Without the support of the American people, the President could not escalate the war any further.
The Vietnam War was the most televised war in history. 90% of the US population had a television in 1965, and most watched the war play out, unedited, on their screens in their living room.
Although the US public was generally supportive of the war in the early stages (64% believed that America was right to send troops to Vietnam, compared to 21% that disagreed), by 1966 this was starting to turn. The number of people that saw the war as a “mistake” climbed by 10 points in 1966, then in 1968, things further deteriorated. In early 1968, the Vietnamese communists launched their largest assault of the Vietnam War. During the Vietnamese holiday of Tet, 67,000 Vietcong and NLF troops attacked more than 100 targets across cities in the South of Vietnam. The American public suddenly saw their TV screens filled with images of US Marines battling Vietnamese forces in major cities across the South. Cities that the US government had been adamant were under US and South Vietnamese control. This was a turning point. Following the 1968 Tet offensive, public opinion shifted against the war. The influential journalist, Walter Cronkite even appeared on national news and openly called for President Johnson to seek a ceasefire with Northern Vietnam. For the Johnson administration to lose the support of “Uncle Walter”, it meant he had lost most of America. Without the support of the US population, the President was unable to escalate the war in Vietnam. He was unable to commit increasing troop numbers without the approval of Congress, and few representatives wanted to jeopardise their chances of re-election by endorsing an escalation of the war in Vietnam. Without escalating the war, Johnson had little chance of defeating the communists. He needed more troops to replace casualties. He also needed to extend the battlefield to include operations in Cambodia to disrupt Vietcong supply lines.
Johnson had the support of the public before 1968. The US arrived in Vietnam in 1965. That means US forces had three years of fighting with some 500,000 American troops with extensive aerial and naval support. But those three years had produced nothing but casualties and a stalemate. The war was already on a losing path before the public turned against it. The public turned against the war in 1968 because Tet showed that after three years, the US and South Vietnamese forces had not even been able to secure the major cities, including the Southern capital, Saigon.
Without the public support, Lyndon Johnson could not get Congress to approve an escalation of the war. Without an escalation of the war, Johnson could not defeat the communists.
Enter the technical rejections of the premises here ...