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.
Special Forces (United States Army)
Their new headdress was first worn at a retirement parade at Fort Bragg on 12 June 1955 for Lieutenant General Joseph P. Cleland, the now-former commander of the XVIII Airborne Corps. Onlookers thought that the commandos were a foreign delegation from NATO. In 1956 General Paul D. Adams, the post commander at Fort Bragg, banned the wearing of the distinctive headdress, (although members of the Special Forces continued to wear it surreptitiously). This was reversed on 25 September 1961 by "Department of the Army Message 578636", which designated the green beret as the exclusive headdress of the Army Special Forces.