navajo_code_talkers.html
Below there are some navajo's code that used in WW2 (DECLASSIFIED UNDER DEPARTMENT OF DEFENSE DIRECTIVE 5200.9). ...
NAMES OF VARIOUS
ORGANIZATIONS NAVAJO WORD LITERAL TRANSLATION
CORPS DIN-NEH-IH CLAN
DIVISION ASHIH-HI SALT
REGIMENT TABAHA EDGE WATER
BATTALION TACHEENE RED SOIL
COMPANY NAKIA MEXICAN
PLATOON HAS-CLISH-NIH MUD
SECTION YO-IH BEADS
SQUAD DEBEH-LI-ZINI BLACK SHEEP
COMMANDING GEN. BIH-KEH-HE (G) WAR CHIEF
MAJOR GEN. SO-NA-KIH TWO STAR
BRIGADIER GEN. SO-A-LA-IH ONE STAR
COLONEL ATSAH-BESH-LE-GAI SILVER EAGLE
LT. COLONEL CHE-CHIL-BE-TAH-BESH-LEGAI SILVER OAK LEAF
MAJOR CHE-CHIL-BE-TAH-OLA GOLD OAK LEAF
CAPTAIN BESH-LEGAI-NAH-KIH TWO SILVER BARS
LIEUTENANT BESH-LEGAI-A-LAH-IH ONE SILVER BAR
COMMANDING OFFICER HASH-KAY-GI-NA-TAH WAR CHIEF
EXECUTIVE OFFICER BIH-DA-HOL-NEHI THOSE IN CHARGE
source : http://www.history.navy.mil/faqs/faq61-4.htm