There's always more to be said, but in the meantime you may find some answers in this thread about the political/ethical dilemma of US ties to South Vietnam by u/enygma9753.

South Vietnam was formed in the aftermath of the First Indochina War, with various elected civilian and dictatorial military governments. The minority Catholic elites who had held influence under French rule formed the nucleus of South Vietnam's ruling class. They were Western-educated and anti-communist, which made them acceptable to US officials fearful of Chinese or Soviet intervention in southeast Asia.

The U.S. covertly engineered or approved military coups to oust uncooperative leaders during this time. Even under civilian rule, so-called free elections were generally rigged with rampant voter fraud and elected leaders often behaved like dictators. Cronyism and corruption were widespread. Promotions in the army depended on political or familial connections, not merit. The government held large numbers of political prisoners and were increasingly repressive with its majority Buddhist population. In contrast, the Viet Cong tried to win over the countryside with communist propaganda and social programs. They also employed terror tactics, ruthlessly assassinating government officials and district heads seen as collaborating with Saigon.

Although the South Vietnamese army was well-trained by the US, it relied heavily on American air power -- a weakness exposed after 1973 when the US withdrew from the region and left the fighting to the South Vietnamese. On its own, South Vietnam was unable to defend itself from NVA and Viet Cong invasion. Saigon fell to communist forces in April 1975.