A lot of questions. US joined the war to prop up the South Vietnamese government to CONTAIN the expansion of Communist forces, aka Democratic Republics, Peoples Republics, Popular Fronts and the like.
Since the end of WWII, the US was actively competing with the USSR, who had taken over Poland, Bulgaria, Romania, Hungary, East Germany. US and Western forces stopped Communist in Greece and Turkey, Italy and West Germany.
In Asia, China fell to Communists in fighting that raged from 1930 too 1949, North Korea in 1949 and N Vietnam after the French were defeated in 1954. US policy attempted to CONTAIN Communism from Thailand, Phillipines, Indonesia, India. It failed in South Vietnam, Laos and Cambodia. So around 1960 we sent advisors and helicopters to support the fight. After the Bay of Tonkin incident, President Johnson committed more and more troops in the mistaken belief that the war was on the verge of victory against the Viet Cong and N Vietnamese. That illusion was shattered in January 1968, with the Tet Lunar New Year offensive. The N Vietnamese attacks eventually failed and cost many lives on their side. But the damage was done to the US civilian psyche, as the TV images shattered the propaganda that victory was close. War opposition, confined to draft demonstrations and student strikes became bigger and more severe after early 1968, with the outcome of ending Johnson's presidency, bringing in Nixon, and the long end game with more conflict within the US asa well as the continuing expense of the war.