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.
First Special Service Force
In 1952 Colonel Aaron Bank (an operator for the Office of Strategic Services [OSS] during WW II) created another elite unit using the training, strategies, and lessons learned from various OSS team missions as well as the experiences of the First Special Service Force (FSSF). This force evolved into the United States Army Special Forces (aka the Green Berets) which draws its lineage from both the OSS and the FSSF. In Canada Military Intelligence and Logistical Operations 1952–1988, the Canadian Airborne Regiment 1968–1995 which formed part of the Special Service Force 1977-1995 and today's Canadian Special Operations Regiment, like United States Army Special Forces, trace their roots to the FSSF. Just like in World War II, Canada's elite JTF2 and the United States' elite SFOD-D operators were united once again into a special assignment force for the 2001 invasion of Afghanistan.
In 2006 the Canadian members of the 1st Special Service Force received the United States Army's Combat Infantryman Badge for participation in front-line combat. On February 3, 2015, the FSSF was awarded the Congressional Gold Medal, the highest award congress can give to civilians.