Fifth Column, a name given to enemy agents or disloyal persons working within their own country to undermine its will to resist an outside enemy. The term originated during the Spanish Civil War (1936–39) when General Emilio Mola, a subordinate of the rebel leader Francisco Franco, boasted that his four columns (troop formations) advancing upon Madrid would be supplemented by a “fifth column,” consisting of Franco supporters, within the city.

During World War II Nazi and Fascist fifth columns operated in many countries to weaken opposition to Axis aggression. The most notable fifth columnist was Vidkun Quisling, a Norwegian army officer who betrayed his country to the Germans in 1940.