Fifth column.

“Fifth column” is commonly traced to the opening months of the Spanish Civil War (1936). As Nationalist forces advanced on Madrid with four military columns, the term arose to describe clandestine supporters inside the city who would aid the attackers through sabotage, intelligence, and demoralization—an internal force supplementing the external assault. The phrase quickly became a widely used label for covert subversion and internal collaborators in wartime and politics, spreading internationally in the late 1930s. Although often attributed to Nationalist general Emilio Mola, the precise wording and the exact occasion of his alleged remark are difficult to document in a definitive primary source.

The expression condenses a strategic idea: an enemy can be weakened as much from within as from direct military pressure. A “fifth column” implies hidden organization, ideological sympathy, or opportunism operating inside a targeted society—spying, spreading rumors, undermining morale, and preparing the way for takeover. The phrase’s enduring power lies in its metaphorical clarity: it turns internal dissent or covert collaboration into a military unit, making subversion seem coordinated and imminent. Because of that, it has also been used polemically, sometimes to stigmatize political opponents as traitors rather than to describe verifiable clandestine activity.