1965 was the point when the U.S. committed ground troops in a combat situation. That LBJ & McNamara committed to a ground war without ever thinking out a long term strategy was criminal stupidity. With no end game they should have just accepted eventual defeat, and continued to provide logistical support, and training. No vital American interests were involved. At no time did they establish objectives for the war, and ask the JCS to come up with plans to achieve them. The U.S. Military had no strategy other then an open ended battle of attrition.Because 1965 was already a different situation from what they were going for when they first went in. But they quickly fell victim to the sunk cost fallacy. Of course in this case the sunk cost wasn't just resources and lives, but ideology and international prestige. But equally it's not that simple either sometimes things don't go your way exactly as you'd like and you do persevere at a cost.
I absolutely agree. But the alternative (general invasion into Laos and N.Vietnam), might in theory solve that issue but it will create other ones*. That's my point. My point is also the US knew this from the very beginning so it was never going to happen. So we can sit here and say oh well if they'd done 'A' it would have solved a problem, all we like, but if they were never going to do 'A' for other reasons it's moot as a realistic option.
Untimely the US were not looking to get into conquering SEAsia for capitalism situation
*that is now war on two countries and giving promoting a more unified response, it will escalate things in Cambodia, you likely bring Thailand in in some fashion since they're now the anchor point for you new lines. China is very much going to react to a massive escalation into what they see as their general sphere, and people around the world will point to "American neo colonialism" and you know what it will very much look like that no matter what the rationale is. US deployment will have increase to cover all that. You've basically turned a 'oh it's not a war war we're just helping out ally with a internal policing/security action' into a general regional war. No one likes general regional wars, certainly not during the cold war, when proxy wars are supposed to be safe way to slowly manage a global confrontation without it spilling over. (One of thd lessons of Korea was that it's really hard to keep a war contained when there are interested parties)
Cutting the HCM Trail also pretty much ends the war in Cambodia, because the Communists can't get there from the North. Instead of escalating the situation in Cambodia it deescalates it, since the war was caused by the North Vietnamese invading the country, and recruiting, and arming local insurgents. The Cambodian Government was trying to play both sides against the middle. When you ride the Tiger you can end up getting eaten. Ending the invasion ends the conflict. The Americans cutting the HCM Trail would be the best thing to happen for Cambodia in it modern history, it would've saved them from the Killing Fields.