Special forces and special operations forces are military units trained to conduct special operations. 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.