Kennedy blundering into the assassination of Diem in November 1963 is what I’d call the turning point as well. LBJ gets the blame for sending in combat troops the next year but as far as I can see the chaos set in motion by Diem’s death forced LBJ into choosing between a major American military role or a decision to abandon South Vietnam. Where I fault both Johnson and Nixon is in their refusal to go for the destruction of North Vietnam.
JFK sent in 16,000 troops, no JFK, then no American Vietnam war.
No JFK and America would have survived, and not had the 1960s.
Johnson, I can agree with you there (especially when he outright rejected his Joint Chiefs when they suggested exactly that). Nixon, not so much. Actually, when he took office, he did bombing raids on various key weapons centers and supply depots within North Vietnam, and even managed to mine the harbor. Nixon certainly did a lot more on that front that Johnson did, which acted as a black mark to the latter. In fact, what Nixon did was EXACTLY what the Joint Chiefs suggested that Johnson do. Though to be fair to Johnson, he also was worried about a potential nuclear war breaking out if they did that based on some of his (otherwise obscene) comments to the Joint Chiefs.
Besides, if we tried a direct invasion into North Vietnam, we’d probably be pushed back anyways, like what happened when MacArthur tried to make his push into North Vietnam and China.