The Geneva Conference of 1954 ended France's colonial presence in Vietnam and partitioned the country into two states at the 17th parallel pending unification on the basis of internationally supervised free elections. Ngô Ðình Diệm, a former mandarin with a strong Catholic and Confucian background, was selected as Premier of the State of Vietnam by Bảo Đại. While Diệm was trying to settle the differences between the various armed militias in the South, Bảo Ðại was persuaded to reduce his power. Diệm used a referendum in 1955 to depose Bảo Đại and declare himself President of the Republic of Vietnam (South Vietnam). The Republic of Vietnam (RVN) was proclaimed in Saigon on October 26, 1955. The United States began to provide military and economic aid to the RVN, training RVN personnel, and sending U.S. advisors to assist in building the infrastructure for the new government.
Also in 1954, Vietminh forces took over North Vietnam according to the Geneva Accord. Two million North Vietnamese civilians emigrated to South Vietnam to avoid the imminent Communist regime. At the same time, Viet Minh armed forces from South Vietnam were also moving to North Vietnam, as dictated by the Geneva Accord. However, some high ranking Viet Minh cadres secretly remained in the South to follow the local situation closely. The most important figure among those was Lê Duẩn.
The Geneva Accord had promised elections to determine the government for a unified Vietnam. However, as only France and the Viet Minh had signed the document, the United States and Ngô Đình Diệm's government refused to abide by the agreement, fearing that Hồ Chí Minh would win the election due to his war popularity, establishing Communism in the whole of Vietnam. Ngô Đình Diệm took some strong measures to secure South Vietnam from perceived internal threats. He eliminated all private militias from the Bình Xuyên Party and the Cao Đài and Hòa Hảo religious sects. In October 1955, he deposed Bao Dai and proclaimed himself President of the newly established the Republic of Vietnam, after rigging a referendum.[12][13] He repressed any political opposition, arresting the famous writer Nguyễn Tường Tam, who committed suicide while awaiting trial in jail.[14] Diệm also acted aggressively to remove Communist agents still remaining in the South. He formed the Cần Lao Nhân Vị Party, mixing Personalist philosophy with labor rhetorics, modeling its organization after the Communist Party, although it was anti-Communist and pro-Catholic. Another controversial policy was the Strategic Hamlet Program, which aimed to build fortified villages to lock out Communists. However, it was ineffective as many communists were already part of the population and visually indistinguishable. It became unpopular as it limited the villagers' freedom and altered their traditional way of life.
In 1960, at the Third Party Congress of the Vietnamese Communist Party, ostensibly renamed the Labor Party since 1951, Lê Duẩn arrived from the South and strongly advocated the use of revolutionary warfare to topple Diệm's regime, unifying the country, and build Marxist-Leninist socialism. Despite some elements in the Party opposing the use of force, Lê Duẩn won the seat of First Secretary of the Party. As Hồ Chí Minh was aging, Lê Duẩn virtually took the helm of war from him. The first step of his war plan was coordinating a rural uprising in the South (Đồng Khởi) and forming the National Front for the Liberation of South Vietnam (NLF) toward the end of 1960. The figurehead leader of the NLF was Nguyễn Hữu Thọ, a South Vietnamese lawyer, but the true leadership was the Communist Party hierarchy in South Vietnam. Arms, supplies, and troops came from North Vietnam into South Vietnam via a system of trails, named the Ho Chi Minh Trail, that branched into Laos and Cambodia before entering South Vietnam. At first, most foreign aid for North Vietnam came from China, as Lê Duẩn distanced Vietnam from the "revisionist" policy of the Soviet Union under Nikita Khrushchev. However, under Leonid Brezhnev, the Soviet Union picked up the pace of aid and provided North Vietnam with heavy weapons, such as T-54 tanks, artillery, MIG fighter planes, surface-to-air missiles, etc.
