I think somebody somewhere was a bit too optimistic on WMDs and it snowballed. Who that is, and how high they were up on the ladder, I don't know.
When I first heard the administration beating the WMD drum instead of the "look, our troops have been tied up since '91, we need to finish this, and Saddam is up there with Stalin and Hitler in regards to intent and brutality, even if his numbers weren't there" argument I got worried. I think a lot of resources were tied up looking for WMD, when they should have been focused on Saddam and Bin Laden.
I will say this in defense of Bush, I think that he used the intelligence and advice that was available to him, and it's hard to fault him for that, because it probably appeared sound. Whoever was pushing the WMD line probably either didn't think things would go as far and well as they did, or they got into a hole they couldn't back out of, and had to keep the WMD story going. How big the WMD issue is in October and November, it's hard to predict, but the mistakes that some made convincing Bush of the WMD issue could cost him his job.
Actually, I think they were right too early, but forced to act by 9/11.
Inspections ended in 1998, and despite the press spin, Hussein began rebuilding his strategic arms in 2000. The estimate was based on what had not been accounted for in 1998, the materials being brought into Iraq since then and the deals being made (significantly with Syria and N. Korea).
Had 9/11 not happened, the deal for Scuds/manufacturing with N. Korea would have gone through and the start up of weapons production would be farther along. Scrutiny intensified after 9/11 and what we're seeing is the extent of a year's worth of getting restarted. That doesn't jive with the expectations of "tons" of bio/chem agents/precursors stored in a bunch of 55 gal drums or a dozen Scuds ready to fire.
Waiting for that would have made the threat more "imminent" and justifiable. It also would have been an incredible act of malfeasance.
Did hawks "sell" the war. Yep. Did they oversell the threat? Some did, and believed what they were saying.
That's why I think simple rationale, like Saddam's lack of disclosure, Saddam's intent on maintaining Iraqi control, and Saddam's shell game of deceit could explain the error and as JG suggests, a top head sacrificed would satisfy the cry for blood as a result of the miscalculations. Tennet said, in effect, intelligence is acting on the best available evidence. Remember, if I don't deny I have a weapon or can prove I can't easily be armed, it's not a good idea to neglect the possibilities.
Same with the Al Queida (mis)link. If the evidence was sufficient to suggest a link - and I believe it was - but there lacks absolute PROOF, the action taken is a far safer option than inaction. Even should that "link" have been well down near the end of the chain, the old saying "An ounce of prevention" aptly fits....