The idea of invading/occupying Iraq was a disaster in and of itself, no matter how it was managed. There simply is no return to be had on the thousands of lives and $2 to $6 trillion that will ultimately be spent. None. In fact, it made the world worse off. Nobody came out a winner there.
Anyone who doesn't see that needs to be kept far away from presidential politics. There is no room here to say "Yeah but if they had only done it this way..." It's akin to saying that repeatedly hitting yourself in the head with a hammer is a good idea, so long as you manage that activity well. No. It isn't. No matter how it is managed.
Just imagine how much better off we would be if we had spent that money on our own infrastructure, subsidizing college education for those who cannot afford it, or anything else other than invading and occupying Iraq. Hell, we could probably could have spent $2 to $6 trillion constructing schools throughout the Middle East and educating children so as to make them resistant to religious radicalization, and even that would have been money better spent.