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1936, from Gen. Emilio Mola's comment at the siege of Madrid during the Spanish Civil War that he would take the city with his four columns of troops outside it and his "fifth column" (quinta columna) in the city.
People willing to cooperate with an aggressor against their own country. The term originated in a remark by Francisco Franco, the Spanish dictator, that he was marching on Madrid with four columns of troops, and that there was a “fifth column” of sympathizers within the city ready to help.
clandestine group or faction of subversive agents who attempt to undermine a nation's solidarity by any means at their disposal. The term is credited to Emilio Mola Vidal, a Nationalist general during the Spanish Civil War (1936-39). 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