Iraq had nothing to do with 9/11. It has been the worst thing to happen to the middle east since Syria came along, and very few people who actually live there feel it has benefited their lives. In fact, almost no one does.
The worst part is that the United States might be dealt a retaliation from it in some way. Illegally invading a country without any planning or strategy, destroying the very fabric of society and law and order and directly being responsible for the deaths of tens of thousands of innocent civilians and the further feeding of the feeling of subjugation by western hegemonic powers--that mobilizes radicalization. The war in Iraq has created way more radicals than it removed, and the result of that is the newest generation of extremist militants, which Obama has been doing a fairly decent job at trying to undo. The United States helped cause 9/11 on itself for this very same cycle through it's interventions in Israel, which was why Bin Laden did what he did. If you listen to some of the speeches he has given, he isn't as "insane" as some people would have you believe, there is a twisted level of logic to his madness that mirrors the attitude a lot of American extremists have. If you go around trying to control the world, often in the pursuit of self interest on some level, and in the course of it don't really give a real shit about the people who live there, you are going to create a lot of enemies, for very legitimate and understandable reasons.
The US can't simply "go in and take out the bad guys" in Syria, because it's not that simple, just as it was in Iraq, there are multiple levels of social, political, and even religious, contexts to consider, plus the sheer logistics of such an operation, and the fact that things are more grey than they are black and white. A complex civil war/revolution isn't something you can solve with just a big stick, even though that is a useful tool to have, such bluntness is ineffectual against something as complex as this. I'm not really sure what the solution is, to be honest, but if you want to kick the hornets nest you are for sure going to get stung. Complex problems require complex solutions that are a lot less satisfying than just saying "well, why don't we just take them out?", which was something a simpleton moron like George Bush didn't understand and why he now has the blood of 110,000 civilian deaths on his hands.