One of the major events that historians use to date the beginning of the U.S. involvement in the war in Vietnam is the Gulf of Tonkin Resolution, signed on 10 August 1964. This resolution was proposed by President Johnson on 5 August 1964 after two North Vietnamese attacks on U.S. Navy ships, one on 2 August 1964 and the other two days later on 4 August 1964 (see Map 7). While the first encounter was well-documented, the North Vietnamese denied the second attack ever took place. In 2005, a National Security Agency study from the time was declassified that cast doubt on the second attack. But, coincidentally, several declassified documents held at the Lyndon Baines Johnson Presidential Library in Austin, Texas, now appear to show that the second attack really did happen.

American participation in the Vietnam War began in early August 1964, when the Congress approved a Southeast Asia Resolution by a vote of 88 to 2 in the Senate and 416 to 0 in the House of Representatives. On 10 August 1964, President Johnson signed into law the Gulf of Tonkin Resolution. By this action, Congress “approves and supports the determination of the President, as Commander in Chief, to take all necessary measures to repel any armed attack against the forces of the United States and to prevent further aggression.” Critics of this resolution have since argued that while the first attack on 2 August 1964 was undoubtedly valid, the second attack on 4 August was not, which makes the resolution's wording suspect: “Whereas naval units of the Communist regime in Vietnam, in violation of the principles of the United Nations and of international law, have deliberately and repeatedly attacked United States naval vessels lawfully present in international waters […]”

The first attack occurred on 2 August 1964 when three North Vietnamese torpedo boats attacked the USS Maddox. This U.S. Navy destroyer was cruising in the Gulf of Tonkin in waters claimed by North Vietnam as within its 12-mile line of control, a claim not recognized by the U.S. government at the time which only recognized 3 nautical miles. After returning fire and expending six torpedoes of its own, all misses, Maddox left the engagement undamaged.