The purported conspiracy theory reason. The official reasoning of WMDs were generally believed to be a nonsense casus belli presented as legal justification. More people thought it had something to do with 9/11 than WMDs.
In this case, much of the real purpose was laid out in articles and papers written just a few years earlier by a bunch of the same people crafting and implementing the policy. There may have been more reasons than that, but some were frankly stated in public documents.
Their publications (including an open letter addressed to Clinton) lay out a case for preventative (as distinct from preemptive—what we in fact did was preventative, with some bullshit to sell it as preemptive) war over concern that Saddam might do some of the stuff we lied about to claim he was already doing it.
Pure realpolitik damn-the-legality chess-playing stuff. He seems risky (to our interests in the region—who cares about his own people?) so we better depose him, that kind of thing.
This is the explanation that checks out with me. Add some impulse for Bush Jr to finish what Bush Sr started and you have yourself a ridiculous waste of resources.
