It began by helping the French after WWII and moved on to protecting the world against communism.
The US never invaded Vietnam.
South Vietnam was considered an ally of the US.
US Military personnel were in Vietnam from 1955 thru 1975.
See: Vietnam
Vietnam
The US was concerned about the development of Communism in Vietnam
We were at war with Vietnam because they disagreed with us about something.
The French were fighting in Vietnam in 1950 not the US.
The fear of being invaded was not a justification for the increase in US involvement in Vietnam. The US withdrew from Vietnam in 1975.
The US wanted N. Vietnam to stop fighting in S. Vietnam.
Yes. Before the US committed to the Vietnam War, many Americans had not even heard of Vietnam.
Even though Vietnam is a communist country, the US does business with Vietnam. The US promotes good business. Friends? Sure, why not.