Afghanistan was invaded to allow the construction of an oil pipeline to the Indian Ocean to serve the growing demand from China and India, and to get to their heroin. Cheney etc were trying to negotiate a deal during the Clinton era with the Taliban, but they kicked them out. They were warned then that they should allow the pipeline and that they could either be carpeted in gold, or carpeted in bombs. Many central asian countries with the oil/gas are just the usual puppet states with terrible human rights issues. No companies would get involved unless Afghanistan had a "proper" government, hence the need for 9/11 to get things kicked off.
On the subject of troop numbers, read a book called Blackwater, you'll find the real truth there- the "official" numbers there are only about half of the actual numbers, due to "contractors" AKA mercenaries. Much easier to do things with corporate armies, you don't then need to report to parliament/congress on casualties/deaths. A pretty covert operation in both Afghanistan and Iraq, nice and convenient to allow these psycho mercenaries to act with impunity as they're not subject to any laws of war, shooting to kill anyone who gets too close....
