What are we fighting for in Afghanistan?

I just saw a Frontline briefing on the war in Afghanistan. The situation is that the Taliban have regrouped. They are flush with opium money and are making a comeback. And even though they are not very popular, we are even more unpopular.

One reason is that there are too few troops on the ground. The military has to rely on air strikes, which tend to hit civilians and non-military targets alienating the population. The humanitarian aid we offer is simply inadequate to bridge the gulf. The nature of insurgency and mountain warfare (where the attackers blend into the population and the terrain) makes it very easy for the Taliban to identify and kill anyone who collaborates with the US, even if they are a contractor we have hired to build a road, dig a well, or any other improvement that might win over the hearts and minds of the Afghan people.

Without a massive commitment of troops, the political and military situation will continue to deteriorate. And we will eventually be driven out like the Soviets before us.

Despite the fact that most of the country is technologically in the 13th Century, the Afghans have successfully repelled invaders since the days of Alexander the Great. Apparently, it is just too large and too rugged to be governed by any form of central government. Consequently most of the country is ruled by autonomous local groups—warlords, opium growers, ethnic factions, bandits, the Taliban, and anyone else that can hold their turf by force of arms.

It looks as though Al Qaeda has fled to Pakistan, which is an even larger and more rugged country (and our ally besides). So, what in the world are we doing in Afghanistan?