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.

The covert association of Visitors who do not agree with the Leader's tactics. Secretly, they work against the cause and often aid the Resistance. Source: Various

a subversive group that supports the enemy and engages in espionage or sabotage; an enemy in your midst

a group of people which clandestinely undermines a larger group to

a group within a country secretly aiding an enemy attacking from without

A group of people living in one country but secretly working or spying for the enemy. Some people thought that there was a Fifth Column in Britain during the Second World War but the government did not agree