March/April '73; all US personnel by April '75.
Unable to stop the flow of communist men and material from entering South Vietnam and with the support from it's own citizens (fellow countrymen) at home now gone, the US was UNABLE to continue the fight. So it withdrew from it.
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.
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.