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
The 1st Special Forces Group on Okinawa provided a number of troopers to act as extras in director Samuel Fuller's "Merrill's Marauders" (1961) and were credited at film's start. On U.S. television, a March 1962 episode of "Surfside 6" titled "The Green Beret" featured SF training. Henry Fonda appeared in, and narrated, a 1962 "Special Forces" episode of "The Big Picture" series of U.S. Army-produced films that found their way to U.S. television. In 1963, a Green Beret appeared in the episode "In Praise of Pip" of "The Twilight Zone" though the U.S. Army told the CBS television network to not name the Southeast Asian country where the story occurred. The Green Beret's first Hollywood appearance is in the futuristic thriller film "Seven Days in May" (1963) wherein Andrew Duggan is a Special Forces officer loyal to the U.S. President, not the traitorous JCS Chief Burt Lancaster; the film also gave the U.S. filmgoer a first glimpse of the M16 rifle.