I have seen this said by several people - We should not have gone into Vietnam when we did, and we shouldn’t have left when we did either.
What we should have done is told the French to f**k off and suggested that they should give the land back to the natives as the French left them at the mercy of the Japanese.
I was working for a Congressman at the time of the Gulf of Tonkin Resolution vote. It was very controversial and there was a lot of discussion and argument in the office. I was not sure what the better answer might be so I read the very long article on Vietnam in the Encyclopedia Britannica. After reading it I decided the arguments between North and South Vietnam were 700 years old, preventing being swallowed by China was long standing issue, thus the Domino Theory probably was not valid. I then felt we should bow out of that war as gracefully as possible. I think that Kennedy was assassinated because he wanted to pull out of Vietnam, and there were $billion dollar contracts for construction by important Texas construction firms who had Johnson connections. I think they wanted Johnson in office.