It`s amazing how many really clever people got this question wrong.
Given the nature of the US operation there and Iraq`s culture and history, the attack and invasion, all the brave talk of nation building and democracy, it was highly likely to fail, at least in the short term. The new Iraq was always very fragile as a country, with a divisive prime minister and government, and a corrupt and ineffective state. The jihadists in Syria, finding the regime in Damascus tougher than they thought, cannot have failed to notice that next door. What happens long term is difficult to know. I think probably for now the civil war becomes much worse. For now, Iraq`s 'failed state' status becomes more and more obvious.
The sectarian divide, Shia, Sunni, Kurd, Christian, and other pagan minorities, predates the 2003 war by centuries, but it was aggravated by the security vacuum, post Saddam Hussein. it was also perfect for extremists of all kinds to flourish.
It might have been less bad if the US/UK had stayed a lot longer, but that would have been politically impossible in both countries. But it looks like they`re both being dragged back into the civil war anyway!