Bush was warned

The Bush admininstration's foreign-policy can be summed up as "faith-based". They set their minds on a particular view of reality (for example, that Saddam has a vast array of WMD, or that Iraqis would greet invaders with flowers and rosewater), and act on that view, regardless of the facts on the ground or any evidence to the contrary. So it's really no surprise to learn that Bush was warned that Iraq would go to hell in a handbasket and that the only winners would be Al-Qaeda, but ignored that warning and invaded anyway:

The same intelligence unit that produced a gloomy report in July about the prospect of growing instability in Iraq warned the Bush administration about the potential costly consequences of an American-led invasion two months before the war began, government officials said Monday.

The estimate came in two classified reports prepared for President Bush in January 2003 by the National Intelligence Council, an independent group that advises the director of central intelligence. The assessments predicted that an American-led invasion of Iraq would increase support for political Islam and would result in a deeply divided Iraqi society prone to violent internal conflict.

One of the reports also warned of a possible insurgency against the new Iraqi government or American-led forces, saying that rogue elements from Saddam Hussein's government could work with existing terrorist groups or act independently to wage guerrilla warfare, the officials said. The assessments also said a war would increase sympathy across the Islamic world for some terrorist objectives, at least in the short run, the officials said.

And the rest, as they say, is history.