fifth column

The phrase "fifth column" refers to a secretive and disloyal group or individuals within a country or organization that work to undermine and sabotage it from within, usually during times of war or conflict. These individuals or groups typically collaborate with or sympathize with the enemy or opposing side, secretly gathering information, spreading propaganda, or engaging in acts of sabotage to destabilize and weaken their own side. The term "fifth column" originated during the Spanish Civil War in the 1930s and has since been used to describe similar situations in different contexts.