cinquième colonne
“Cinquième colonne” is a French idiom that means “fifth column.” It translates as “traitor, enemy spy organisation, spy, or secret service for a foreign country.”
The idiom dates back to 1936, during the Spanish Civil War. The Nationalists of General Franco announced that the capital of Spain, Madrid, would be attacked by five columns of the enemy. The first four columns were troops coming along the four main roads that led to Madrid. The fifth column was made of enemy forces who were already within Madrid, given the task of helping the attackers.
The idiom comes from the Spanish “quinta columna” which also means “fifth column.” It entered popular usage immediately and is used to refer to enemies within a territory that use techniques such as espionnage, terrorist attacks, propaganda or attempts at destabilisation to undermine the area or authority in which they are.
The current French term is “ennemi intérieur,” or “internal enemy.”
The English equivalent of this saying is “fifth column,” which is a direct translation of the Spanish and the French.