The history of man is a history of taking things from others, or of protecting things from others.
In the case of Afghanistan, we're not trying to take anything, and we don't have anything over there that needs to be protected. In that sense, it is exactly like Vietnam: we don't want it, but we REALLY don't want the OTHER guy to get it. In this case, rather than Communism, the "other" is the Taliban. And unlike Vietnam, if it falls, then terrorists will once again have a protected home base in a sovereign nation from which to operate unhindered.
So it's worth fighting for, right? I mean, if we pull out and let it fall once again to the Taliban (and make no mistake, it WILL fall to the Taliban, whether it takes a few weeks or a few months), then it will just be a matter of no time at all until al Qaeda backs up the U-Hauls to the caves and trucks all their stuff back over into Afghanistan. And that can't end happily for America or Americans, not to mention the rest of the Western world.
There's just one problem with that rationale for fighting: can you name the Evil Empire that employed that very same strategy in the decades following the close of World War II? Yes, the Soviet Union. They were not interested in "owning" East Germany, Hungary, Poland, Czechoslovakia, or other Eastern European nations. They were interested in NOT owning them, NOT having them part of Mother Russia, but just as importantly, NOT being allied with the West. Because, in the heart of Russia lies the conviction born of millennia of East-West conflict that the West wants to kill the East. They want to own the East. And Russia, in the eyes of the West, is the East.
Does the U.S. want Canada to buddy up to Iran? Does it want Mexico to be chummy with China? And non-hypothetically (since we already saw what happens when this is attempted), do we get all warm and fuzzy when the Russians try to park a bunch of missiles in Cuba? No. And Russia didn't want Eastern Europe to host an American presence for the very same reason. The same reason that we won't allow Afghanistan to fall to the Taliban.
It's probably an impossible strategy to sustain in the long term due to lack of popular support in America. And by long term, I mean longer than a generation or two. That's how long the Soviets were able to sustain it in Eastern Europe, and even less than that in its own Afghanistan debacle. We could only execute that strategy for roughly 10 years in Vietnam before crumbling under pressure from our own citizenry.
The question is, this time, can we stick around long enough for there to be no Taliban for Afghanistan to fall to? Can our citizens' resolve outlive Osama bin Laden and his aging top lieutenants? It can if it is presented in those terms. It's taken 60 years, but we've outlasted Castro in Cuba, and could be on the verge of normalizing relations with the island nation 90 miles from our border. If we take the long view instead of the 4-year election cycle view, I believe we can be successful in keeping Afghanistan out of the hands of the Taliban and al Qaeda for decades to come, and therefore, forever. But so far, the argument in favor of continued American heavy presence in that country has yet to be loudly or persuasively presented from this perspective.