A fifth column is a group of people who clandestinely undermine a larger group such as a nation from within. A fifth column can be a group of secret sympathizers of an enemy that are involved in sabotage within military defense lines, or a country's borders. A key tactic of the fifth column is the secret introduction of supporters into the whole fabric of the entity under attack. This clandestine infiltration is especially effective with positions concerning national policy and defense. From influential positions like these, fifth-column tactics can be effectively utilized, from stoking fears through misinformation campaigns, to traditional techniques like espionage.
The term originated with a 1936 radio address by Emilio Mola, a Nationalist General during the 1936–39 Spanish Civil War. As his army approached Madrid, a message was broadcast that the four columns of his forces outside the city would be supported by a "fifth column" of his supporters inside the city, intent on undermining the Republican government from within (see Siege of Madrid). The term was used as the title of Ernest Hemingway's only play, which he wrote while the city was being bombarded; the play was published in 1938 in his book The Fifth Column and the First Forty-Nine Stories