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)
Shortly after this attack, while supervising a group of United States Navy Seabees clearing part of the jungle around the base camp and evacuating the civilians in preparation for a potential Viet Cong attack, Sergeant Muldoon notices an ARVN soldier pacing out distances in the camp and knocks him out. When Captain Nghiem interrogates him, he discovers a silver Zippo lighter in the ARVN soldier's possession. The lighter belonged to a Green Beret medical specialist, a friend of Kirby's, recently murdered by the Viet Cong. After Beckworth sees Nghiem torture the Viet Cong suspect to get a confession from him, he confronts Kirby about it. The Colonel justifies the interrogation by telling Beckworth about the cigarette lighter, and says the Viet Cong are ruthless killers who deserve no legal protections of any sort in this new kind of war.