Berets of the United States Army
U.S. Army Special Forces wear the green beret because of their link to the British Commandos of World War II. The first Ranger unit, commonly known as Darby's Rangers, was formed in Northern Ireland during the summer of 1942. On completion of training at the Commando Training Depot at Achnacarry Castle in Scotland, those Rangers had the right to wear the British Commando green beret, but it was not part of the regulation uniform at the time and was disallowed by the U.S. Army. 
The 10th Special Forces Group (Airborne) had many veterans of World War II and Korea in its ranks when it was formed in 1952. Members of the 10th SFG began to unofficially wear a variety of berets while training, some favoring the red or maroon airborne beret, the black beret, or the green commando beret. In 1953, a beret whose design was based on that of the Canadian Army pattern, and which was rifle-green in color, was chosen for wear by Special Forces units.
United States Army Special Forces in popular culture
"Ballad of the Green Berets" had many cover versions ranging from Ennio Morricone and Duane Eddy to "drugstore records" on labels such as Diplomat and Wyncote records. Hanna-Barbera Records released a children's LP "The Story of the Green Beret" available to members of the G.I. Joe club. The album was a tie-in with the release of the G.I. Joe Green Beret "action figure" (doll) that had appeared in 1966. The record had an album cover of Special Forces in action and a picture of the Medal of Honor. The record started off with a cover version of "Ballad of the Green Berets" but was a spoken account with sound effects of Colonel Pat Lawrence (Mike Road) taking two small boys (Andy and George) to visit Fort Bragg, North Carolina, to learn about the training and capabilities of "The Green Berets". The album then featured an exciting account of the Battle of Nam Dong where Captain Roger Donlon received the first Medal of Honor in the Vietnam War.