The first B-52 Arc Light bombing mission was carried out on 18 June 1965. On this mission, 27 B-52F bombers of the 7th and 320th Bombardment Wings based at Guam were used to attack a Viet Cong jungle redoubt with conventional 750-pound and 1,000-pound bombs.
General William C. Westmoreland, commander of U.S. forces in South Vietnam, was convinced the B-52 could play an effective role in defeating the North, and he called for more bombing missions. From June through December, the 7th, 320th, and 454th Bombardment Wings completed over 100 missions to South Vietnam. These B-52s were used primarily in saturation bombing of Viet Cong base areas, but later they were used in direct tactical support of the Marine Corps' Operation Harvest Moon and the First Cavalry Division's fight in the Ia Drang Valley.
In 1966, the Strategic Air Command (SAC) continued to support military actions in Southeast Asia with B-52 conventional bombing missions. By late June, after one year of participation in the war, the B-52s were dropping approximately 8,000 tons of bombs each month. Missions were flown in all types of weather, night and day. In 1966, over 5,000 B-52 sorties were flown to support operations against the enemy. Although the B-52s were used primarily against targets in South Vietnam, they were also used to bomb the approaches to the Mu Gia Pass in North Vietnam on 12 and 26 April 1966. The objective here was to stop the infiltration of enemy troops who, after leaving the Mu Gia Pass, crossed over into Laos and made their way down the Ho Chi Minh Trail.
Along with the overall growth of U.S. military operations in Southeast Asia, SAC B-52 conventional bombing activity increased tremendously in 1967. During that year, the B-52s flew approximately 9,700 effective bombing sorties, almost twice the number flown in 1966. Most of this bombing was aimed at supporting ground troops who were in close contact with the enemy. A great deal of attention was also devoted to enemy troop concentrations and supply lines in the Ashau Valley.
The defense of Khe Sanh in 1968 developed into the largest and most significant air campaigns to date in Southeast Asia. Around-the-clock strikes were made against enemy forces besieging the base, with SAC bombers accounting for approximately 60,000 tons of bombs being dropped. With fighter-bomber support being limited by the monsoon season, the B-52 was particularly valuable in countering enemy aggression. In conducting this bombing, the B-52 crews relied upon ground-based radar to direct them to their targets, where they destroyed tons of North Vietnamese supplies. These air attacks helped break the siege on Khe Sanh and force the North Vietnamese to withdraw.
In 1969, the B-52 conventional bombing operations in Southeast Asia continued at a steady pace. Greater emphasis was placed on harassment and disruption of enemy operations than in previous years. Potential and actual enemy forces were hampered in South Vietnam, particularly around Saigon. SAC bombers also continued to hit enemy supply dumps, base areas, troop concentrations, and the infiltration network that supplied enemy forces in the south. The number of sorties flown in support of Arc Light bombing operations declined from November 1969 through April 1970.
See the AFHSO publication by Jacob Van Staaveren: Gradual Failure: the Air War Over North Vietnam, 1965-1966.