Here's why: for starters, Afghanistan is not Iraq. Our invasion there was justified in international law, and came with the force of a UN resolution. The American citizenry does not see this war as the foolhardy fraud they saw Iraq, and neither for that matter does the international community. Secondly, Afghanistan would in all probability go back to the Taliban short of US military support for Karzi and the warlords, and would again become an Al Qaeda redoubt. Accepting that outcome short of trying to affect it militarily, diplomatically, would be a huge blow to US morale, and would likely cause popular uproar. The upshot could doom Obama's Presidency, and all of its aims domestic and otherwise, immediately. It is difficult for me to see how any of that is in US interest, although I'd be interested to hear someone try.

Depends on what you mean by interest. You've made some good points here, from a political perspective. However, in the long term statying in Afghanistan may very well be damaging to US interests. If we are fighting a war (or skirmish, or whatever it should be called) that ultimately can not be won, then why isn't it better to cut one's losses and go home?

The bottom line is that if the US cannot keep states from sheltering terrorists that want to harm her interest and slaughter her citizens, it will lose one of the most effective tools for keeping a lid on such violence, if not the most: cooperative foreign governments.

We simply disagree here. The formal governement of Afghanistan is not harboring the Taliban. The Taliban is fighting an insurgency. I agree we need to have cooperative foreign governements, but our behavior in Afghanistan and the region generally has been counter-productive by this measure. We have angered Pakistan (a nuclear state) by conducting raids and bombing missions on her territory without her permission. We have inflamed Arab and Muslim anger toward us with our callous disregard of civilian casualties (in Iraq and Afghanistan, as well as in Gaza). We essentially opened up a new country for al-Qaeda, for a time, when we invaded Iraq and botched the occupation.

You get foreign governments to cooperate by, first of all, not doing things which make life difficult for them, and secondly, by finding ways to reward cooperation. Afghanistan is more an example of what not to do in this regard.

And it may very well be the case that increased troop presence is a good way to decrease civilian casualties.

Well, that would be a first. More soldiers usually translates to more violence and death. True, it doesn't have to be, but I'd say history puts the burden of proof on your side. I'd be happy if future events prove me wrong, however.

In any case, I think the new administration is afforded latitude to deal with this issue- certainly a Taliban run Afghanistan would eventually be a source for far more civilian casualties.

Again, I disagree. The Taliban actually ruled Afghanistan for a while, and for all accounts people preferred their rule to the immediately preceeding period where the warlords just duked it out among themselves, with the civilian population often caught in the crossfire. The Taliban brought stability and relative calm, and could do so again.

Given the last eight years, they probably would be much more hostile toward the US this time. But even then, what could they really do? We would not be taken by surprise again, and Afghanistan is a hopelessly poor country. I doubt they would be able to hurt us much, even if they wanted to.