A fifth column is a group of people who clandestinely undermine a larger group such as a nation from within.
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).[1] 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.[2]
In fact, this supposed "fifth column" did not prove very effective, as demonstrated 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 Nazis.
