Let me clarify my thinking a bit. As was said by a journalist to the nascent leader of Pakistan " Most of the Muslim terrorists who have been apprehended in the UK since 2001 have been Pakistani, and what do you plan to do? ". Note: not Afghani. The usual story is that Pakis are going to Afghanistan to be trained up, but interestingly Pakistan is our friend in the war against terrorism. Anyone old enough to remember the IRA when it comes to threats to us? If not, google chronology of terrorist attacks on mainland UK. The Afghans, and I am outside PC but inside truth, are a feral pack of individuals who will spend the rest of eternity killing each other because they will never know much better. Why should anyone else ever bother with that? They are not in some mad war against Britain, they are in an eternal war against each other in their own country. Leave em to it.
Ungoverned space. Afghanistan was (is?) an ungoverned expanse of land where naughty people with extreme ideas could hide. This tended to suck in young Pakistani men educated in Madrassas in Northern Pakistan and nutters from around the Middle East and Central Asia. Add centuries of inter-tribal warfare and battle-hardened veterans of the Russian invasion into the mix and you get a country which, under the leadership of the Taliban, enabled a wealthy Saudi and his two mates to take their ideology and make it flesh in the form of paramilitary operations across international borders. Pakistan's most pressing problem is introducing formal law and order to the North West of the country where unpleasant people are still able to hide out of the reach of ISAF or Pakistani Security Forces.
When you look back it is surprising that it was only an act on the scale of 9/11 that prompted us into doing something in Afghanistan. A UN stabilisation force would have been most useful there 10 years ago, if not immediately after the Russian withdrawal.
Scrofula: If you haven't already, read On the Road to Kandahar by Jason Burke for an introduction to life in the region.
Parts of the Taliban leadership were Afghan refugees educated in religious madrasses set up in the Pakistani tribal areas after the Soviet invasion in 1979. However, most of today's foot soldiers will be Pakistanis (or second generation Afghans).