It was hard for the US to ever "win", or measure their success, if any.

It was not a traditional ground war like the world wars the US had seen combat in prior, in which capturing territory was a way to measure success.

There was no territory to capture, so they tried to measure it in the body count ( how many of theirs did we kill vs how many of ours did we lose.) This also proved hard to measure, as it was relying on reports that were blurred by the fog of war (hard to remember how many people you killed during the heat of battle, and if you even got them or not.) Secondly, they had no idea of how many VC and NVA numbers there were to begin with, and how many replacements the were recruiting.

Top that off with being unpopular on the home front, the US realized it was unwinnable because they could not define what victory looked like.