Following the Gulf of Tonkin incident in 1964, the US Congress passed a resolution that gave President Lyndon B. Johnson authority to increase military presence without a declaration of war.

Within the US, the war gave rise to Vietnam syndrome, a public aversion to American overseas military involvement,[61] which, with the Watergate scandal, contributed to the crisis of confidence that affected America throughout the 1970s.

[74] In March 1945, Japan, losing the war, overthrew the French government in Indochina, establishing the Empire of Vietnam and installing Vietnamese Emperor Bảo Đại as its figurehead leader.

[25]: 26 [80] In September 1950, the US further enforced the Truman Doctrine by creating a Military Assistance and Advisory Group (MAAG) to screen French requests for aid, advise on strategy, and train Vietnamese soldiers.

[96]: 570–571 US President Eisenhower wrote in 1954: I have never talked or corresponded with a person knowledgeable in Indochinese affairs who did not agree that had elections been held as of the time of the fighting, possibly 80% of the population would have voted for the Communist Ho Chi Minh as their leader rather than Chief of State Bảo Đại.

[97]According to the Pentagon Papers, which commented on Eisenhower's observation, Diệm would have been a more popular candidate than Bảo Đại against Hồ, stating that "It is almost certain that by 1956 the proportion which might have voted for Ho - in a free election against Diem - would have been much smaller than 80%.

[109]: 24 The North Vietnamese Communist Party approved a "people's war" on the South at a session in January 1959,[25]: 119–120  and, in May, Group 559 was established to maintain and upgrade the Ho Chi Minh trail, at this time a six-month mountain trek through Laos.

Although they were intended for use behind front lines after a conventional Soviet invasion of Europe, Kennedy believed guerrilla tactics employed by special forces, such as the Green Berets, would be effective in a "brush fire" war in Vietnam.

On 21 August 1963, the ARVN Special Forces of Colonel Lê Quang Tung, loyal to Diệm's younger brother Ngô Đình Nhu, raided pagodas, causing widespread destruction and leaving a death toll into the hundreds.

Chief among the proposed changes was removal of Diệm's younger brother Nhu, who controlled the secret police and special forces, and was seen as being behind the Buddhist repression and the architect of the Ngô family's rule.

[137]: 78  The resolution granted the president power "to take all necessary measures to repel any armed attack against the forces of the United States and to prevent further aggression" and Johnson relied on this as giving him authority to expand the war.

Massive aerial bombardment against the Pathet Lao and PAVN forces was carried out by the US to prevent the collapse of the Royal central government, and deny use of the Ho Chi Minh Trail.

[78]: 349–351 General William Westmoreland informed Admiral U. S. Grant Sharp Jr., commander of U.S. Pacific forces, that the situation was critical,[78]: 349–351  "I am convinced that U.S. troops with their energy, mobility, and firepower can successfully take the fight to the NLF (Viet Cong)".

[158] Preparations were underway for the Tet Offensive, with the intention of Văn Tiến Dũng forces to launch "direct attacks on the American and puppet nerve centers—Saigon, Huế, Danang, all the cities, towns and main bases ..."[159] Le Duan sought to placate critics of the stalemate by planning a decisive victory.

[173] Westmoreland requested 200,000 additional troops, which was leaked to the media, and the fallout combined with intelligence failures caused him to be removed from command in March 1968, succeeded by his deputy Creighton Abrams.

But revelations of the 1968 My Lai Massacre,[25]: 518–521  in which a US Army unit raped and killed civilians, and the 1969 "Green Beret Affair", where eight Special Forces soldiers, were arrested for the murder[185] of a suspected double agent,[186] provoked national and international outrage.

Racial incidents, drug abuse, combat disobedience, and crime reflected growing idleness, resentment, and frustration ... the fatal handicaps of faulty campaign strategy, incomplete wartime preparation, and the tardy, superficial attempts at Vietnamization.

[A 11] In March 1970, Sihanouk was deposed by his pro-American prime minister Lon Nol, who demanded North Vietnamese troops leave Cambodia or face military action.

[25]: 606–637  The US Navy also initiated Operation Pocket Money in May, an aerial mining campaign in Haiphong Harbor that prevented North Vietnam's allies from resupplying it with weapons and aid by sea.

Lê Đức Thọ and Henry Kissinger, along with the PRG Foreign Minister Nguyễn Thị Bình and a reluctant Thiệu, signed the Paris Peace Accords on 27 January 1973.

[7] In the summer of 1962, Mao Zedong agreed to supply Hanoi with 90,000 rifles and guns free of charge, and starting in 1965, China began sending anti-aircraft units and engineering battalions, to repair the damage caused by American bombing.

The ethnic minority peoples of South Vietnam, like the Montagnards in the Central Highlands, the Hindu and Muslim Cham, and the Buddhist Khmer Krom, were actively recruited in the war.

The Cambodians under pro-China Sihanouk and pro-American Lon Nol, supported their fellow co-ethnic Khmer Krom in South Vietnam, following an anti-ethnic Vietnamese policy.

The tribunal found the US and its allies guilty of acts of aggression, use of weapons forbidden by the laws of war, bombardment of targets of a purely civilian character, mistreatment of prisoners, and genocide.

[240] Such practice, which involved the assumption that anyone appearing in the designated zones was an enemy combatant that could be freely targeted by weapons, was regarded by journalist Lewis Simons as "a severe violation of the laws of war".

[248]: 77  During their visit to the Con Son Prison in 1970, US congressmen Augustus Hawkins and William R. Anderson witnessed detainees either confined in minute "tiger cages" or chained to their cells, and provided with poor-quality food.

[294][295] According to Amnesty International, this figure varied depending on different observers: "... "50,000 to 80,000" (Le Monde, April 1978), "150,000" (Reuters from Bien Hoa, November 1977), "150,000 to 200,000" (The Washington Post, December 1978), and "300,000" (Agence France Presse from Hanoi, February 1978).

[151] Secretary of Defense McNamara wrote to President Johnson his doubts: "The picture of the world's greatest superpower killing or seriously injuring 1,000 noncombatants a week, while trying to pound a tiny backward nation into submission on an issue whose merits are hotly disputed, is not a pretty one.

Further cinematic representations were released during the 1970s and 1980s, the most noteworthy examples being Michael Cimino's The Deer Hunter (1978), Francis Ford Coppola's Apocalypse Now (1979), Oliver Stone's Platoon (1986) and Stanley Kubrick's Full Metal Jacket (1987).

[352]: 374  Michael Allen in Until The Last Man Comes Home accuses Nixon of mythmaking, by exploiting the plight of the National League of POW/MIA Families to allow the government to appear caring, as the war was increasingly considered lost.