The Vietnam War (1945–1975)

by: History SparkNotes

Quagmire and the Tet Offensive: 1966–1968


January1967United States reaches nearly 400,000 troops in Vietnam
June1967CIA initiates Phoenix Program
January 1968NVA attacks U.S. Marine base at Khe SanhNorth Vietnamese launch Tet Offensive
February1968McNamara resigns as secretary of defense
March1968Westmoreland causes uproar by requesting 200,000 more troops U.S. soldiers kill 500 Vietnamese civilians in My Lai Massacre

Key People

Lyndon B. Johnson - 36th U.S. president; insistence that the United States was winning the war in Vietnam led to the development of the “credibility gap”
Robert McNamara - Johnson’s secretary of defense; had initially supported escalation but began to question U.S. involvement and resigned in early 1968
William C. Westmoreland - Commander of U.S. military in Vietnam; made enormous political blunder by requesting Congress for 200,000 more troops after Tet Offensive of 1968
William Calley - U.S. Army lieutenant in charge of company that killed 500 Vietnamese civilians at My Lai; was court-martialed in 1971 but paroled in 1974

Changing Strategies

By 1966, both sides in the Vietnam War started changing their strategies. General Nguyen Chi Thanh, a top Viet Cong commander, began to push for a general offensive. Meanwhile, General William C. Westmoreland’s search-and-destroy operations were fully under way. Although many of Westmoreland’s campaigns were successful in killing Viet Cong forces, they also required large numbers of U.S. troops.

By the end of 1966, nearly 400,000 U.S. soldiers were in Vietnam; this number would reach 500,000 by the end of 1968. President Johnson also authorized the use of chemical weapons such as napalm, a thick gasoline-based gel that can be sprayed and burns at high temperatures, and Agent Orange, a chemical defoliant that was used to destroy jungle vegetation to expose Viet Cong hideouts. Although both of these weapons were effective, they inflicted horrific devastation, and Agent Orange in particular caused unforeseen health problems among both troops and Vietnamese civilians, the effects of which have persisted for decades.


In late 1966, U.S. forces began to search for the so-called Central Office of South Vietnam, or COSVN—the Viet Cong command center that U.S. officials insisted existed somewhere in the jungle, directing Viet Cong operations throughout Vietnam. The existence of the COSVN has never been confirmed, however, and it is likely there never really was such a command center at all. Nonetheless, General Westmoreland initiated a series of search campaigns in the so-called Iron Triangle, a sixty-square-mile area north of Saigon. Although several thousand Viet Cong were killed in a campaign that lasted until 1967, U.S. forces failed to locate COSVN or make any progress in encircling or rooting out the Viet Cong.

The “Credibility Gap”

Despite the numerous setbacks, Johnson and other U.S. officials, citing increased troop numbers and redefined objectives, again claimed to be making headway in the war. Photos and video footage of dead American soldiers in newspapers and on evening news programs, however, indicated otherwise. Moreover, U.S. spending in support of the war had reached record levels, costing the government an estimated $3 billion a month. As a result, many people in the United States began to speak of a “credibility gap” between what Johnson and the U.S. government were telling the American people and what actually was transpiring on the ground.

Khe Sanh

Throughout 1967, Viet Cong guerrillas stepped up their attacks on U.S. servicemen. Then, in late January 1968, the North Vietnamese Army launched a major offensive against the U.S. Marine base at Khe Sanh, just below the DMZ. U.S. commanders, determined to hold the base, sent 50,000 men as reinforcements. Though one of the largest battles of the war, Khe Sanh was essentially a diversion planned by the Viet Cong in an effort to weaken American forces farther south, paving the way for a more significant offensive.

The Tet Offensive

Indeed, with U.S. forces still north at Khe Sanh, the Viet Cong launched the Tet Offensive, the large “general offensive” that Ho Chi Minh and the Vietnamese Communists had been planning for years. On January 30, 1968, on the Vietnamese new year holiday of Tet, separate Viet Cong and NVA cells attacked twenty-seven different U.S. military installations throughout South Vietnam at the same time.