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
Mattel toys made "Guerrilla Fighter" playsets in 1962 containing a commando green beret with an interesting tin "Guerrilla Fighter" badge depicting the crossed arrows insignia of the Special Forces, (formerly worn by the 1st Special Service Force, and before that the U.S. Army Indian Scouts) and a jungle knife in front of a parachute. The set also contained the Mattel Dick Tracy automatic cap firing "tommy gun" or "Scattergun" (the Dick Tracy cap firing but no longer water firing riot shotgun) toy guns, both now in military camouflage plastic, a military camouflaged poncho, and in some sets, a rubber Ka-Bar knife and a tripwire booby trap. Mattel later made the "M-16 Marauder", in 1966, which appeared in "The Green Berets" film wherein an enraged John Wayne smashes one against a tree.