In 1954, their main goal was to help the French, in which they did. They were later attacked 5 years after by Vietnam. In addition, They feared communism would spread too quickly so they wanted to prevent that. This is known as the Domino Theory.
The French need help, let us do it!
In 1964, Vietnam had stumbled on a U.S warship, it was attacked by them with torpedoes. This was now when America entered the war. The Gulf of Ton-kin Resolution gave Vietnam the go-to attack the warship.
We're under attack!
In 1968, during their Tet, Vietnamese New Year, Americans thought they wouldn't attack, but the complete opposite happened and it was horrible.
This was unexpected! Everyone retreat!
The people are losing trust in us! What do we do?!
In 1969-1971, After the Tet offensive, information was leaked out to the public. People lost hope in the war and slowly retreated.
Welcome to ARVN Training Camps. You will train here for the time being.
ARVN troops were deployed, but they were defeated easily. In 1974 they signed a treaty, America withdrawn from the war.
In 1975, North Vietnam then breezed through troops and had successfully captured South Vietnam as their own, having full communism and united Vietnam. That is how it ended.