That's the thing alot of our hatters and enemies don't seem to realize don't realize, we're not trying to win. We could steam roll Al Quida in a day if we just said screw the civs and dropped a bunch of MOABs on every square inch of Afghanistan and Pakistan.
Even if the US would try (the Russians and English had their turns as well), they would never be able to conquer Afghanistan. The terrain is just too BIG, with too many mountains to hide away, they would never be able to get to the enemy, as all the enemy would do is hide out in caves. I agree however, that the objective of the US would not be to conquer Afghanistan but to track down the Taliban and El Qaeda, not only for rooting them out, but also to gather intelligence of intended El Qaeda operations that may be plotted for harming the United States, or key organizers who can provide access to those who may be planning operations.
