Hanoi's response to the fall of the Diem regime was a subject of intense debate at the Ninth Plenum of the VWP Central Committee held in December 1963. It appeared that the new administration of President Lyndon B. Johnson (who assumed office following the assassination of President Kennedy on November 22) was not planning to withdraw from Vietnam but, rather, to increase its support for the new Saigon government. The VWP leadership concluded that only armed struggle would lead to success and called for an escalation of the war. The critical issues then became the reactions of the United States and the Soviet Union. Hanoi clearly hoped that the United States would opt for a compromise solution, as it had in Korea and Laos, and the party leaders believed that a quick and forceful escalation of the war would induce it to do so. Hanoi's decision to escalate the struggle was made in spite of the risk of damage to its relations with Moscow, which opposed the decision. The new policy also became an issue in the developing rift between Beijing and Moscow because China expressed its full support for the Vietnamese war of national liberation. As a result, Moscow's aid began to decrease as Beijing's grew.

Escalation of the war resulted in some immediate success for the struggle in the South. By 1964 a liberated zone had been established from the Central Highlands to the edge of the Mekong Delta, giving the communists control over more than half the total land area and about half the population of the South. PLAF forces totaled between 30 and 40 battalions, including 35,000 guerrillas and 80,000 irregulars. Moreover, with the completion of the so-called Ho Chi Minh Trail (see Glossary) through Laos, the number of PAVN troops infiltrated into the South began to increase. ARVN control was limited mainly to the cities and surrounding areas, and in 1964 and 1965 Saigon governments fell repeatedly in a series of military and civilian coups.

The Johnson administration remained hesitant to raise the American commitment to Vietnam. However, in August 1964, following the reputed shelling of United States warships in the Gulf of Tonkin off the North Vietnamese coast, Johnson approved air strikes against North Vietnamese naval bases. At President Johnson's urgent request, the United States Congress passed the Gulf of Tonkin Resolution, which gave the president the power "to take all necessary measures to repel any armed attack against the forces of the United States and to prevent further aggression." This tougher United States stance was matched in Moscow in October when Leonid Brezhnev and Aleksey Kosygin took over control of the government following the fall from power of Nikita Khrushchev. The new Soviet government pledged increased military support for Hanoi, and the NLF set up a permanent mission in Moscow.

United States support for South Vietnam, which had begun as an effort to defend Southeast Asia from the communist threat, developed into a matter of preserving United States prestige. The Johnson administration, nevertheless, was reluctant to commit combat troops to Vietnam, although the number of United States military advisers including their support and defense units had reached 16,000 by July 1964. Instead, in February 1965 the United States began a program of air strikes known as Operation Rolling Thunder against military targets in North Vietnam. Despite the bombing of the North, ARVN losses grew steadily, and the political situation in Saigon became precarious as one unstable government succeeded another. General William C. Westmoreland, commander of MACV from June 1964 to March 1968 urged the use of United States combat troops to stop the Communist advance, which he predicted, could take over the country within a year. The first two battalions of U.S. Marines (3,500 men) arrived in Vietnam in March 1965 to protect the U.S. airbase at Da Nang. The following month, Westmoreland convinced the administration to commit sufficient combat troops to secure base areas and mount a series of search and destroy missions (see Glossary). By late 1965, the United States expeditionary force in South Vietnam numbered 180,000, and the military situation had stabilized somewhat. Infiltration from the north, however, had also increased, although still chiefly by southerners who had gone north in 1954 and received military training. PLAF strength was estimated to be about 220,000, divided almost equally between guerrillas and main force troops, the latter including units of PAVN regulars totalling about 13,000 troops.

The United States decision to escalate the war was a surprise and a blow to party strategists in Hanoi. At the Twelfth Plenum of the Central Committee in December 1965, the decision was made to continue the struggle for liberation of the South despite the escalated American commitment. The party leadership concluded that a period of protracted struggle lay ahead in which it would be necessary to exert constant military pressure on the Saigon government and its ally in order to make the war sufficiently unpopular in Washington. Efforts were to be concentrated on the ARVN troops, which had suffered 113,000 desertions in 1965 and were thought to be on the verge of disintegration. In early 1965, Hanoi had been encouraged by Moscow's decision to increase its economic and military assistance substantially. The resulting several hundred million dollars in Soviet aid, including surface- to-air missiles, had probably been tied to a promise by Hanoi to attend an international conference on Indochina that had been proposed by Soviet premier Kosygin in February. As preconditions for these negotiations Hanoi and Washington, however, had each presented demands that were unacceptable to the other side. The DRV had called for an immediate and unconditional halt to the bombing of the north, and the United States had demanded the removal of PAVN troops from the South. Although both Hanoi and Washington had been interested in a negotiated settlement, each had preferred to postpone negotiations until it had achieved a position of strength on the battlefield.

By mid-1966 United States forces, now numbering 350,000, had gained the initiative in several key areas, pushing the communists out of the heavily populated zones of the south into the more remote mountainous regions and into areas along the Cambodian border. Revolutionary forces in the South, under the command of General Nguyen Chi Thanh, responded by launching an aggressive campaign of harassment operations and full-scale attacks by regiment-sized units. This approach proved costly, however, in terms of manpower and resources, and by late 1966 about 5,000 troops, including main force PAVN units, were being infiltrated from the North each month to help implement this strategy. At the same time, North Vietnam placed its economy on a war footing, temporarily shelving non-war-related construction efforts. As a consequence of the heavy United States bombing of the North, industries were dismantled and moved to remote areas. Young men were conscripted into the army and their places in fields and factories were filled by women, who also served in home defense and antiaircraft units. Such measures were very effective in countering the impact od the bombing on the North's war effort. The Johnson administration, however, showed no sign of willingness to change its bombing strategy or to lessen its war effort (see fig. 8; fig. 9).

During this difficult period, the communists returned to protracted guerrilla warfare and political struggle. The party leadership called for increased efforts to infiltrate moderate political parties and religious organizations. The underground communist leadership in Saigon was instructed to prepare for a general uprising by recruiting youths into guerrilla units and training women to agitate against the city's poor living conditions and the injustices of the Saigon government. Total victory, according to the party leadership, would probably occur when military victories in rural areas were combined with general uprisings in the cities.

In mid-1967, with United States troop levels close to the half million mark, Westmoreland requested 80,000 additional troops for immediate needs and indicated that further requests were being contemplated. United States forces in Tay Ninh, Binh Dinh, Quang Ngai, and Dinh Tuong provinces had initiated major offensives in late 1966 and in early 1967, and more troops were needed to support these and other planned operations. As a result of these deployments, United States forces were scattered from the DMZ to the Mekong Delta by mid-1967. Opposition to the war, meanwhile, was mounting in the United States; and among the Vietnamese facing one another in the South, the rising cost of men and resources was beginning to take its toll on both sides. The level of PLAF volunteers declined to less than 50 percent in 1967 and desertions rose, resulting in an even greater increase in northern troop participation. Morale declined among communist sympathizers and Saigon government supporters alike. In elections held in South Vietnam in September 1967, former generals Nguyen Van Thieu and Nguyen Cao Ky were elected president and vice president, respectively. A number of popular candidates, including Buddhists and peace candidates, were barred from running, and newspapers were largely suppressed during the campaign. Even so, the military candidates received less than 35 percent of the vote, although the election took place only in areas under the Saigon government's control. When proof of widespread election fraud was produced by the defeated candidates, students and Buddhists demonstrated and demanded that the elections be annulled.

Data as of December 1987