Refers to any clandestine group or faction of subversive agents who attempt to undermine a nation's solidarity.

   Emilio Mola Vidal, a Nationalist general during the Spanish Civil War (1936-39), originally coined the term. As four of his army columns moved on Madrid, the general referred to his militant supporters within the capital as his "fifth column," intent on undermining the loyalist government from within.

   So the fifth column is a group of secret sympathizers or supporters of an enemy that engage in espionage or sabotage within defense lines or national borders. Recent conflicts have had their fifth columns: Iraqi insurgents in the Gulf War, Cuban rebels in the Bay of Pigs. Those columns didn't fare quite as well.