[The Fifth Column] 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, he broadcast a message 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).[1]
In fact, this supposed "fifth column" did not prove very effective, as evidenced by the fact that Madrid held out until 1939 despite very heavy fighting. Nevertheless, the term caught on and was used extensively, especially by those fighting the Fascists and *****. It was especially in wide use in Britain in the early stages of the Second World War. Their fear of the "fifth Column" was used as justification for the mass internment, on the Isle of Man, of German nationals who resided in the United Kingdom. The United States and Canada interned Japanese citizens around the same time (early 1940s) using similar justification.
