Viet cong
The Viet Cong was an insurgency in the South of Vietnam that fought the established anti-communist government, to unite the two Vietnams into one communist state.
It had a regular army but submerged into the civil population and used guerilla tactics to stage attacks. The Viet Cong relied on their own tactics, including ambushes, raids, sniping, tunnel warfare as well as their tools, such as land mines and elaborate but deadly booby traps.
North Vietnam trained and provided military aid to the Viet Cong and sent them back to the South on the famous Ho Chi Minh trail that lead through Laos and Cambodia.