Unwinnable and a quagmire.
The capital of South Vietnam , Saigon , fell to the communists April 30th 1975 .
In 1974 the north Vietnamese broke the treaty,and in 1975 they completely took over south Vietnam.
The US never invaded Vietnam.
The North overran the South and then began to murder anyone who was deemed an enemy of the Communist North. Some were reeducated (About 300,000) but most leaders in the South were eliminated. Viet Nam is a Communist nation ruled with an iron fist, and is not a Democracy.
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.