There's always more to be said, but in the meantime you may find some answers about America's decision to initially pursue a sustained aerial bombing campaign in Vietnam and the transition to primarily a ground war strategy here by u/enygma9753.

The US view was that they were only there to "assist" South Vietnam in repelling a communist insurgency, as part of the West's attempt to contain the spread of communism in southeast Asia. South Vietnam was created as a democratic nation in the aftermath of the First Indochina War between France and communist insurgents, where France lost its colonial foothold in the region. On paper, the U.S. allying with South Vietnam seemed to be the prudent and logical choice. The leadership of South Vietnam was pro-West, anti-communist and formed from the ruling class of Catholic elites who remained. Its leaders were however extremely corrupt, increasingly undemocratic and oppressive to the non-Catholic majority in South Vietnam.

Domestically, it became harder as the war wore on for the Johnson and Nixon administrations to continue to justify their propping up of such a corrupt regime. It was seen as a necessary evil in the greater context of the Cold War and the belief that Vietnam falling to communism would cause a chain reaction or domino effect in the region, as other nations (in theory) could fall under Soviet or Chinese influence.

The US trained the South Vietnamese army in the hopes that they would take up more of the fighting gradually. This process of "Vietnamization" accelerated under Nixon. South Vietnamese forces, while well-trained, were heavily dependent on US air power. Once the US withdrew its forces from the region by 1973, South Vietnam found itself overwhelmed and unable to stop the North Vietnamese army and Viet Cong incursions. Saigon fell to North Vietnamese forces in April 1975.