Meanwhile, in South Vietnam, although Ngô Đình Diệm personally was respected for his nationalism, he ran a nepotistic and authoritarian regime. Elections were routinely rigged and Diem discriminated in favour of minority Roman Catholics on many issues. His religious policies sparked protests from the Buddhist community after demonstrators were killed on Vesak, Buddha's birthday, in 1963 when they were protesting a ban on the Buddhist flag. This incident sparked mass protests calling for religious equality. The most famous case was of Venerable Thích Quảng Đức, who burned himself to death to protest. The images of this event made worldwide headlines and brought extreme embarrassment for Diem. The tension was not resolved, and on August 21, the ARVN Special Forces loyal to his brother and chief adviser Ngô Đình Nhu and commanded by Le Quang Tung raided Buddhist pagodas across the country, leaving a death toll estimated to range into the hundreds. In the United States, the Kennedy administration became worried that the problems of Diệm's regime were undermining the US's anti-Communist effort in Southeast Asia. On November 1, 1963, confident the US would not intervene or cut off aid as a result, South Vietnamese generals led by Dương Văn Minh engineered a coup d'état and overthrew Ngô Đình Diệm, killing both him and his brother Nhu.
Between 1963 and 1965, South Vietnam was extremely unstable as no government could keep power for long. There were more coups, often more than one every year. The Communist-run NLF expanded their operation and scored some significant military victories. In 1965, US President Lyndon Johnson sent troops to South Vietnam to secure the country and started to bomb North Vietnam, assuming that if South Vietnam fell to the Communists, other countries in the Southeast Asia would follow, in accordance with the Domino Theory. Other US allies, such as Australia, New Zealand, South Korea, Thailand, the Philippines, and Taiwan also sent troops to South Vietnam. Although the American-led troops succeeded in containing the advance of Communist forces, the presence of foreign troops, the widespread bombing over all of Vietnam, and the social vices that mushroomed around US bases upset the sense of national pride among many Vietnamese, North and South, causing many to become sympathetic to North Vietnam and the NLF.
In 1965, Air Marshal Nguyễn Cao Kỳ and General Nguyễn Văn Thiệu took power in a coup, and presided over a stable junta, and promised to hold elections under US pressure. In 1967, South Vietnam managed to conduct a National Assembly and Presidential election with Lt. General Nguyễn Văn Thiệu being elected to the Presidency, bringing the government to some level of stability. However, in 1968, the NLF launched a massive and surprise Tết Offensive (known in South Vietnam as "Biến Cố Tết Mậu Thân" or in the North as "Cuộc Tổng Tấn Công và Nổi Dậy Tết Mậu Thân"), attacking almost all major cities in South Vietnam over the Vietnamese New Year (Tết). NLF and North Vietnamese captured the city of Huế, after which many mass graves were found. Many of the executed victims had relations with the South Vietnamese government or the US (Thảm Sát Tết Mậu Thân). Over the course of the year the NLF forces were pushed out of all cities in South Vietnam and nearly decimated. In subsequent major offensives in later years, North Vietnamese regulars with artillery and tanks took over the fighting. In the months following the Tet Offensive, an American unit massacred civilian villagers, suspected to be sheltering Viet Cong guerillas, in the hamlet of My Lai in Central Vietnam, causing an uproar in protest around the world.
In 1969, Hồ Chí Minh died, leaving wishes that his body be cremated. However, the Communist Party embalmed his body for public display and built the Ho Chi Minh Mausoleum on Ba Đình Square in Hà Nội, in the style of Lenin's Mausoleum in Moscow.
Although the Tết Offensive was a catastrophic military defeat for the Việt Cộng, it was a stunning political victory as it led many Americans to view the war as unwinnable. U.S. President Richard Nixon entered office with a pledge to end the war "with honor." He normalized US relations with China in 1972 and entered into détente with the USSR. Nixon thus forged a new strategy to deal with the Communist Bloc, taking advantage of the rift between China and the Soviet Union. A costly war in Vietnam begun to appear less effective for the cause of Communist containment. Nixon proposed "Vietnamization" of the war, with South Vietnamese troops taking charge of the fighting, yet still receiving American aid and, if necessary, air and naval support. The new strategy started to show some effects: in 1970, troops from the Army of the Republic of Vietnam (ARVN) successfully conducted raids against North Vietnamese bases in Cambodia (Cambodian Campaign); in 1971, the ARVN made an incursion into Southern Laos to cut off the Ho Chi Minh Trail in Operation Lam Son 719, but the operation failed as most high positions captured by ARVN forces were recaptured by North Vietnamese artillery; in 1972, the ARVN successfully held the town of An Lộc against massive attacks from North Vietnamese regulars and recaptured the town of Quảng Trị near the demilitarised zone (DMZ) in the centre of the country during the Easter Offensive.
