Bush dropped the ball at every level of the game. There was no way that Kerry could have known that Powell's State Department would have been summarily ignored and Rumsfeld's Pentagon given full control of the show.
Secondly, Kerry made the argument based on Saddam's long-term threat, not on anything imminent:
"Every nation has the right to act preemptively, if it faces an imminent and grave threat, for its self-defense under the standards of law. The threat we face today with Iraq
does not meet that test yet. I emphasize "yet." Yes, it is grave because of the deadliness of Saddam Hussein's arsenal and the very high probability that he might use these weapons one day if not disarmed. But it is
not imminent, and no one in the CIA, no intelligence briefing we have had suggests it is imminent."
Kerry's argument was that Saddam was a loose cannon, and after 9/11 we could not afford to have loose cannons running around. Kerry's position is absolutely consistent with what he was saying back in 1998. And also with what Bill Clinton was saying back then:
"Other countries possess weapons of mass destruction and ballistic missiles. With Saddam, there is one big difference: He has used them. Not once, but repeatedly. Unleashing chemical weapons against Iranian troops during a decade-long war.
Not only against soldiers, but against civilians, firing Scud missiles at the citizens of Israel, Saudi Arabia, Bahrain and Iran. And not only against a foreign enemy, but even against his own people, gassing Kurdish civilians in Northern Iraq.
The international community had little doubt then, and I have no doubt today, that left unchecked, Saddam Hussein will use these terrible weapons again.
...
Iraq repeatedly blocked UNSCOM from inspecting suspect sites...Iraq repeatedly restricted UNSCOM's ability to obtain necessary evidence...It tried to stop an UNSCOM biological weapons team from videotaping a site and photocopying documents and prevented Iraqi personnel from answering UNSCOM's questions...Prior to the inspection of another site, Iraq actually emptied out the building, removing not just documents but even the furniture and the equipment...Iraq has failed to turn over virtually all the documents requested by the inspectors. Indeed, we know that Iraq ordered the destruction of weapons-related documents in anticipation of an UNSCOM inspection.
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Instead of the inspectors disarming Saddam, Saddam has disarmed the inspectors. The
credible threat to use force, and when necessary, the actual use of force, is the surest way to contain Saddam's weapons of mass destruction program, curtail his aggression and prevent another Gulf War.
...
Heavy as they are, the costs of action must be weighed against the price of inaction. If Saddam defies the world and we fail to respond, we will face a far greater threat in the future. Saddam will strike again at his neighbors. He will make war on his own people.
And mark my words, he will develop weapons of mass destruction. He will deploy them, and he will use them.
http://www.nrc.nl/W2/Lab/Irak/clinton.html