On 2 August, against the background of open warfare in Laos and increasing infiltration across the North/South Vietnamese border, North Vietnamese torpedo boats attacked the destroyer USS Maddox in international waters in the Gulf of Tonkin.  The destroyer was cruising along a patrol line in the northern region of the Gulf in order to gather intelligence as part of Operation Plan 34A.  This was a covert campaign that started in February 1964 and intended to deter the North Vietnamese from interfering with the South.  One of the torpedo boats that attacked the Maddox was sunk by a flight of F-8s led by Cdr James Stockdale of VF-51 from USS Ticonderoga.  During the night of 4/5 August USS Maddox, now reinforced by USS Turner Joy, returned to its station off the North Vietnamese coast to listen for radio traffic and monitor communist naval activity.  Shortly after a covert South Vietnamese attack on a coastal radar station near Cua Ron, the two destroyers tracked what they took to be enemy torpedo boats on their radar.  Debate still rages whether there really was any North Vietnamese boats in the vicinity of the two destroyers.  Apparently no attack developed and no boats were seen by the pilots of the aircraft launched to provide air cover.  However, the incident was enough to goad President Johnson into ordering Operation Pierce Arrow, a limited retaliatory raid on military facilities in North Vietnam.  On 10 August the US Congress passed what came to be known as the Gulf of Tonkin Resolution which was as close as the US ever came to declaring war on North Vietnam but which actually fell far short of that.  The Gulf of Tonkin Incident also resulted in a major increase in US air strength in the Southeast Asian theatre and saw US involvement change from an advisory role to a more operational role, even though US aircraft and airmen had been participating in operations ever since they first arrived in the region. 

The political and physical restrictions on the basing of US aircraft in South Vietnam was to some extent solved by the permanent stationing of aircraft carriers in the South China Sea.  By the end of August four aircraft carriers, the USS Bon Homme Richard, Constellation, Kearsarge and Ticonderoga, had arrived in position in the Gulf and started a pattern of line duty that continued until August 1973.  The carriers and their protecting forces constituted the US 7th Fleet’s Task Force 77 which in March developed a pattern of positioning carriers at Yankee Station in the South China Sea off Da Nang from which to launch attacks against North Vietnam.  On 20 May TF 77 established Dixie Station 100 miles southeast of Cam Ranh Bay from where close air support missions could be mounted against South Vietnam.  The carriers developed a system that normally kept each ship on line duty for a period of between 25 and 35 days after which the carrier would visit a port in either the Philippines, Japan or Hong Kong for rest and replenishment of supplies.  Each carrier would normally complete four spells of duty on the line before returning to its homeport for refitting and re-equipping.  However, the period spent on line duty could vary considerably and some ships spent well over the average number of days on duty.  The establishment of Dixie Station required the assignment of a fifth carrier to the Western Pacific to maintain the constant presence of at least two carriers at Yankee Station and one at Dixie Station.  By the summer of 1966 there were enough aircraft based in South Vietnam to provide the required air power and Dixie Station was discontinued from 4 August.

The USAF also increased its forces in Southeast Asia following the Gulf of Tonkin Incident.  Immediate reinforcements consisted of six F-102 interceptors (from the 509th FIS at Clark) to Da Nang and a further six (from the 16th FIS at Naha AB, Okinawa) to Tan Son Nhut; eight F-100s (from the 615th TFS temporarily deployed to Clark) to Da Nang and another 10 (from the 405th FW at Clark) to Takhli; 36 B-57Bs (also from the 405th) to Bien Hoa, six RF-101Cs to Tan Son Nhut; eight KB-50s (421st ARS) split equally between Tan Son Nhut and Takhli; and eight F-105Ds (36th TFS) to Korat.  Other aircraft, including a large number of F-100s, deployed to Clark to form a composite task force that would move to South Vietnam or Thailand as required.