By coincidence, this is basically my current block of instruction in my CGSC course. (Command and General Staff College).

The bottom line is that the hills, valleys and cities are worthless, or nearly worthless, in and of themselves, unless there is a plan to exploit the fact that the land is posessed.

There are basically two phases to the war, the Westmoreland phase, where the going phrase was "seek and destroy", and the Abrams phase, where the slogan was "clear and hold". It is commonly theorised that the Westmoreland phase, the first few years, was incorrectly focused on body counts and attrition, while Abrams correctly more focused on the individuals.

If you can find it, there's a wonderful assessment by Dale Andrade from the US Army's Center for Military History, entitled "Westmoreland was right", which lays out pretty clearly the dual problem facing the US in the early part of the vietnam war: The US was facing both the conventional North Vietnamese Forces (PAVN), and the internal insurgency, the NLF (later with the PLAF/Viet-Cong). With two axis of attack to deal with, the conventional threat and the internal threat, Westmoreland needed to prioritise one over the other. He didn't have the assets to defeat both immediately, while the enemy only needed one of its two prongs to eventually succeed in order to win. In other words, the US needed to win both fights, Hanoi only needed to win one.

Westmoreland concluded that the pressing threat was the NVA conventional forces. As long as they remained in existance and could sweep down into the South, all the political/pacification work (the 'oil spot' theory) would be pointless. He considered the insurgent forces to be like 'termites', something that could well destroy the house eventually, but that they could wait until after the 'bully boys with crow bars' who were across the street were dealt with first.

Thus came the plan to attrit the PLAV of its manpower. Holding land was not the goal. Killing NV soldiers was, wherever they could find them. If the area being fought over was not within the sphere of influence of South Vietnam's government's pacification program, then there was no point in holding the ground. The South Vietnamese would focus on their local problems first, and then work outwards.

It would seem that the end result was, as far as it went, successful. After the Tet offensive of 1968, the PLAV's capability was pretty much exhausted. It was not something which could be ignored, but it was no longer the looming threat which it had been. About this time also was the time that Abrams rotated in, and, seeing that Westmoreland had done the work of deleting one of the two threats, he set about trying to work with the other threat, that of the insurgents who would come along when the regular forces had left. Hence, clear-and-hold. And he now had the forces to do this, since they weren't being used any more for the conventional seek-and-destroy operations against North Vietnam's regular forces.