Kevin Boylan in the NYT:
[T]he revisionist case rests largely on the assertion that our defeat in Vietnam was essentially psychological, and that victory would therefore have been possible if only our political leadership had sustained popular support for the war. But although psychological factors and popular support were crucial, it was Vietnamese, rather than American, attitudes that were decisive. In the United States, popular support for fighting Communism in South Vietnam started strong and then declined as the war dragged on. In South Vietnam itself, however, popular support for the war was always halfhearted, and a large segment (and in some regions, a majority) of the population favored the Communists.
The corrupt, undemocratic and faction-riven South Vietnamese government — both under President Ngo Dinh Diem, who was assassinated in a 1963 coup, and under the military cliques that followed him — proved incapable of providing its people and armed forces a cause worth fighting for. Unfortunately for the United States and the future happiness of the South Vietnamese people, the Communists were more successful: By whipping up anti-foreign nationalist sentiment against the “American imperialists” and promising to reform the corrupt socio-economic system that kept most of the country’s citizens trapped in perpetual poverty, they persuaded millions to fight and die for them.