I think Praetorian has a point in that the USA did achieve a significant level of tactical success in the Vietnam war, but this does not mean that they won. War is essentially a political act, as Clausewitz put it; āwar is a continuation of policy by other meansā. The policy objective of the US was to prevent South Vietnam from becoming a communist country, and this it failed to do. Even during the US presence in Vietnam, massive swathes of the south vietnamese countryside were under effective communist control; yes, the communists melted away when the troops arrived in an area, but the rest of the time, they were there, running schools, indoctrinating peasants and even collecting taxes! IMHO the failure of the US was to appreciate that it was a very political war; both sides wanted control of the vietnamese countryside and its inhabitants, but in order to gain that kind of control you really need to gain the trust and loyalty of the civilian population; and this is where the US failed. They also failed to stop the Communists from gaining said trust and loyalty from much of the population.
A big part of the problem seems to have been that the Diem (and subsquent) regimes that the US were supporting did not have the support and resources they claimed to.
I am by no means trying to insult the GIās who fought there, but I feel they were let down by there high command and political masters, who did not appreciate what they were getting into.