Barrier troops, blocking units, or anti-retreat forces are military units that are located in the rear or on the front line (behind the main forces) to maintain military discipline, prevent the flight of servicemen from the battlefield, capture spies, saboteurs and deserters, and return troops who flee from the battlefield or lag behind their units. According to research by Jason Lyall, barrier troops are more likely to be used by the militaries of states that discriminate against the ethnic groups that comprise the state's military.