The Gulf of Tonkin incident (Vietnamese: Sự kiện Vịnh Bắc Bộ) refers to a naval confrontation in the Gulf of Tonkin off the coast of North Vietnam, which led to the United States engaging more directly in the Vietnam War. On 2 August 1964 there was a clash between a destroyer of the United States Navy that was collecting signals intelligence close to North Vietnamese waters, and three North Vietnamese naval vessels. On the night of 4 August, two US destroyers reported they were attacked by North Vietnamese vessels and that they were returning fire. Later investigation revealed that the 4 August attack did not happen; no North Vietnamese vessels had been present. Shortly after the events, the National Security Agency, an agency of the US Defense Department, deliberately skewed intelligence to create the impression that an attack had been carried out.

On the night of 30 July, South Vietnamese commandos raided a North Vietnamese radar station on the island of Hòn Mê in the Gulf of Tonkin. The next day the destroyer USS Maddox commanded by Commander Herbert L. Ogier began patrolling near the North Vietnamese coast. On 2 August, the Maddox, while performing a signals intelligence patrol as part of DESOTO operations, near one of the islands that had been shelled two nights before, was approached by three North Vietnamese Navy torpedo boats of the 135th Torpedo Squadron. Maddox fired warning shots and the North Vietnamese boats attacked with torpedoes and machine gun fire. In the ensuing engagement, one US aircraft (which had been launched from aircraft carrier USS Ticonderoga) was damaged, three North Vietnamese torpedo boats were damaged, and four North Vietnamese sailors were killed, with six more wounded. There were no US casualties. Maddox was "unscathed except for a single bullet hole from a [North] Vietnamese machine gun round".

On 3 August, destroyer USS Turner Joy joined Maddox and the two destroyers continued the DESOTO mission. On the evening of 4 August, the ships opened fire on radar returns that had been preceded by communications intercepts, which US forces claimed meant an attack was imminent. The commodore of the Maddox task force, Captain John Herrick, reported that the ships were being attacked by North Vietnamese boats when, in fact, there were no North Vietnamese boats in the area. While Herrick soon reported doubts regarding the task force's initial perceptions of the attack, the Johnson administration relied on the wrongly interpreted National Security Agency communications intercepts to conclude that the attack was real.

While doubts regarding the perceived second attack have been expressed since 1964, it was not until years later that it was shown conclusively never to have happened. In the 2003 documentary The Fog of War, the former United States secretary of defense, Robert S. McNamara, admitted that there was no attack on 4 August. In 1995, McNamara met with former North Vietnamese Army General Võ Nguyên Giáp to ask what happened on 4 August 1964. "Absolutely nothing", Giáp replied. Giáp confirmed that the attack had been imaginary. In 2005, an internal National Security Agency historical study was declassified; it concluded that Maddox had engaged the North Vietnamese Navy on 2 August, but that the incident of 4 August was based on bad naval intelligence and misrepresentations of North Vietnamese communications. The official US government claim is that it was based mostly on erroneously interpreted communications intercepts.

The outcome of the incident was the passage by US Congress of the Gulf of Tonkin Resolution, which granted US president Lyndon B. Johnson the authority to assist any Southeast Asian country whose government was considered to be jeopardized by communist aggression. The resolution served as Johnson's legal justification for deploying US conventional forces to South Vietnam and the commencement of open warfare against North Vietnam in early 1965.

The Geneva Conference in 1954 was intended to settle outstanding issues following the end of hostilities between France and the Viet Minh at the end of the First Indochina War. Neither the United States nor the State of Vietnam signed anything at the 1954 Geneva Conference. The accords, which were signed by other participants including the Viet Minh, mandated a temporary ceasefire line, which separated southern and northern Vietnam to be governed by the State of Vietnam and the Viet Minh respectively. The accords called for a general election by July 1956 to create a unified Vietnamese state. The accords allowed free movement of the population between the north and south for three hundred days. They also forbade the political interference of other countries in the area, the creation of new governments without the stipulated elections, and foreign military presence. By 1961, South Vietnamese President Ngo Dinh Diem faced significant discontent among some quarters of the southern population, including some Buddhists who were opposed to the rule of Diem's Catholic supporters. Viet Minh political cadres, who were legally campaigning for the promised elections between 1955 and 1957, were suppressed by the government. In March 1956, the North Vietnamese leadership approved tentative measures to revive the southern insurgency in December 1956. A communist-led uprising began against Diem's government in April 1957. The North Vietnamese Communist Party approved a "people's war" on the South at a session in January 1959, and on 28 July, North Vietnamese forces invaded Laos to maintain and upgrade the Ho Chi Minh trail, in support of insurgents in the south. The rebellion, headed by the National Front for the Liberation of South Vietnam (NLF, or Viet Cong) under the direction of North Vietnam, had intensified by 1961. About 40,000 communist soldiers infiltrated the south from 1961 to 1963.

The Gulf of Tonkin Incident occurred during the first year of the Johnson administration. While U.S. President John F. Kennedy had originally supported the policy of sending military advisers to Diem, he had begun to alter his thinking by September 1963, because of what he perceived to be the ineptitude of the Saigon government and its inability and unwillingness to make needed reforms (which led to a U.S.-supported coup which resulted in the death of Diem). Shortly before Kennedy was assassinated in November 1963, he had begun a limited withdrawal of 1,000 U.S. forces before the end of 1963. Johnson's views were likewise complex, but he had supported military escalation as a means of challenging what was perceived to be the Soviet Union's expansionist policies. The Cold War policy of containment was to be applied to prevent the fall of Southeast Asia to communism under the precepts of the domino theory. After Kennedy's assassination, Johnson ordered in more U.S. forces to support the Saigon government, beginning a protracted United States presence in Southeast Asia.

A highly classified program of covert actions against North Vietnam, known as Operation Plan 34-Alpha, in conjunction with the DESOTO operations, had begun under the Central Intelligence Agency (CIA) in 1961. In 1964, the program was transferred to the Defense Department and conducted by the Military Assistance Command, Vietnam Studies and Observations Group (MACV-SOG). For the maritime portion of the covert operation, a set of fast patrol boats had been purchased quietly from Norway and sent to South Vietnam. In 1963, three young Norwegian skippers traveled on a mission in South Vietnam. They were recruited for the job by the Norwegian intelligence officer Alf Martens Meyer. Martens Meyer, who was head of department at the military intelligence staff, operated on behalf of U.S. intelligence. The three skippers did not know who Meyer really was when they agreed to a job that involved them in sabotage missions against North Vietnam.