Adding very briefly, and maybe making even more explicit what Dr. Rubin has said, if you look back at the history of Pakistan's relations with Afghanistan, it's hard to say that stability and progress in Afghanistan has been at the top of Pakistan's agenda. I agree entirely with what Dr. Rubin has said. The most desirable thing from a Pakistani geostrategic point of view is that Afghanistan have a friendly government. The second is instability, which it has been prepared to live with. The third, obviously, as he said, are the enemies ruling Afghanistan.
I really think the critical thing here--and this takes one back into one's history lessons--is that how the British drew the Durand Line was guaranteed to make this an area of instability. If you read Margaret MacMillan's Paris 1919, you'll see this in a number of other areas as Britain drew boundaries. A lot of it was done to avoid the creation of strong powers and to put unstable situations out beyond where the empire ended.