The Northern Alliance looks more and more like power-hungry jerks
We're not their hired Air Force. Our job is to destroy a source of terrorism and the state sponsoring it. period. Of course we're willing to help mediate between various Afghan groups, and we'd be very happy to see the next Afghan government NOT support terrorism, but the bottom line is that the goodness or badness of the next Afghan state will depend mostly on how much the Afghan people demand of it.
If the Northern Alliance wants Kabul, let them take it the hard way - let them earn it.