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.
Commandos Marine
Commando training is the gateway to the Special Operations Forces for the Naval Riflemen. Conducted at the Fusilier Marins school at Lorient on the Atlantic coast, it provides upon successful completion entry to the commandos and the right to wear the green beret. Lasting 20 weeks, it includes one week of commando testing, six weeks of screening and preparatory training, four weeks of evaluation, the actual SOF course for seven weeks, and two weeks of parachute training. During this period, any mistake can instantly disqualify the candidate. The ultimate goal of this training is to detect individuals with the physical, intellectual and psychological potential needed to serve in the Commandos Marine. The historical roots of commando training date back to the Second World War, when Fusilier-Marins volunteers from the Free French Navy went to the Commando training center in Achnacarry, Scotland. Since then, the Commandos Marine have kept by tradition the green beret pulled right with the bronze shield badge on the left, the only such exception in the French armed forces. They have retained the principle of exceptional training without compromise, based on immersion in a highly stressful environment, close to the conditions of combat operations.