Will anyone ever defeat Afghanistan?

No. There are too many hiding places. Those mountains and caves spread out for what seems like an eternity. All the taliban/Al Quada will ever do is just hide out or take off for a while and come back. One effective way to win though would be to give stability back to the country and let them fight them off themselves. They would have the upper hand knowing the terrain, being used to the climate, and would fight harder knowing they are fighting to preserve their way of life.