[rhino369]

By the time the evacuation of Saigon happened the war was over.

Why they lost before that point can be debated. Communist nations funding of the North increased while US funding was cut by Congress.

Some claim the US agreed to sacrifice South Vietnam to the Chinese in exchange for reestablishment of relations and trade.

Others claim it was because of Watergate and the war weariness of the American people.

Probably a lot of things.

[user_0001]

People fighting for their homeland and existence tend be in it for the long haul and quite determined - they don't have a choice

An aggressive, invading, external military just whilst it is makes financial sense.