We still would have needed to CONQUER and OCCUPY North Vietnam. Rail lines can be fixed quickly, and the stuff coming in by sea could have been diverted through China via rail. Giving warning was totally absurd. LBJ was an asshole, but he could not have won no matter what he did, and he knew it. The South Vietnamese were totally unwilling to fight for their freedom, so they lost it. IF they had been willing to fight, the USA would not have needed to send in large ground forces, just supplies, training, and air support. The Vietnam war was lost at the Battle of Dien Bien Fu. It was downhill after that.

Incidently, while the battle of Dien Bien Fu raged, the French asked for American air support. The President called in the Chairman of the Senate Foreign Relations Committee, who told him that the USA should stay the hell away from any involvement in Vietnam, because it was a lost cause. Oh, by the way, the name of the Chairman of the Senate Foreign Relations Committee at the time way Lyndon Baines Johnson.