After extensive preparation, if the US had invaded the North in '65, the DMZ would have moved to the border of China and a geographically unified Viet Nam. Unlike Korea, we would have been prepared for any Chinese incursion, and the vast majority of their forces would have been destroyed at or near the border. There is no way that Chinese ground forces could have held up under a massive B52 and naval air campaign. How could they get their supply lines out from underneath a continuos rain of 2000lb bombs? Where would the Chinese troops hide? In holes in the ground? How could they stop advancing US troops by doing that? Obviously the US ground invasion would not have started at the old North-South border. But much further up the coast near Hai Phong, so that the port could be captured early and used for support. Ho would have fled to China, or back to his pals in Moscow. After some period, elections would have been held. The North had no culture of communism, it was after all under the influence of the French until 1954. It is far from clear that communists would have won in a countrywide general election, in fact it is more likely that they would have been defeated. Not just communists would have sported the nationalist banner. Anyway, the mass of US forces would have left after the election, except for those forces necessary to keep the Chinese from crossing the border, and prevent supplies from reaching any potential insurgents. Most of our forces might have been withdrawn by '68-9. Moreover, we might have been able to accomplish the reunification of Viet Nam with fewer than the 50,000+ killed that we suffered in a useless police action confined to the South that lasted until 72.
Dude, no offense, but that plan is long on optimism and "American the invinvible" idealism, and short on strategy.
First and foremost, after The Cuban Missile Crisis, the superpowers not very willing to tempt precipitating a global thermonuclear war again. They plaed their Cold War games by proxy.
Mao called our bluff in Korea and won. He had no reason to believe that under the same circumstances the same tactic would not work again.
Then there is the result of your "invasion" should by some miracle you pull it off. "Uniting Vietnam under one government. Let's rewind back to the 50s. France had Vietnam united under one government (theirs) and the Viet Minh ran their asses out of town. South Vietnam's government couldn't control South Vietnam, but you want to give them a WHOLE country?
Then there's your completely underestimating your enemy. China's army outnumbered ours anywhere from 3-5. That a BIG difference; especially, when you as the smaller force are on the offense attempting to invade a numerically superior army on defense.
Bomb them you say? The Chinese army was on the CHINESE side of the border. You are going to commit a unilateral act of aggression against a sovereign nation with an unprecipitated attack on its armed forces? In the world's eyes, WHATEVER China chose to do in retaliation would be justified.
So you don't bomb then UNTIL you invade. That alone violates the partition of Vietnam by the UN. It also takes away almost ALL your strategic air superiority insofar as wholesale carpet bombing is concerned. You can't just cut loose on an enemy engaged with your own troops.
You'd need at least 500K frontline combat troops to invade just to cut China's superiority to 2-to-1. Where are you going to get them? And even IF you could scrounge them up, you think you're going to move a Naval fleet the size required to support 500K men into the GUlf of Tonkin and NOBODY would notice and think something was amiss?
At the very worst for yourself, you could have your military force annihilated. At the worst for the world, you could precipitate a thermonuclear war. And you really don't stand to gain much. North Vietnam? There'd be a North Vietnamese insurgent behind every tree.
So, even if you managed to prevail, what exactly would you have gained? Rarely did US troops engage NVA regular troops during the war. They fought an insurgency for the most part. The VC backed by North Vietnam. You don't think the N Vietnamese would not have just moved into China and continued to do the same thing?
So, what you would have gained is an insurgency twice the size as the one you started out with.