The United States Army Special Forces (SF), colloquially known as the "Green Berets" due to their distinctive service headgear, are a special operations force of the United States Army. The Green Berets are geared towards nine doctrinal missions: unconventional warfare, foreign internal defense, direct action, counterterrorism, counterinsurgency, special reconnaissance, information operations, counterproliferation of weapons of mass destruction, and security force assistance. The unit emphasizes language, cultural, and training skills in working with foreign troops; recruits are required to learn a foreign language as part of their training and must maintain knowledge of the political, economic, and cultural complexities of the regions in which they are deployed. Other Special Forces missions, known as secondary missions, include combat search and rescue (CSAR), counter-narcotics, hostage rescue, humanitarian assistance, humanitarian demining, peacekeeping, and manhunts. Other components of the United States Special Operations Command (USSOCOM) or other U.S. government activities may also specialize in these secondary missions. The Special Forces conduct these missions via seven geographically focused groups. Many of their operational techniques are classified, but some nonfiction works and doctrinal manuals are available. As special operations units, Special Forces are not necessarily under the command authority of the ground commanders in those countries. Instead, while in theater, SF units may report directly to a geographic combatant command, USSOCOM, or other command authorities. The Central Intelligence Agency's (CIA) highly secretive Special Activities Center, and more specifically its Special Operations Group (SOG), recruits from U.S. Army Special Forces. Joint CIA–Army Special Forces operations go back to the unit MACV-SOG during the Vietnam War, and were seen as recently as the War in Afghanistan (2001–2021). Mission The primary mission of the Army Special Forces is to train and lead unconventional warfare (UW) forces, or a clandestine guerrilla force in an occupied nation. The 10th Special Forces Group was the first deployed SF unit, intended to train and lead UW forces behind enemy lines in the event of a Warsaw Pact invasion of Western Europe. As the U.S. became involved in Southeast Asia, it was realized that specialists trained to lead guerrillas could also help defend against hostile guerrillas, so SF acquired the additional mission of Foreign Internal Defense (FID), working with Host Nation (HN) forces in a spectrum of counter-guerrilla activities from indirect support to combat command. Special Forces personnel qualify both in advanced military skills and the regional languages and cultures of defined parts of the