Briefly summarizing, they began when, in 1946, Truman refused Ho Chi Minh’s request for help in evicting the French colonial occupiers. He helped the French, instead. This all but assured that the U.S. would never “win the hearts and minds” of the Vietnamese people.
The errors continued when, in 1955, Eisenhower set up a “South” Vietnam in order to evade the national elections that had been agreed to in settling the French defeat. Eisenhower stated bluntly, “Our guys would have lost.”
Still more mistakes were to come. Eisenhower foisted an alien ruler on his new country, a wealthy, Catholic, urban, mandarin from New Jersey, Ngo Diem. The Vietnamese were poor, Buddhist, rural peasants. Then, U.S. stood by as Diem took people’s land and gave it to his wealthy friends. Ho Chi Minh took land from the French and distributed it to the people.
All of these moves only served to harden the Vietnamese people’s conviction that “South” Vietnam and its government were simply puppets for a different Western colonial occupier. No such lackey regime could ever achieve political legitimacy. And without political legitimacy, there could never be a long-term solution to the War.
Compounding the political failings were the failures of intelligence. The most obvious of these was the confusion of nationalism with communism. Vietnam was first and foremost a struggle for national independence. The Vietnamese wanted the foriegn occupiers out of their country. The Americans should have understood this. They had once fought a war of national independence to get the foreign occupiers out of their country.
Much more at Why the US Lost the Vietnam War | By | Common Dreams