Nguyn Vn Thiu (Vietnamese: [n vn tw] (listen); 5 April 1923 29 September 2001) was a Vietnamese military officer and politician who was the president of South Vietnam from 1967 to 1975. He was a general in the Army of the Republic of Vietnam (ARVN), became head of a military junta in 1965, and then president after winning an election in 1967. He established rule over South Vietnam until he resigned and left the nation and relocated to Taipei, Taiwan a few days before the fall of Saigon and the ultimate North Vietnamese victory.

Born in Phan Rang in the south central coast of Vietnam, Thieu joined the communist-dominated Vit Minh of H Ch Minh in 1945 but quit after a year and joined the Vietnamese National Army (VNA) of the French-backed State of Vietnam. He gradually rose up the ranks and, in 1954, led a battalion in expelling the communists from his native village. Following the withdrawal of France, the VNA became the ARVN and Thiu was the head of the Vietnamese National Military Academy for four years before becoming a division commander and colonel. In November 1960, he helped put down a coup attempt against President Ng nh Dim. During this time, he also converted to Roman Catholicism and joined the regime's secret Cn Lao Party; Dim was thought to give preferential treatment to his co-religionists and Thiu was accused of being one of many who converted for political advancement.

Despite this, Thiu agreed to join the coup against Ng nh Dim in November 1963 in the midst of the Buddhist crisis, leading the siege on Gia Long Palace. Dim was captured and executed and Thiu made a general. Following Dim's death, there were several short-lived juntas as coups occurred frequently. Thiu gradually moved up the ranks of the junta by adopting a cautious approach while other officers around him defeated and sidelined one another. In 1965, stability came to South Vietnam when he became the figurehead head of state, while Air Marshal Nguyn Cao K became prime minister, although the men were rivals. In 1967, a transition to elected government was scheduled; and, after a power struggle within the military, Thiu ran for the presidency with K as his running mateboth men had wanted the top job. To allow the two to work together, their fellow officers had agreed to have a military body controlled by K shape policy behind the scenes. Leadership tensions became evident, and Thiu prevailed, sidelining K supporters from key military and cabinet posts. Thiu then passed legislation to restrict candidacy eligibility for the 1971 election, banning almost all would-be opponents, while the rest withdrew as it was obvious that the poll would be a sham; Thiu won more than 90 percent of the vote and the election was uncontested, while K retired from politics.

During his rule, Thiu was accused of turning a blind eye to and indulging in corruption, and appointing loyalists rather than competent officers to lead ARVN units. During the 1971 Operation Lam Sn 719 and the communists' Easter Offensive, the I Corps in the north of the country was under the command of his confidant, Hong Xun Lm, whose incompetence led to heavy defeats until Thiu finally replaced him with Ng Quang Trng. After the signing of the Paris Peace Accordswhich Thiu opposedand the US withdrawal, South Vietnam resisted the communists for another two years until the communists' final push for victory, which saw the South openly invaded by the entire North Vietnamese army. Thiu gave contradictory orders to Trng to stand and fight or withdraw and consolidate, leading to mass panic and collapse in the south of the country. This allowed the communists to generate much momentum and within a month they were close to Saigon, prompting Thiu to resign and leave the country. He eventually settled near Boston, Massachusetts, preferring not to talk to the media. He died in 2001.

The Vietnam War (Vietnamese: Chiến tranh Việt Nam), also known as the Second Indochina War, was a conflict in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. It was the second of the Indochina Wars and was officially fought between North Vietnam and South Vietnam. North Vietnam was supported by the Soviet Union, China, and other communist allies; South Vietnam was supported by the United States and other anti-communist allies. The war is widely considered to be a Cold War-era proxy war. It lasted almost 20 years, with direct U.S. involvement ending in 1973. The conflict also spilled over into neighboring states, exacerbating the Laotian Civil War and the Cambodian Civil War, which ended with all three countries becoming communist states by 1975.

