I've said before, there are many realities about Afghanistan that make our war there more difficult than is/was the case in Iraq. Some examples:
-While Afghanistan is both bigger and more populous than Iraq, they are close enough that I don't think either of these really makes a difference. More important is the landscape. Afghanistan is extremely rugged, mountainous, inhospitable, and rural. Iraq, on the other hand, is very flat and most of the population urbanized. As far as geography goes, Afghanistan was always more similar to Vietnam than was Iraq, even though people were quick to call Iraq "Bush's Vietnam".
-Our enemy in Iraq, for the most part, did not have a safe haven outside of Iraq. Thus it was just a matter of ejecting the insurgents from within by getting their protecting population to turn against them. In Afghanistan, our enemy simply runs across the border to Pakistan when things get too dicey. There is no evidence that the Pakistani military is going to be a reliable partner in going after the Taliban, either. Sure, they're going after the Taliban right now; the Taliban are waging full scale war against Pakistan. It makes one wonder, however, what the Taliban perception of the US presence long term is, that they are apparently unafraid to open up a second front while we're still around. Maybe they assume we're leaving soon. If we don't, they could just decide to stop fighting the Pakistanis for now (reach another truce), and are we going to be sure the Pakistanis will continue to fight them? History says they won't. Again, Vietnam was like Afghanistan in that our enemies had a safe haven, North Vietnam, where they were relatively safe, and where they could rest, re-arm, and recruit.
-Iraq has had a strong central government for a long time now so the people there are used to living under a strong central government. Afghanistan has been almost in anarchy for years, with a very decentralized power system. This makes our strategy of empowering the central Afghan government seem like a questionable one.
-In Afghanistan we are confronted with an ideological enemy, and those seem to be the most difficult to defeat. The Taliban are motivated by strong religious conviction in most cases, although arguably some are bought. In Iraq, we were able to quell a large part of the insurgency simply by putting them on our bankroll. I suspect that Taliban will be more expensive to buy off than former Baathist secular Iraqis. Again, we look to Vietnam. Our enemy there was motivated also by ideology, communism, which made them stubbornly resistant to our attacks.
All of these things being said, I do believe the United States *could* pacify Afghanistan and stabilize the country if it wanted to. With Iraq just about wrapped up, we have the troops. America definitely knows how to do counter-insurgency, thanks to experience gained in Iraq. It would take several years, but we could do it. So what then is the real problem? I see two of them:
1. Is it even worth it? I question whether it is. Afghanistan is just not a strategically important country and maybe won't ever be. The best argument that I've seen repeated in multiple places is that a Taliban revival will destabilize Pakistan. I'm not sure about this. Pakistan got on fine with the Taliban before 9/11, and anyway, Pakistan is just too big of a country for the Taliban to pose an existential threat to. Other arguments about an al Qaeda safe haven, America's reputation, et cetera have been thoroughly debunked over the last few weeks so I'll ignore those.
2. The Karzai government. Corrupt, disliked by the people, part of the problem, and I definitely don't trust the guy. Here is the kicker - whats the alternative? There is a run-off election coming up. If Karzai wins, we have the same problems. If Karzai doesn't win, then all of the sudden the leader of Afghanistan is not an ethnic Pashtun. That would be a serious problem, since Pashtuns are the largest ethnic group by far in Afghanistan and are the main component of the Taliban. Heck, "Afghan" is just another name for "Pashtun". So basically, we're stuck with a corrupt and ineffective government or one that will stoke even more insurgency just by virtue of what it is.