This paper is a few years old, but it outlines the reason why we are still in Afghani-land.

What happens if we cut and run from Afghanistan? India might get a pipeline full of energy from Iran, thats what! Uncle Sam says "hell to the no" to that prospect. Boy oh boy, its getting harder and harder to be a global hegemon these days.

www.pnl.gov/aisu/pubs/tapvatan.pdf