The conflict emerged from the First Indochina War between the French colonial government and a left-wing revolutionary movement, the Viet Minh. After the French military withdrawal from Indochina in 1954, the U.S. assumed financial and military support for the South Vietnamese state. The Việt Cộng (VC), a South Vietnamese common front under the direction of North Vietnam, initiated a guerrilla war in the south. North Vietnam had also invaded Laos in 1958 in support of insurgents, establishing the Ho Chi Minh Trail to supply and reinforce the Việt Cộng.: 16  By 1963, the North Vietnamese had sent 40,000 soldiers to fight in the south.: 16  U.S. involvement escalated under President John F. Kennedy through the MAAG program, from just under a thousand military advisors in 1959 to 23,000 in 1964.: 131 In the Gulf of Tonkin incident in August 1964, a U.S. destroyer clashed with North Vietnamese fast attack craft. In response, the U.S. Congress passed the Gulf of Tonkin Resolution and gave President Lyndon B. Johnson broad authority to increase U.S. military presence in Vietnam. Johnson ordered the deployment of combat units for the first time and increased troop levels to 184,000. The People's Army of Vietnam (PAVN) (also known as the North Vietnamese Army or NVA) engaged in more conventional warfare with U.S. and South Vietnamese forces (Army of the Republic of Vietnam (ARVN)). Despite little progress, the U.S. continued a significant build-up of forces. U.S. and South Vietnam forces relied on air superiority and overwhelming firepower to conduct search and destroy operations, involving ground forces, artillery, and airstrikes. The U.S. also conducted a large-scale strategic bombing campaign against North Vietnam.: 371–4 The communist Tet Offensive throughout 1968 caused U.S. domestic support for the war to fade. The VC sustained heavy losses during the Offensive and subsequent U.S.-ARVN operations.: 481  The CIA's Phoenix Program further degraded the VC's membership and capabilities. By the end of the year, the VC insurgents held almost no territory in South Vietnam, and their recruitment dropped by over 80%, signifying a drastic reduction in guerrilla operations, necessitating increased use of PAVN regular soldiers from the north. In 1969, North Vietnam declared a Provisional Revolutionary Government (the PRG) in the south to give the reduced VC a more international stature, but from then on, they were sidelined as PAVN forces began more conventional combined arms warfare. By 1970, over 70% of communist troops in the south were northerners, and southern-dominated VC units no longer existed. Operations crossed national borders: North Vietnam used Laos as a supply route early on, while Cambodia was also used starting in 1967; the U.S. bombed the Laotian route starting in 1964, and the Cambodian route in 1969. The deposing of the monarch Norodom Sihanouk by the Cambodian National Assembly resulted in a PAVN invasion of the country at the request of the Khmer Rouge, escalating the Cambodian Civil War and resulting in a U.S.-ARVN counter-invasion.

In 1969, following the election of U.S. President Richard Nixon, a policy of "Vietnamization" began, which saw the conflict fought by an expanded ARVN, with U.S. forces sidelined and increasingly demoralized by domestic opposition and reduced recruitment. U.S. ground forces had largely withdrawn by early 1972 and support was limited to air support, artillery support, advisers, and materiel shipments. The ARVN, with U.S. support, stopped the first and largest mechanized PAVN offensive during the Easter Offensive of 1972. The offensive failed to subdue South Vietnam, but the ARVN itself failed to recapture all lost territory, leaving its military situation difficult. The Paris Peace Accords of January 1973 saw all U.S. forces withdrawn; the Case–Church Amendment, passed by the U.S. Congress on 15 August 1973, officially ended direct U.S. military involvement.: 457  The Peace Accords were broken almost immediately, and fighting continued for two more years. Phnom Penh fell to the Khmer Rouge on 17 April 1975, while the 1975 Spring Offensive saw the Fall of Saigon by the PAVN on 30 April; this marked the end of the war, and North and South Vietnam were reunified the following year.

By 1970, the ARVN was the world's fourth largest army, and the PAVN was not far behind with approximately one million regular soldiers.: 770  The war exacted an enormous human cost: estimates of the number of Vietnamese soldiers and civilians killed range from 966,000 to 3 million. Some 275,000–310,000 Cambodians, 20,000–62,000 Laotians, and 58,220 U.S. service members also died in the conflict, and a further 1,626 remain missing in action.The Sino-Soviet split re-emerged following the lull during the Vietnam War. Conflict between North Vietnam and its Cambodian allies in the Royal Government of the National Union of Kampuchea, and the newly formed Democratic Kampuchea began almost immediately in a series of border raids by the Khmer Rouge, eventually escalating into the Cambodian–Vietnamese War. Chinese forces directly invaded Vietnam in the Sino-Vietnamese War, with subsequent border conflicts lasting until 1991. The unified Vietnam fought insurgencies in all three countries. The end of the war and resumption of the Third Indochina War would precipitate the Vietnamese boat people and the larger Indochina refugee crisis, which saw millions of refugees leave Indochina (mainly southern Vietnam), an estimated 250,000 of whom perished at sea. Within the U.S., the war gave rise to what was referred to as Vietnam Syndrome, a public aversion to American overseas military involvements, which together with the Watergate scandal contributed to the crisis of confidence that affected America throughout the 1970s.