Gulf of Tonkin Incident
The major landmark in the escalation of U.S. involvement in Vietnam is the “Gulf of Tonkin incident” in August 1964. In the aftermath of this incident, President Lyndon Johnson obtained a Joint Resolution of Congress approving "take all necessary measures to repel any armed attack against the forces of the United States and to prevent further aggression." Retaliatory bombing followed, and in early 1965 Operation Rolling Thunder initiated sustained bombings. Marines landed at Da Nang, and the war quickly escalated.
What happened in the Gulf of Tonkin? On August 2, the destroyer USS Maddox came under machine gun fire during a reconnaissance mission in international waters off the coast of North Vietnam. Two days later, a second attack was reported, and it was in the wake of this event that Johnson sought authorization for retaliatory measures.
The reality of this second attack has long been challenged, and the consensus view now is that no second attack ever occurred. Even Secretary of Defense McNamara, who presented the evidence in to Johnson in the first place, now admits this. In late 2005, the National Security Agency declassified documents which conclusively show that there was no second attack. What remains are lingering questions over whether the false reporting was due to confusion or subterfuge, and if so on whose part.
The U.S. naval vessels were in waters near North Vietnam in support of aggressive intelligence-gathering maneuvers, so that the claims of "unproved aggression" are overstated. It is also interesting to note that these operations were under the aegis of OPLAN 34-A, which itself was approved by the Joint Chiefs of Staff in December 1963, after NSAM 273 of November 26 authorized such activities. Thus there is a direct connection between the decisions reached within days of Kennedy’s murder, and the later escalation of the Vietnam war.
Tonkin Gulf Intelligence "Skewed" According to Official History and Intercepts at National Security Archive.
The Gulf of Tonkin Incident, 40 Years Later at National Security Archive.
Gulf of Tonkin NSA Documents Released on 11/30/05 on National Security Agency website.