the US was absolutely able to knock the NVA out of the war....
The NVA couldnt continue with the conflict personnel and equipment-wise....
the US "could"....
internal politics was the issue not the "ability" to continue to conduct war....
the military had a strategic objective.
The US government had a political one...
the military's objective was obtainable and they were close to it...
the government had ulterior motives influenced by factors completely outside of the war itself
Clausewitz: 'war is the continuation of politics by other means'.War is politics!
The military do not wage war to please itself; it wages war to reach a political objective defined by the government! The US military was in Vietnam to realize a political objective, it was unable to deliver.
War is a political operation. So, the only valuable metric to identify a winner and a loser in war is to ask who reached his objective. The US had an objective, the NV had an objective. They both use military means to reach that objective. Only Vietnam reached his. It won.
They were close since they beginning... They had more than 5 years. They could... Well, in REALITY, they did not. That's the only thing that matters because it's certain. The US was unable after more than 5 years to destroy the NV fighting will and capability.
No, the main reason for the american retreat had to do to with war: it became too unpopular due to the bodybags and the length. And those two factors were caused by the NV on the battlefield. The war became politically too heavy. The idea that the US left for reasons unrelated to how the war was going is a lie.
In fact, as long as the US did not invade north vietnam, it was an unwinnable war. And the US was shook to invade north vietnam.
Till these days kids are born with deformations in Vietnam, Cambodia and Laos. The US did flex its muscles and fought a dirty war; the vietnamese just did not give up 
