That statement is part of what I like to call the Lost Cause myth of the Vietnam War. It's something that isn't quite new in military history, since it's very close to the stab in the back myth of post-WWI Germany or the views of professional French soldiers after the French Indochina War in Algeria. One can say that it comes down to this: "The domestic population and the politics gave us up. If we only had more men/weapons and invaded the North, we would have won the war! Hell, we won every single engagement during the war."

With hindsight, we can say that it's a ridiculous statement. The contemporary view, however, is understandable since it genuinely did seem like the US never lost during the Vietnam War. However, this view is incorrect. I often like to say that the statement 'the US never lost a single engagement in the Vietnam War' is the same as to say that 'slavery was not the cause of the American Civil War' - both are complex topics, but the simplified statement is absolutely incorrect. When it comes to the Vietnam statement, those who say it treat it as a war of conventional means. The idea is that there was a clear picture of who won and who was defeated. The truth, however, is that the Vietnam War can't really been seen in that way. First off, the Vietnam War wasn't a conventional war. Second of all, how does one measure victory in a conventional war? Through the defeat of the opposing army, moving forward on the front lines (capturing ground) and taking strategic and tactical objectives.

This is not how it works in a counterinsurgency. There were multitude, literally thousands of engagements in which there was no clear victory. These engagements are sometimes completely unheard of and some are completely forgotten except as a page in the notes of a late NCO. Most of the engagements that the US infantry soldiers were involved in were not on their initiative, but on that of their enemies. The NVA/VC would not face the US in a pitched battle and found it better to surprise them, strike quickly and then disappear. They had no intention to stay. In their minds, if they had produced a couple of casualties on the Americans, then that's a success. If they lost a few men in the process, then well, that's what happens.

Even during the war itself, there was no clear indication on how to "read" the war, how to measure victory. For example, as historian Gregory Daddis argues, there were an understanding of counterinsurgency goals in amongst the professional officers and senior leadership in the US Armed Forces. It is an argument that has only recently been made and remains not entirely convincing in the light of how the more conventional minded leaders measured victory; that only way the US military leadership could measure victories were through the infamous body count system which was genuinely rather useless in measuring victory. The concept was completely based on collecting bodies after engagements and using that as evidence for your supposed victory. This would then be included in a sort of quota which the commanding officer would then send up the ranks. This unfortunate system only worked as a tool for the military leadership to fool themselves into believing that they were winning, while the reality on the ground was different. You can't win a counterinsurgency war through superior firepower and maneuver.

Right off the top of my head, Kham Duc was a tactical defeat for the US, so there's already your loss.

Winning a battle and achieving your objectives are two different things. Tactical victory implies you held the field and drove the enemy off, but tactical victories do not matter as strategic objectives might be left unfulfilled. For instance, at Khe Sanh, the US managed to drive off the initial Vietnamese attack (the tactical victory), but their position was unsustainable and they were forced to withdraw, thus conceding more territory to the NVA and giving the NVA the strategic victory (because they accomplished their overall strategic objectives).