I think fundamentally, this was a stupid war-of-choice, started by a stupid person. Weapons of mass destruction, 9/11, Saddam's brutality toward his own people, etc., all just the reasons of the month. The real reason we went into Iraq was that Bush and his cadre were (and apparently still are) convinced that they can take over a major oil producer, and install a western-style democracy and capitalist economy. It's a dangerous mixture of neocon fantasy, and oil baron selfishness...
Iraq had nothing to do with 9/11. Saddam Hussein was a brutal tyrant, but he was no islamofascist idealogue. Iraq was basically a secular country before we invaded... funny thing about ruthless dictators... they tend to clamp down on religious extremism rather well.
So far, this war-of-choice has cost the U.S. over 3,500 lives and $450 billion dollars. And don't forget that modern battlefield medicine has saved the lives of tens of thousands more, but left them living with horrific injuries and dependent on a VA system that is in shambles.
Anti-western extremism has exploded as a result, and it's taught many other countries a valuable lesson... Does anyone think we would have invaded Iraq if we knew they had nuclear weapons? If you have nuclear weapons, you won't get invaded. Iran and North Korea were watching carefully.
Edit... fixed the typo... 2,500 -> 3,500.