We couldn't, because it's impossible to prove a negative. What we could be sure of is that he violated 17 U.N. resolutions and the peace treaties following Gulf War I, that he had and used WMD, and that he never met his burden of proving to us that he had disposed of the weapons. In fact, he behaved as a leader who still had those weapons, as he played cat and mouse with the inspectors and filed a bogus 12,000-page "compliance" report.
How could President Bush have done anything other than attack Iraq? Our intelligence agencies and foreign intelligence services said he had WMD, and Saddam himself behaved as if he had WMD.
Moreover, Saddam had a history of sponsoring terrorists, including the families of the Palestinian suicide bombers. We know he hated the United States, loved terrorists, either had or was trying to produce WMD, and would have -- had we permitted him to remain in power -- handed those off to terrorists to use against the United States or its allies.
President Bush announced the Bush Doctrine shortly after 9-11, and it involved taking the fight to the terrorists and their sponsoring nation states and, if necessary, doing so preemptively -- attacking them before they had a chance to attack us. Based on the information President Bush had, indeed, based on the information we have now, he had to take out Saddam. And you can be sure that if he hadn't and we were later attacked by terrorists with WMD acquired through Saddam, Democrats would crucify President Bush for not doing enough.
While John Kerry says President Bush has hurt our credibility with foreign nations, a nation doesn't gain credibility by making nice, but by following up on its promises, threats, and commitments, and by enforcing resolutions, as the president has done.
The United States is, or will be, safer in the long run because President Bush had the wisdom and courage to dethrone Saddam Hussein. God bless President Bush.