The excuse was to defeat Al Qaeda and punish the Taliban for not handing them over and for allowing them to train in their country. The reason we went and are still there is because we see it in our national interest to have influence in the region. Afghanistan gives us a spot from where we can influence events in Iran, Pakistan and all of Central Asia and their mineral and oil reserves.
The problem, as I see it, is we had this idea and then made some promises to keep us there post Taliban. But, as usual, we failed in the execution of those promises and the reality of the situation on the ground made us make changes in our approach that also affected those promises.
Some examples:
We promised to help the Afghan Government get formed and promised to help it be a viable, legitimate nation in the region. As part of that we promised to reform the Afghan Army into a force that could defend the country. We set the number at 70,000. . .but then dictated it would be only infantry. We decided after the fact that we would not provide armor or artillery or logistics or helicopters etc, just an Army of 70k Infantrymen. We can debate whether we should have made such a promise, but any fool could have predicted that the Russians would try to come in and peddle their stuff. Never-the-less, we were surprised when a Russian IL-76 landed in Dec 04 with a gift of Russian 1/4 tons Gaz trucks (like a jeep) and the promise of providing them with a helicopter fleet if they wanted it. Suddenly Rummy offered to provide a complete combined armed force, thus changing that part of the mission.
We promised to help the government reform and rid itself of corruption. But, the fact is they are so dirt poor that corruption is a way of life. Initially we placed Provincial Reconstruction Teams (PRT) in strategic locations to help gain some influence in areas where we needed to root out the high level Talibs and Al Qaeda. For example, we put a PRT (along with an ODA) in Gardez to control/influence a key former Taliban area where we thought we would find a lot of former Taliban leaders and some of the key AQ. But this promise of helping them with the corruption then made it necessary to deploy more PRTs, put Infantry Companies out in forward operating bases around the country and increase the size of the force dramatically. Afghanistan has always had what we would call corruption but they would just call "doing business" and this is an impossible promise.
And really, the average Afghan doesn't really care. For most of the Pashtun's they are what we called Taliban Hillbillies. They do their thing in their village and are generally pretty peaceful, but when the Revenuers (i.e. Afghan or US Army) come to break up their still, they pick up their AK and fight for their village. We leave them alone and they work their farm.
Oh, yeah, then we decided that NATO was losing relevance so we thought it would be good to just give them the mission. Another stupid idea that just complicated the entire mission again and made the US have to do more just to make up for the blunders that ISAF would do.
Then let's add the UK anti-drug effort that gets no publicity. They have an independent force that goes in and razes farms and takes away livelihoods of dirt farmers and drug lords, gets everyone riled up and then our PRTs and FOBs have to deal with the aftermath. Afghan opium goes to Europe, US gets its opium from the Golden Triangle, so it is not in our national interest to fight the drug war in Afghanistan.
The bottom line is we failed to adhere to any real strategic theory. I think we should have stuck to the Powell Doctrine and set an achievable mission gotten to the endstate and then left. We should have said, "hey, we'll get you to your first national election but from that point on you are on your own." They would have accepted that. I was there for the first Presidential election and they were pretty set at that point. We could have moved out and maybe left a token force and some intelligence capabilities to look for AQ. Everyone else could be home right now and the situation would be about the same.
Just my thoughts on the situation, sorry for the rant.