At the same time, Nixon was pressing both Hanoi and Saigon to sign the Paris Peace Agreement of 1973, for American military forces to withdraw from Vietnam. The pressure on Hanoi materialized with the Christmas Bombings in 1972. In South Vietnam, Nguyễn Văn Thiệu vocally opposed any accord with the Communists, but was threatened with withdrawal of American aid.
Despite the peace treaty, the North continued the war as had been envisioned by Lê Duẩn and the South still tried to recapture lost territories. In the U.S., Nixon resigned after the Watergate scandal. South Vietnam was seen as losing a strong backer. Under U.S. President Gerald Ford, the Democratic-controlled Congress became less willing to provide military support to South Vietnam.
In 1974, South Vietnam also fought and lost the Battle of Hoàng Sa against China over the control of the Paracel Islands in the South China Sea. Neither North Vietnam nor the U.S. interfered.
In early 1975, North Vietnamese military led by General Văn Tiến Dũng launched a massive attack against the Central Highland province of Buôn Mê Thuột. South Vietnamese troops had anticipated attack against the neighboring province of Pleiku, and were caught off guard. President Nguyễn Văn Thiệu ordered the moving of all troops from the Central Highland to the coastal areas, as with shrinking American aid, South Vietnamese forces could not afford to spread too thin. However, due to lack of experience and logistics for such a large troop movement in such a short time, the whole South Vietnamese 2nd Corps got bogged down on narrow mountain roads, flooded with thousands of civilian refugees, and was decimated by ambushes along the way. The South Vietnamese First Corp near the DMZ was cut off, received conflicting orders from Saigon on whether to fight or to retreat, and eventually collapsed. Many civilians tried to flee to Saigon via land, air, and sea routes, suffering massive casualties along the way. In early April 1975, South Vietnam set up a last ditch defense line at Xuân Lộc, under commander Lê Minh Đảo. North Vietnamese troops failed to penetrate the line and had to make a detour, which the South Vietnamese failed to stop due to lack of troops. President Nguyễn văn Thiệu resigned. Power fell to Dương Văn Minh.
Dương Văn Minh had led the coup against Diệm in 1963. By the mid 1970s, he had leaned toward the "Third Party" (Thành Phần Thứ Ba), South Vietnamese elites who favored dialogues and cooperation with the North. Communist infiltrators in the South tried to work out political deals to let Dương Văn Minh ascend to the Presidency, with the hope that he would prevent a last stand, destructive battle for Saigon. Although many South Vietnamese units were ready to defend Saigon, and the ARVN 4th Corp was still intact in the Mekong Delta, Duong Van Minh ordered a surrender on April 30, 1975, sparing Saigon from destruction. Nevertheless, the reputation of the North Vietnamese army towards perceived traitors preceded them, and hundreds of thousands of South Vietnamese fled the country by all means: airplanes, helicopters, ships, fishing boats, and barges. Most were picked up by the U.S. Seventh Fleet in the South China Sea or landed in Thailand. The seaborne refugees came to be known as "boat people". In a famous case, a South Vietnamese pilot, with his wife and children aboard a small Cessna plane, miraculously landed safely without a tailhook on the aircraft carrier USS Midway.
During this period, North Vietnam was a Socialist state with a centralized command economy, an extensive security apparatus to carry out Dictatorship of the Proletariat, a powerful propaganda machine that effectively rallied the people for the Party's causes, a superb intelligence system that infiltrated South Vietnam (spies such as Pham Ngoc Thao climbed to high military government positions), and a severe suppression of political opposition. Even some decorated veterans and famed Communist cadres, such as Trần Đức Thảo, Nguyễn Hữu Đang, Trần Dần, Hoàng Minh Chính, were persecuted during the late 1950s Nhân Văn Giai Phẩm events and the 1960s Trial Against the Anti-Party Revisionists (Vụ Án Xét Lại Chống Đảng) for speaking their opinions. Nevertheless, this iron grip, together with consistent support from the Soviet Union and China, gave North Vietnam a militaristic advantage over South Vietnam. North Vietnamese leadership also had a steely determination to fight, even when facing massive casualties and destruction at their end. The young North Vietnamese were idealistically and innocently patriotic, ready to give the ultimate sacrifice for the "liberation of the South" and the "unification of the motherland".