Why us lost in Vietnam war?

Flow of men/material couldn't be stemmed. War couldn't be isolated due to geography/communist superpowers.

The USA was not beaten by the Communists in Vietnam. For example, in December 1972, American B-52s were bombing North Vietnam and there was very little North Vietnam could do about it. The USA gave up on the inept and corrupt government of South Vietnam. South Vietnam was unable to defend itself, as South Korea has since 1953.