fifth column noun

A clandestine subversive organization working within a country to further an invading enemy's military and political aims.

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.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.