As a result of the Viet Cong escalation, U.S. President Lyndon B. Johnson authorized Operation Rolling Thunder, a limited-scale bombing of North Vietnam, which began on March 2, 1965, with the stated aims of boosting South Vietnamese morale, deterring North Vietnam from supporting the Viet Cong/NLF, and stopping North Vietnamese forces from entering South Vietnam. Initially planned to last only 8 weeks, the bombing campaign became an incremental, sustained effort that lasted 44 months, ending in November 1968.
(Taken from Vietnam War – Wars of the 20th Century – Twenty Wars in Asia – Vol. 5)
Under Operation Rolling Thunder, President Johnson required that the U.S. military’s list of potential targets be subject to his approval, which generated great consternation among the generals who wanted an all-out, large-scale strategic bombing campaign of North Vietnam. U.S. planes also were only allowed to hit targets (such as road and rail systems, industries, and air defenses) inside a designated radius away from Hanoi and Haiphong, as well as from a buffer zone from the North Vietnam-China border. Some of these restrictions would be lifted later.
The incremental nature of Operation Rolling Thunder allowed North Vietnam enough time to strengthen its air defenses. Thus, by 1968, Hanoi, Haiphong, and other vital centers were bristling with 8,000 Soviet-supplied anti-aircraft guns and 300 surface-to-air missile batteries, supported by 350 radar facilities, as well as scores of Soviet MiG-21 fighter planes and 15,000 Soviet air-defense advisers. In February 1965, the Soviet Union further increased its military support to North Vietnam when an American bombing attack coincided with the visit of Soviet Deputy Premier Alexei Kosygin to Hanoi. Previously, the Soviet government had sought a diplomatic resolution to the Vietnam War (despite providing military support to North Vietnam). Ultimately, by the end of Rolling Thunder, the United States lost over 900 planes, while North Vietnam continued to deliver even larger amounts of weapons to South Vietnam through the Ho Chi Minh Trail.
Throughout the war, the United States launched other aerial operations (Steel Tiger, Tiger Hound, and Commando Hunt) on the Ho Chi Minh Trail to try and stop the flow of men and materiel from North Vietnam to South Vietnam, but all of these ultimately proved unsuccessful.
Over the course of the war, the Ho Chi Minh Trail system expanded considerably into an elaborate network of small and wide roads, foot and bike paths, and concealed river crossings across a vast and ever-increasing area in the eastern regions of Laos and Cambodia. With 43,000 North Vietnamese and Laotian laborers, dozens of bulldozers, road graders, and other road-building equipment working day and night, by December 1961, the Trail system allowed for truck traffic, which became the main source of transporting men and supplies for the rest of the war. Apart from construction crews, other units in the Ho Chi Mnh Trail were tasked with providing food, housing, and medical care, and other services to soldiers and transport crews moving along the system. To counter U.S. air attacks, which intensified as the war progressed, the Trail system was massively fortified with air defenses, eventually bristling with 1,500 anti-aircraft guns. Supply convoys also traveled only at night to lessen the risk of U.S. air attacks.
But because of the U.S. air campaign, American bases came under greater threat of Viet Cong retaliatory attacks. Thus, in March 1965, on President Johnson’s orders, 3,500 U.S. Marines arrived to protect Da Nang air base. These Marines were the first U.S. combat troops to be deployed in Vietnam. Then in April 1965, when the U.S. government’s offer of economic aid to North Vietnam in exchange for a peace agreement was rejected by the Hanoi government, President Johnson soon sent more U.S. ground forces, raising the total U.S. personnel strength in Vietnam to 60,000 troops. At this point, U.S. forces were authorized only to defend American military installations.
Then in May 1965, in a major effort to overthrow South Vietnam, Viet Cong and North Vietnamese forces launched attacks in three major areas: just south of the DMZ, in the Central Highlands, and in areas around Saigon. U.S. and South Vietnamese forces repulsed these attacks, with massive U.S. air firepower being particularly effective, and in mid-1965, Viet Cong and North Vietnamese forces retreated, and the danger to the Saigon government passed. By that time also, President Johnson agreed to the U.S. military’s request and sent more troops to Vietnam, raising the total to 184,000 by the end of 1965. More crucially, he now authorized U.S. forces to not merely defend U.S. facilities, but to undertake offensive combat missions, in line with American military doctrine to take the war to the enemy.