In the times surrounding the Vietnam war, the US was primarily focused on preventing nations from becoming communist and the rivalry with the Soviets. There was no way that we were not getting involved in Vietnam at the time. This was also a chance for the Americans to beat the Soviets since they were backing the North Vietnamese.
America's war in Vietnam was justified, even though many Americans and Vietnamese lost their lives in a struggle that seemed to be in vain. American helped the country not turn to communism, which helped many other nations in Asia. If it wasn't for America's involvement in the war, many other Asian countries would have turned to communism.
American had no business in Vietnam. Many citizens in South Vietnam supported the National Liberation Front (Vietcong) and wanted a reunified communist or not. Diem, our ally, was a faulty leader and abused C.I.A. Power to arrest "suspected communists", so it's not surprising how many people in the South supported the North. This conflict was not our business.
For goodness sake, America didn't have to stick their nose in it, the war should've never come to America. Kennedy tried to stop it before it got worse, buy he was killed! Then Johnson increased it, this was a pathetic war fought, it's terrible how many people fought for nothing.
The only reason that we were even in Vietnam was because the French could not hold on and manage their territories. We got involved was to keep the spread of communism from spreading. People were fine with this in Korea because it was so soon after World War II. But during Vietnam it was a new America that did not think that the price we would have to pay was worth the price of a third world country falling to terrorism.