Fifth Column

fifth column

1. (originally) a group of Falangist sympathizers in Madrid during the Spanish Civil War who were prepared to join the four columns of insurgents marching on the city

2. any group of hostile or subversive infiltrators

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Fifth Column 

a term used to designate a group of General Franco’s agents who operated in the Spanish Republic during the National Revolutionary War of 1936–39.

The term “fifth column” originated in early October 1936, when the Francoist general E. Mola declared on the radio that the rebels were conducting an offensive on Madrid using four columns, while the fifth would strike from the rear at the decisive moment. The fifth column spread panic and engaged in sabotage, espionage, and diversionary activity. During World War II the term was used to designate groups of Nazi agents in various countries who aided the fascist troops in the capture of these countries.