Republic of Vietnam
Formally named the Republic of Vietnam (RVN) and commonly called South Vietnam (SVN), this is the political entity created by the Geneva Accords of 1954 that partitioned former French Indochina, and whose existence ended with the forcible occupation of the South by Communist forces of the North.
While the dominant ideology of the South (i.e., the Democratic Republic of Vietnam (DRV) or North Vietnam (NVN) was clearly Communist, of a Stalinist flavor, the Government of Vietnam (GVN) can be characterized as anticommunist. Beyond that generalization, it is hard to have one consistent view of its political system. At various times, its wasbriefly led by a figurehead "Emperor", Bao Dai; led by what can most gently called Confucianist authoritarians such as Ngo Dinh Diem, giving preference to a Catholic minority; and by a series of military governments. Widespread democratic institutions never evolved, and there remained, regardless of the leadership, an endemic problem with government corruption.
Military
Government
The main ground force of the RVN was commonly called the Army of the Republic of Vietnam. There were smaller separate Air Force and Navy branches. Airborne, Ranger, and Marine units served as strategic reserves and "fire brigades", without clear identities as services.
To varying extents, some of the leadership came from roles in the French military, or from groups that resisted the World War II Japanese invasion.
Vietnamese Special Forces did have a conceptual politicomilitary and village-oriented function similar to United States Army Special Forces, but, especially under Diem, they acted as a political police rather than as a nation-building organization. Even so, there were competent individuals that improved specific situations.
Sectarian or private groups
Especially in the fifties and sixties, there were a number of factions that had light forces of their own, including the Cao Dai and Hoa Hao, essentially religious groups. The Binh Xuyen could fairly be described as pirates, but that would enter into tactical alliances.
Montagnards, or tribal groups of the Central Highlands, were often technologically primitive, but with a strong warrior culture. With guidance and leadership, as by United States Army Special Forces, they could be excellent light infantry within missions suited to their skills.
The Nungs, of Chinese origin, were often called "mercenaries", but good and loyal service, when fairly and promptly paid, was a cultural trdition. They did not have the long history with as specific national employer as, for example, the Gurkhas, but should not be confused with the individual opportunists such as the Europeans under Bob Denard or Mike Hoare.
Antigovernment
Some forces, either Communist or communist-dominated, were in the south but clearly opposed to the GVN. When these were formed as opponents to the French or Japanese, they were known as Viet Minh. Some Viet Minh later became regular members of the Northern or Southern militaries.
In the South, other former Viet Minh, as well as younger individuals opposed to the GVN, formed the military wing of the National Front for the Liberation of South Vietnam (better known as the National Liberation Front). These were generally called Viet Cong, although some regarded the term as a derogatory contraction of "Vietnamese Communist".