actually i believe you are talking about fundamentalist Islamic groups who would be perfectly happy is the US wasn't supporting Dictators in their own countries with weapons. They didn't even think of attacking the US until they had failed to overthrow their own governments. (recall please Osama Bin Laden is a Saudi Citizen, Saudi Arabia is ruled by a king who recieves militrary aid from the US)
Now i know they actually have an objection to western culture which they feel is corrupting their own people and is supported by these dictators (because the US likes to get great advantage out of their 'friends' by having them import american goods/services with all those US dollars they've gotten for their oil - not that this is inherently bad, it's a smart move and i don't question it morally) AND they would rather the US didn't exist at all, but they would lose most of their support if the US didn't interfere with (as they see it) their own cultures, and by being in afghanistan they have the advantage that you've moved your people into range of their guns, and dropped bombs on towns full of people who will now grow up to hate America, (because they aren't educated, and don't know any better and HAVE seen American weapons killing their people)
Now I can't say whether you should be in Afghanistan at present or not, (nor whether offering money to warlords if they would give up 'taliban' fighters was a good idea because they could round up whoever they wanted) Right now you ARE in Afghanistan and can't just leave, though that country has a long and bloody history, and like Vietnam i can't see the US getting a mission acomplished there (though i think perhaps it will be easier to build a country in Iraq, because they had on there to begin with, so the people there have an idea of how to run a country, and trust that a system can work, and don't want to live in fear - they would rather have a stable system)