Agreed. Had I known then what I know today, I still would have invaded Iraq, but based on the intelligence we had at the time, it was a no-brainer. Liberalism is about thowing wet spaghetti at the wall until something sticks. First it was "Afganistan has never been conquered. Then it was "no blood for oil." Then it was "no unilateralism." Then it was "where are the weapons of mass destruction?" Currently it is "Iraq is in the middle of a civil war." Where it stops, nobody knows.
IMO the mistake was not invading Iraq but staying there. Had the US withdrawn in May '03 after Saddam was overthrown, or in December after his capture, they could have left as victors having delivered a clear message that an aggressive regime will not be tolerated. By choosing to engage in "nation building" after the victory, defeat is all but inevitable now because there is no honourable way out.