The answer is Vietcong.
The Vietcong was a military group who were supported by the communist North Vietnamese government during the Vietnam War. This group used guerilla warfare. Guerilla warfare is a type of military fighting in which small groups of individuals ambush, raid, and sabotage their enemy. In this case, the Vietcong was very difficult to identify, as many of them were not dressed in military uniforms. Rather, they looked like normal civilians, making it difficult for the US and Southern Vietnamese army to differentiate between who is part of the Vietcong and who is a citizen.
