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.
The Green Berets (film)
Film commentator Emanuel Levy noted in his review that Wayne was not attempting to promote the cause of the Vietnam War as much as he was trying to portray the Special Forces in their unique role in the military: "Wayne said his motive was to glorify American soldiers as the finest fighting men 'without going into why we are there, or if they should be there.' His 'compulsion' to do the movie was based on his pride of the Special Forces, determined to show 'what a magnificent job this still little-known branch of service is doing.' ... 'I wasn't trying to send a message out to anybody,' he reasoned, 'or debating whether it is right or wrong for the United States to be in this war.'"