"They see a mess in Iraq and aren't prepared to put up with it any longer."
What they see is what the media spoon feeds them. They don't get the whole picture.
Anyway, guess when Al Qaeda takes over Iraq and has access to billions in oil revenues each year, Americans will again feel contented.
AQ won't take over in Iraq no matter what. The US has been there long enough that the Kurds and Shiites have the decisive upper hand.
AQ can be as terroristic as they like, but the real victims from this point are going to be the Sunni, on which the rest will take their revenge against any AQ outrage. They are a minority, now lacking power and having no organizational or human resource advantages.
Given a free hand, the Iraqi government, whatever it will eventually be, will crush AQ, even if it means plenty of "ethnic cleansing".
What the US has done cannot now be undone, the change in that situation is permanent.
The real question is the nature of the Shiite government and the amount of Sunni blood that will be shed.