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.
Robin Moore
Thanks to connections with Harvard classmate Robert F. Kennedy, Moore was allowed access to the U.S. Army Special Forces to write about this elite unit of the United States Army. It was General William P. Yarborough who insisted that Moore go through special forces training in order to better understand "what makes Special Forces soldiers 'special'." He trained for nearly a year, first at "jump school" for airborne training before completing the Special Forces Qualification Course or "Q Course", becoming the first civilian to participate in such an intensive program. Afterwards, Moore was assigned to the 5th Special Forces Group on deployment to Vietnam. His experiences in-country formed the basis for "The Green Berets", a bestseller that helped secure him international acclaim (see United States Army Special Forces in popular culture).