Under the Gulf of tonkin resolution, all necessary military force.
The US asked them to.
A reluctance to commit to foreign adventure (for awhile).
No, the US was committed to not losing, which was entirely different then winning. It was never an attempt at winning a war, just deny the North a victory and prop up the South.
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
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 US was concerned about the development of Communism in Vietnam
The fear of being invaded was not a justification for the increase in US involvement in Vietnam. The US withdrew from Vietnam in 1975.