The United States Army Special Forces (SF), colloquially known as the "Green Berets" due to their distinctive service headgear, is the special operations branch of the United States Army. Although technically an Army branch, the Special Forces operates similarly to a functional area (FA), in that individuals may not join its ranks until having served in another Army branch. The core missionset of Special Forces contains five doctrinal missions: unconventional warfare, foreign internal defense, direct action, counterterrorism, and special reconnaissance.
https://en.wikipedia.org/wiki/United_States_Army_Special_Forces