The goal was supposedly prevent the spread of communism. That was an objective failure since the results of the war were: Viet Nam became communist, Cambodia became communist, and Laos became communist. In what way did the US win? They failed in their primary objective.
You also cannot just look at the war in a vacuum. You need to look at the whole picture of American involvement. The US first got involved in the area to preserve the French colony. When that failed they ignored Viet Nam’s declared independence earlier to try to legitimize a pretty illegitimate government. You can’t say the goal was to preserve a state that only stayed in existence to begin with because of the French and Americans. And, again, as a proxy war it was an objective failure since it didn’t prevent the spread of communism at all.