fifth column

n. [ from a statement during the Spanish Civil War (1936) that the Falange had four columns of soldiers marching on the city, and a fifth column “already there” (i.e. sympathizers inside the Republican lines). ] 1. a group of persons inside the battle lines of a territory engaged in a conflict, who secretly sympathize with the enemy, and who engage in espionage or sabotage; -- sometimes also referred to as a trojan horse. [ RP ]

2. Hence, any faction of persons within a group who secretly sympathize with an enemy, especially those who engage in activities harmful to the group; an enemy in one's midst; a group of traitors. [ RP ]