special forces troops at Umm Qasr during the 2003 invasion of Iraq
Special forces and special operations forces are
military units trained to conduct
NATO defines special operations as "military activities conducted by specially designated, organized, trained, and equipped forces, manned with selected personnel, using unconventional tactics, techniques, and modes of employment".
Special forces emerged in the early 20th century, with a significant growth in the field during the
Second World War, when "every major army involved in the fighting" created formations devoted to special operations behind enemy lines.
Depending on the country, special forces may perform some of the following functions:
foreign internal defense,
mobility operations, and
unconventional warfare. The term special forces in the United States refers to the U.S. Army's forces, while the term special operations forces (SOF) refers to all units.
 In Russian-speaking countries special forces are typically called
spetsnaz, an acronym for "special purpose". Likewise, Russian speakers refer to special forces of other nations as spetsnaz; for example, U.S. Special Forces would be referred to as amerikanski spetsnaz in Russian.