A fifth column is any group of people who undermine a larger group or nation from within, usually in favor of an enemy group or another nation. According to Harris Mylonas and Scott Radnitz, "fifth columns" are "domestic actors who work to undermine the national interest, in cooperation with external rivals of the state". The activities of a fifth column can be overt or clandestine. Forces gathered in secret can mobilize openly to assist an external attack. This term is also extended to organized actions by military personnel. Clandestine fifth column activities can involve acts of sabotage, disinformation, espionage, and/or terrorism executed within defense lines by secret sympathizers with an external force.