During World War II Allied code breakers figured out German and Japanese codes -- the secret work done at Bletchley Park and other code breaking sites played a vital role in the eventual Allied victory.
And, of course, the Axis powers had their own code breakers, too.
Navajo Code Talkers. According to the Navajo Code Talkers website,
[The Navajo code] originated as approximately 200 terms—growing to over 600 by war's end—and could communicate in 20 seconds what took coding machines of the time 30 minutes to do. It consisted of native terms that were associated with the respective military terms they resembled. For example, the Navajo word for turtle meant "tank," and a dive-bomber was a "chicken hawk." To supplement those terms, words could be spelled out using Navajo terms assigned to individual letters of the alphabet—the selection of the Navajo term being based on the first letter of the Navajo word's English meaning. For instance, "Wo-La-Chee" means "ant," and would represent the letter "A". In this way the Navajo Code Talkers could quickly and concisely communicate with each other in a manner even uninitiated Navajos could not understand.The Navajo were not the only code talkers, nor was code talking invented for World War II: According to Wikipedia Cherokee and Choctaw code talkers aided Allied efforts in World War I. Hitler knew about these code talkers and, as part of his war preparation, ordered a team of 30 anthropologists to study and learn Native American languages. And because we knew that Hitler knew, Native American code talkers were used mostly in the Pacific Theater (although, Wikipedia adds, there were Comanche code talkers used in the Normandy invasion, landing at Utah Beach, and 27 Meskwaki code talkers -- members of the Fox tribe -- used in the North African campaign).
Navajo Code Talkers Day was set aside in 1982, however, by President Ronald Reagan, specifically to honor the surviving Navajo code talkers. From the linked Wikipedia article:
The Navajo code talkers were commended for their skill, speed and accuracy accrued throughout the war. At the Battle of 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."