Political and economic concerns
The government thought in a different way, of course. President Johnson met the conflicts after Kennedy's death. The Cold War moved to a new phase, the Soviet Union had an advantage in weapons and there was the occasion to fight back. Besides, a local war was the best opportunity to test the newly developped guns, choppers, tanks etc., in a word: to test strategic arms. Apache helicopters were first deployed in Vietnam. The increasing Soviet and Chinese, that is communist influence demanded a capitalist intervention and that was the USA. On the 2nd and 4th August, 1964, the American Maddox destroyer was attacked and that was the cassus belli to enter the war. Hitherto, they only sent advisors, whose number was increased to 17000 during the Kennedy administration. There's no unanimous opinion about the Tonkin Gulf incident either. Some think that was provocation, the others see the aggression from North Vietnam, but it's considered to be a pretext. The US shouldn't have become entangled in long war that served a wrong goal just because of an attack like this - sounded after the war, but not in political circle.
The situation became untenable during the Johnson administration, the most fierce combats, bombings and demonstrations started then. At the end of his office, Johnson tried to initiate discussions, but it failed because of the corrupt South Vietnamese regime, the continuous fights and the unwillingness of North Vietnam. Many politicians and businessmen didn't want the end of the war since it was a big business. The weapon dealers made their fortune and many politicians had concern in an arms company. For the government, the victory would mean that they can rebuild the region that's a great amount of money for the country and the Soviet influence would decrease. They thought that the South Vietnamese needed their help, but when it became blatant that the major part of the people supported by them was against them, the economic and political concern had an important role. Things can't be undone then otherwise they were the loser. At any case, Johnson stopped bombing North Vietnam at the end of his office and wasn't nominated for president at the following election. Richard Nixon, the succeedor, escalated the war to Cambodia and Laos, although he claimed: "We are not in Vietnam as aggressor, but to defend against aggressors. History will note the American participation as a good cause that was undertaken and finished with honor." At least, the publicy calmed down. The war between the USA and North Vietnam turned into a war between North and South Vietnam. Nixon started vietnamizing the war to diminish the protests.
American soldiers were substituted by South Vietnamese personnels. The army was strengthened utmostly. The cut of American infantrymen to 250 thousand in 1971 poured oil on troubled waters. On the other hand, Nixon increased the South Vietnamese army to one million personnels. The total American withdrawal is linked with Nixon, too, but it wasn't his achievement. Anyone would do it since the fight had been lasting for a decade and it was obvious that it's impossible to win the war. There was nothing to do, just to end up with the least damage, casualties and moral loss. Politicians rates the war differently, though. They tried to better the situation since it was 1973 and the loss was great. The US had spent 140 billion dollars on the war, 55 thousand young lost his life on the fronts and nobody knows how many people suffered afterward. The statesmen blame the Vietnamese people and claimed that they had done what they could have done. The further financial support of the South Vietnamese was concealed, though. After the peace-treaty in Paris in 1973, the war still went on as civil war and it was internal affair. It didn't have so great publicity as before. In 1975, North Vietnam overran South and the two regions were united. Gerald Ford claimed unambigously that the war had ended and wouldn't continue any more. Citizens were happy that their son won't be involved in the war. Many people just saw the North Vietnamese victory, and they might be glad at the Vietnamese dream coming true.