The Vietcong fought against both the government of South Vietnam and the army of the United States between 1959 and 1975. Initially a loose grouping of guerrilla fighters, the Vietcong increased markedly strategy and in numbers over the course of the conflict.
The NLF grew quite strong; by 1968, it was able to carry out operations in the Tet Offensive. Unfortunately, it was almost destroyed by the US military in the offensive, and had to be remade.
http://www.britannica.com/EBchecked/topic/628305/Viet-Cong-VC