[Chomsky has long argued that the US didn't really lose the Vietnam War, as below. That seems to me correct. --CGE]
The US succeeded brilliantly in its major war aims, though it didn't attain its maximal goal of conquering Vietnam. The major goals throughout were to prevent the "virus" of "radical nationalism" from "infecting" other reasons (to use the terminology that appeals to high planners) by the demonstration effect of successful development -- the rational version of the "domino theory." That goal was achieved.
Indochina was demolished, with maybe 5 million corpses and huge destruction. The surrounding areas were "inoculated" with vicious and murderous dictatorships. By the early '70s, the business press recognized that the US had basically won the war (that was my view then as well; wrote about it at the time). To call the war a "failure" is to assume that anything less than achievement of maximal goals is a failure. An interesting conception, and it's an interesting sign of US ideological power that the left and peace movement not only accept the standard view but regard it as uncontroversial.
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Chomsky may be correct (he usually is). However, American war-mongers create a trap for themselves when they justify their imperial adventures by evoking the concept of "the good war" as exemplified by WWII. If every adventure is analogous to the war against fascism, then nothing less than total, unequivocal victory is acceptable. Thus, while the U.S. may have achieved its larger goals in Vietnam, it sure didn't appear that way; after all, WWII didn't end with American troops ignominiously fleeing oncoming enemy.