A war that should never have taken place. Saddam Hussein was never a threat to the USA, he did not have links to Al-Qaeda etc. etc.. The major threat in the region is Iran, now the USA is too weakened by Iraq to really do anything but utter empty threats. Iran's theocracy is much more likely to support Islamic Fundamentalism then the secular Hussein ever was (albeit a Shiite group not a Sunni group).
Secondly, Saudi Arabia is another big problem in the region. It's education system is churning out Western civilization hating zealots by the thousands. The USA's addiciton to oil is causing them to turn a blind eye. If an Iran type theocracy ever takes control of Saudi Arabia, then look out.
Bush, Cheney, Rice and Rumsfeld have to shoulder a lot of the blame for Iraq, but the Congress cannot escape either, after all they did vote in favour of it.
Finally, once the war started the US administration did just about everything wrong. Not enough troops, dismissal of the Iraq army, no clear plan on how to manage the country after the fighting was done, believing that they would be welcomed with open arms, etc..
I am frankly surprised the USA only lost 3,900+ soldiers and that the country is fairly calm (except for Basra) after five years despite all these blunders. If McCain had been in power they at least would have been better prepared. If Gore had been in power (or Bush senior for that matter) they never would have entered the god forsaken place at all.