The war was already lost before Johnson put troops on the ground in 1965.
Questions about the fundamental winnability of the Vietnam War have bounced around academic circles since the 1960s.
As soon as America put troops into Vietnam, the war became unwinnable. It enabled the communists to invoke nationalist sentiment, branding the Americans as imperialists, drawing many Southern Vietnamese civilians to their cause. As a result, the communists remained hidden in the South. The South Vietnamese population hid them, fed them, and assisted them where they could. It kept the Americans clutching at ghosts, leaving them totally unable to engage their enemy on their own terms. Lt. Gen. Arthur S. Collins, the troop commander for Central Vietnam from 1970 to 1971, told a historian after the war, “I don’t think there was any way that South Vietnam could survive, no matter what we did for them.” A group of historians labelled the Legitimacists, shared Collins’ view. They assert that victory in Vietnam became unobtainable as early as 1963. When the US helped overthrow Ngo Dinh Diem, it was condemning itself to defeat. Diem was the strongest, most charismatic leader South Vietnam was able to produce. His overthrow caused a power vacuum in the country, which led to instability and ultimately forced the US to commit troops to Vietnam. Diem had a far better understanding of the Southern Vietnamese political landscape than those that came after him. He was also responsible for a period of economic growth and upheld nationalist pride.
Counter arguments come from two major blocs of historians. The Clausewitzians and the hearts-and-minders. Both believe that the war was very much a winnable war. The Clausewitzians suggest that it was policy failings in Washington that led to the fall of Vietnam. Essentially, lawmakers in Washington misunderstood the nature of the conflict. Their policy reflected a lack of understanding of the war as a war of attrition and aggression. Had they deployed every weapon available, and secured the backing of the American people, the war could have been won quickly. The hearts-and-minders, as the name suggests, believe the only way for the US to win in Vietnam was to win the battle for the hearts and minds of the Vietnamese people. The army leadership failed to apply effective counterinsurgency strategies. The search-and-destroy strategy only made enemies among the South Vietnamese population. For all America’s firepower, it was their lack of understanding of the local Vietnamese people that led to their downfall.
No matter what the US did in Vietnam, the outcome would have been the same due to events that occurred before Johnson ever committed ground troops to the country.
In the 1970s, Nixon's narrative of the war rejected the unwinnable war premises. He promoted the "Lost Victory" position, that America threw its chances at winning the war when it lost the public support at home.