Final answer:
The primary ally of South Vietnam during the Vietnam War was the United States, which became increasingly involved in the conflict to prevent the spread of communism.
Step-by-step explanation:
The ally of South Vietnam during the Vietnam War was predominantly the United States. Initially, the United States supported South Vietnam to counter the spread of communism in Southeast Asia and to back up French interests. As time passed, the involvement escalated significantly. By the 1960s, the conflict had grown into a major point of contention in US domestic politics, especially during President Lyndon Johnson's administration, which saw the deployment of hundreds of thousands of American troops. Despite vast military aid, the South Vietnamese government was often seen as inefficient and corrupt by US soldiers.
The Vietnam War was a protracted conflict facing the challenge of guerilla warfare, mainly against the Viet Cong and supported by North Vietnam, and in broader terms, a proxy war involving the Soviet Union and China. The United States' commitment to South Vietnam continued until 1973 when American forces withdrew from the conflict. Following the withdrawal of US support, South Vietnam eventually fell, leading to the reunification of Vietnam in 1975.