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.
Military beret
Berets are also used by the Finnish Border Guard, which is a military organization under the aegis of Ministry of Interior during peacetime.The military beret originated in the French Army, in the form of the wide and floppy headdress worn by the Chasseurs Alpins (mountain light infantry) from their foundation in the early 1880s. The practical uses of the beret were soon known army-wide: the entire Expeditionary Force sent in China in 1900 used berets as headwear A tight-fitting version was subsequently adopted by French armoured troops towards the end of World War I. Between the wars, special fortress units raised to garrison the Maginot Line wore khaki berets as did the 13th Demi-Brigade of Foreign Legion when it was created in 1940. The Vichy Milice of the War period wore a blue beret.