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.
Berets of the United States Army
When visiting the Special Forces at Fort Bragg on 12 October 1961, President John F. Kennedy asked Brig. Gen. William P. Yarborough to make sure that the men under his command wore green berets for the visit. Later that day, Kennedy sent a memorandum that included the line: '"I am sure that the green beret will be a mark of distinction in the trying times ahead". By America's entry into the Vietnam War, the green beret had become a symbol of excellence throughout the U.S. Army. On 11 April 1962 in a White House memorandum to the United States Army, President Kennedy reiterated his view: "The green beret is a symbol of excellence, a badge of courage, a mark of distinction in the fight for freedom". Previously, both Yarborough and Edson Raff had petitioned the Pentagon to allow wearing of the green beret, to no avail.