The Vietminh did much to get rid of the colonial French from Vietnam after WW II while the politicians in the south did just the opposite. Many fought on the side of the French or profited from the war to expel them. Was it any surprise we would have an issue with "hearts and minds" when we appeared for all purposes to just be taking the place of France and installing our own politicians?
Under President Eisenhower when Dienbienphu fell to the Vietminh - bombing by US forces would rightly have been seen as an act of war. And Eisenhower rightly did not take the bait. President Kennedy opened that door a crack and slowly it swung wide open.
Those in power when the French again took over after WW II knew America should not get involved militarily. At best they'd be on the side of a colonial power when we were the poster child for anti-colonialism and the costs were likely to be more along the lines of the Philippines insurrection from 1899 to 1913 than the Korean conflict. Besides Americans were emotionally exhausted from both WW II and Korea.
By the time President Kennedy was elected - Americans were more rested, China had been "lost" to the Communists, and the US was now an acknowledged Super Power. Plus we were willing to believe in illusions. To paraphrase Richard Feynman, "The easiest person to fool is yourself." David Halberstam's The Best and the Brightest tells the story of how America fooled ourselves into the Vietnam war.
Dense history but worth the time.
