A Fifth Column is any group of people who undermine a larger group from within, usually in favour of an enemy group or nation.
The activities of a fifth column can be overt or clandestine. Forces gathered in secret can mobilise openly to assist an external attack. This term is also extended to organised actions by military personnel.
Clandestine fifth column activities can involve acts of sabotage, disinformation, or espionage executed within defence lines by secret sympathisers with an external force.