Spencer Ackerman notes that we now have confirmation that the invasion of Iraq was intended as the overture in a broader regional war, including one aimed at prompting regime change in Teheran. It's worth noting that this isn't just something to file away in the "wacky pre-war predictions" file, but led directly to the administration massively screwing the pooch on several fronts.
First, in Iraq. Whatever it is you're trying to do with Iraq policy, it's always going to be easier to accomplish it if the countries surrounding Iraq -- including Iran and Syria -- are helping you rather than trying to undermine you. Iran and Syria are not, however, run by blithering morons. Thus, when you hint in your public and private statements that one of your ultimate aims in Iraq is to overthrow the governments of Iran and Syria you wind up pushing them heavily into the "undermine" camp and essentially making it impossible to accomplish anything.
Second, in Iran. As we now know, soon after the invasion of Iraq, Iran tried to open talks aimed at a broad US-Iranian diplomatic settlement. On the table would be Iran ending its nuclear program and curtailing its support for Palestinian rejectionists, in exchange for the United States lifting sanctions disavowing a regime change policy, and trying to accommodate Iranian interests in Iraq and Afghanistan. That would have been a very good deal for the USA to take, as anyone with a functioning brain to see. Unfortunately, though, functioning brains were in short supply inside the administration which believed that the Iranian domino was about to fall so there was no need to talk settlement.
Thus we have a major cause of our current mess in Iraq and of our current mess vis-a-vis Iran all wrapped up in one neat package.