A fifth column is a group of people who undermine a larger group (such as a nation or a besieged city) from within, usually in favor of an enemy group or nation. The activities of a fifth column can be overt or clandestine.

Usually, a technique of the fifth column is the infiltration of sympathizers into the entire fabric of the nation under attack and, particularly, into positions of policy decision and national defense. From these key positions, fifth column activists exploit the fears of a people by spreading rumors and misinformation, as well as by employing more standard techniques of espionage and sabotage.

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