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.
Green Berets (Bosnian paramilitary)
Zelene beretke (English, "Green Berets") was a paramilitary organization founded in Sarajevo in early 1992. They were founded by demobilized soldiers and conscripts from the Yugoslav National Army (JNA) who were mostly ethnic Bosniaks and supporters of Bosnian nationalism. They were integrated into a newly founded Army of the Republic of Bosnia and Herzegovina in the second half of 1992. The Green Berets as a gear of choice and name was selected both as a reference to United States Army Special Forces (although no actual relation existed) and as a common colour brand of the predominant ethnic group that composed the unit, namely Bosniaks. Bosnian Green Berets were mostly active during the war in the early part of 1992 in northern and central Bosnia.