There are two very important lessons we learned in Vietnam that we can argue we have ignored in Afghanistan. The most important reason, I think, is that we learned that if we cannot take and hold territory we will simply fight over the same territory repeatedly. And the second lesson is that, in a war of attrition--one that requires us to kill more of the enemy than they kill of us--we have to be willing to expend as many lives as necessary to render the enemy ineffective.
In Vietnam, in part because we were able to move around the country at will in helicopters, we fought the Vietcong (guerrilla fighters) and the North Vietnamese wherever we found them and usually won our battles. But after we won a battle, we got into our helicopters and left the area to fight another battle somewhere else. In some cases, our enemies were occupying the battleground within hours of our departure. Sometime later, we would return to fight on that territory again and then leave again after a fight. In short, we did not have the troops to take and hold the land we fought over, so our enemies routinely reoccupied territory that they lost in previous battles. In WWII, for example, we won the war in part because we took territory from the enemy and held it until the end of the war. "Owning" the land is critical to winning a war.
A second, but equally important lesson, is that we cannot win a war of attrition unless we are willing to kill every enemy combatant or at least enough to make them unwilling to fight any longer. That strategy, of course, assumes that we are willing to lose enough men to accomplish that. For political and cultural reasons in the US, America was unwilling to sacrifice enough troops to win a war of attrition, which made success impossible for us.
One can argue that the Afghanistan War cannot be won, in any sense, if we cannot take and hold territory. As we did in Vietnam, we fight the enemy wherever we find them, but once we win a battle, we leave the area, withdraw to our bases, and wait for the next battle somewhere else. In the meantime, our enemies re-occupy the territory we fought over. We are also, as a culture, not willing to expend the number of troops necessary to win a war of attrition. In other words, we do not believe we should allow an unlimited number of Americans to die in order to kill enough of our enemies to make them ineffective.
One can argue, then, that some crucial lessons of the Vietnam War, unfortunately, are being ignored in Afghanistan. We have already announced our departure from Afghanistan in 2014, and we might see in Afghanistan the same result we saw in Vietnam--that is, once we leave, it's like we were never there.
To make this argument we have to identify a lesson that was learned from Vietnam and is not being heeded today. Perhaps the best such lesson (one can argue) is the lesson that it is very difficult to win the “hearts and minds” of a population through the use of military force.
In Vietnam, we tried to use military force to make people like us more than they liked the Vietcong. This did not work. We can argue that we are trying the same thing in Afghanistan. We are trying to use military force to make people like us more than they like the Taliban. There, too, the tactic seems not to be working.
If we accept this argument, then, we can say that we failed to learn the most crucial lesson of the Vietnam War and that this failure is coming back to haunt us in Afghanistan.