Yo,
Having spoken frequently with guys who have been over there, while the mountains and the caves are a big problem, the terrain is not the problem with subduing Afghanistan - the borders are.
At present the current Taliban/Al Qaeda policy is to establish permanent camps in Pakistan and then raid into Afghanistan, using friendly villages as base camps for operations. When they have pulled a few ambushes, planted some mines, triggered some IEDs, decapitated some teachers, and blown up the new hospital and water supply, they retreat back into their safe areas in Pakistan. Meanwhile, their losses are constantly replaced by new Jihadis being trained in the Pakistani Masjids and Madrassas. To win in Afghanistan, we would have to dismantle that Jihadi support/safety infrastructure on the Pakistani side of the border, which would involve elminating the schools of Islamic instruction on that side - which obviously isn't going to happen.
So instead we will fight an endless hit and run war with them, and as soon as US politics finally forces us to withdraw, they'll gradually tighten the noose around the Afghan government in Kabul, kill all the teachers, bomb the new schools, hospitals, and whatever infrastructure we've added, eliminate the aid organizations, assasinate the local government and then eventually topple the new government and then reestablish the Islamic Talibani government, make the country into one huge Jihadi training camp and return the people of Afghanistan to the endless cycle of oppression, misery and hopelessness they endured throughout the 90s.
We'll lose, because unlike the Jihadis, we refuse to accept we are at war with an ideology and act accordingly, not because of some difficult terrain.
- SEAGOON