Ngo Dinh Diem in the context of "United States in the Vietnam War"

The Vietnam War (1 November 1955 – 30 April 1975) was an armed conflict in Vietnam, Laos, and Cambodia fought between North Vietnam (Democratic Republic of Vietnam) and South Vietnam (Republic of Vietnam) and their allies. North Vietnam was supported by the Soviet Union and China, while South Vietnam was supported by the United States and other anti-communist nations. The conflict was the second of the Indochina wars and a proxy war of the Cold War between the Soviet Union and US. The Vietnam War was one of the postcolonial wars of national liberation, a theater in the Cold War, and a civil war, with civil warfare a defining feature from the outset. Direct US military involvement escalated from 1965 until its withdrawal in 1973. The fighting spilled into the Laotian and Cambodian Civil Wars, which ended with all three countries becoming communist in 1975.

After the defeat of the French Union in the First Indochina War that began in 1946, Vietnam gained independence in the 1954 Geneva Conference but was divided in two at the 17th parallel: the Viet Minh, led by Ho Chi Minh, took control of North Vietnam, while the US assumed financial and military support for South Vietnam, led by Ngo Dinh Diem. The North Vietnamese supplied and directed the Viet Cong (VC), a common front of dissidents in the south which intensified a guerrilla war from 1957. In 1958, North Vietnam invaded Laos, establishing the Ho Chi Minh trail to supply the VC. By 1963, the north had covertly sent 40,000 soldiers of its People's Army of Vietnam (PAVN), armed with Soviet and Chinese weapons, to fight in the insurgency in the south. President John F. Kennedy increased US involvement from 900 military advisors in 1960 to 16,000 in 1963 and sent more aid to the Army of the Republic of Vietnam (ARVN), which failed to produce results. In 1963, Diem was killed in a US-backed military coup, which added to the south's instability.

South Vietnam, officially the Republic of Vietnam (RVN; Vietnamese: Việt Nam Cộng hòa, VNCH), was a country in Southeast Asia that existed from 1955 to 1975. It first garnered international recognition in 1949 as the associated State of Vietnam within the French Union, with its capital at Saigon. Since 1950, it was a member of the Western Bloc during the Cold War. Following the 1954 partition of Vietnam, it became known as South Vietnam and was established as a republic in 1955. Its sovereignty was recognized by the United States and 87 other nations, though it failed to gain admission into the United Nations as a result of a Soviet veto in 1957. It was succeeded by the Republic of South Vietnam in 1975. In 1976, the Republic of South Vietnam and North Vietnam merged to form the Socialist Republic of Vietnam.

The aftermath of World War II saw the communist-led Viet Minh, under Ho Chi Minh, seize power and proclaim the Democratic Republic of Vietnam (DRV) in Hanoi in September 1945, initiating civil conflicts. In 1949, during the First Indochina War, the French and anti-communist nationalists established the State of Vietnam (SVN), led by former emperor Bảo Đại. Returning from exile in June 1954, Ngo Dinh Diem, recognized as the prominent anti-communist and anti-colonialist figure, was appointed prime minister of the SVN.

On November 1, 1963, President Ngô Đình Diệm and the Personalist Labor Revolutionary Party of the Republic of Vietnam (South Vietnam) were deposed by a group of CIA-backed Army of the Republic of Vietnam officers who disagreed with Diệm's handling of the Buddhist crisis and the North Vietnamese and Viet Cong (VC) threat to South Vietnam. During South Vietnam's later years, some referred to the coup as Cách mạng 1-11-1963 (1st November 1963 Revolution).

The Kennedy administration had been aware of the coup planning, but Cable 243 from the United States Department of State to U.S. Ambassador to South Vietnam Henry Cabot Lodge Jr. stated that it was U.S. policy not to try to stop it. Lucien Conein, the Central Intelligence Agency's liaison between the U.S. Embassy and the coup planners, told them that the U.S. would not intervene to stop it. Conein also provided 3 million Vietnamese piastres (approx. $42,000 USD) to reward military units that joined the coup.The decision to support the coup stemmed from the Kennedy administration's growing conviction that Diem was incapable of uniting the South Vietnamese people against communism. Diem's government was also increasingly seen as repressive, particularly after his violent crackdown on Buddhist protests.