A fifth column is a group of people who undermine a larger group or nation from within, usually in favor of an enemy group or another nation. The activities of a fifth column can be overt or clandestine. Forces gathered in secret can mobilize openly to assist an external attack. The term is also applied to organized actions by military personnel. Clandestine fifth column activities can involve acts of sabotage, disinformation, espionage or terrorism executed within defense lines by secret sympathizers with an external force.