Do you know how stupid it is to Monday morning quarterback a decision?
Whatever is known about Iraqi WMDs now, at the time the best intelligence we had available said that Saddam had chemical weapons, he was still trying to develop nuclear weapons, he was supporting terrorists even if there was no strong connection with Al Quaeda, and that if he supplied those weapons to a terrorist organization, not just Al Quaeda, the results could be devastating. 9-11 showed that we couldn't afford to just react to a terrorist attack, we had to be proactive and go after them before the attack. Hence the global war on terror, not a global war on Al Quaeda. AQ may be the most visible terrorist organization, but they are certainly not the only one and they are not the only one that hates the US and has the capability to mount attacks against Americans
After the fact, it was determined that there were flaws in the intell, but everything up till then said it was good