There was no pressing reason to go into Iraq when we did. Should have waited to deal with Saddam until after Afghanistan was settled.
I agree that 9/11 was used as an excuse to invade Iraq, rather than to eliminate some immediate security threat.
GW's well-intentioned, if unrealistic, goal was to create a new kind of Arab democracy that could serve as a model to break the unending conflicts in that region. A more practical (and less costly) goal would have been to depose Saddam and divide that "country"* into several administrative districts which could decide to unify or remain independent at some future date. In the meantime, we would have been able to establish a permanent military presence to counter Iranian threats to that region.
*Created out of thin air by British and French diplomats by drawing lines on a map in 1919 (Treaty of Versailles)