This term for a group of insurgents, traitors, or spies is a calque from Spanish. It dates to 1936 and the Spanish Civil War. That year, Nationalist general Emilio Mola had surrounded Madrid with four military columns and declared that he had a quinta columna within the besieged city. The quotation was widely reported in British and American newspapers and the term quickly caught on and generalized. From the New York Times of 17 October 1936, in reference to the fighting for Madrid:
Prudence counsels the government to forestall as far as possible the activities of this “fifth column.”
And in a more general sense, from the 21 October 1939 edition of War Illustrated:
This looks to me like the Nazis’ “fifth column” in Belgium ready for the invasion.
(Source: Oxford English Dictionary, 2nd Edition)
Copyright 1997-2015, by David Wilton