This is the name of the north vietnamese group that was both official army and guerrilla group during the era of the vietnam war.

Viet Cong

Explanation:

The Viet Cong was a military unit that had both regular army forces and a guerrilla group. They were from the Southern part of Vietnam, but where fighting together with the North, against the US and the South. They were a massive organization, and were very successful on the ground, managing to win large portions of territory and control it, winning the majority of the battles. The US officials were portraying them as communists, though they were not initially, but were just fighting against the South and its policies, and against the US interference. Eventually they were under the command of the North, and started to be active participants in the setting up of the communism in the country.