One thing you hear all of the time is that "Afghanistan is the graveyard of empires". It's taken down everyone from Alexander the Great to the British to the Soviets, so America should probably stay away. Or else just go in all the way just to prove how America is so amazing.
Well, check out the history of Afghanistan on Wikipedia. You'll see that Alexander the Great conquered Afghanistan (then Bactria) just fine, and Greeks ran empires in the region for hundreds of years. Other successful conquerors include the Persians, Scythians, Mongols, Arabs, Sikhs... the list really goes on. Historically, Afghanistan has been the stepping-stone of empire rather than its graveyard.
It's true, the British had some setbacks there. But ultimately both the British and Russians had victories in the region.
But none of this answers the question of whether Afghanistan can be "won" today or not. Iraq, for instance, has historically been very easy to conquer; and, yes, any country today with a sizable standing army probably would have found it reasonably easy to both conquer and pacify Iraq. India or China, if either had the logistics capacities and local defense bases, could probably have done a decent job at it. But it proved to be very difficult for America today nonetheless.
The Soviets, for instance, found Afghanistan tough going--even though the Russians enjoyed some success there over a hundred years ago. The past is very poor at predicting the outcome of insurgency today. Not even the Soviet example is a perfect indicator of how things may go in Afghanistan, as things are very different. There are no Stinger missiles or foreign countries supplying insurgents today, for instance.
Rather than misreading inapplicable historical examples, it would be nice if the media offered some more cost-benefit analysis on this sort of foreign adventure. Here, I'm genuinely agnostic: cutting and running would seriously hurt counter-terrorist operations, but even total success is not worth an infinite amount of money and lives. It seems silly to skimp and scrounge for every last penny on a health reform bill, but then say that because this particular war seems good in comparison to a pretty lousy one, it's worth throwing no end to money towards it.
It also seems odd to slam McChrystal for behaving as a neo-MacArthur in demanding more troops for the mission. Isn't that what Shinseki did before the Iraq War, before getting fired and then applauded for his willingness to speak up? It's probably true in both cases that generals shouldn't be the final determinants of troop levels (at least in wars of choice), because they aren't in a position to weigh the benefits of more troops with costs elsewhere. But really that's just a reason to stop engaging in wars of choice, so that these issues will stop rearing up.
The news already does a pretty poor job of covering international news; maybe if they finally do a poor enough job, no one will care, and these sorts of wars will stop happening.