re: the wrong question
Show me one clear indication that Iraq under Saddam was a clear and present danger to the United States. Show me one clear reason why hundreds of Americans and thousands of Iraqis had to die.
Saddam had a long and consistent history of researching, developing, building, deploying, and using weapons of mass destruction. Under these circumstances, the bar to prove his lack of such weapons is understandably quite a bit higher than if we were accusing, say, Austria of having such weapons.
Spend a little time thinking about what exactly would constitute proof of "a clear and present danger" to your satisfaction. Does it involve hundreds to thousands of Americans dying?
"Some have said we must not act until the threat is imminent. Since when have terrorists and tyrants announced their intentions, politely putting us on notice before they strike? If this threat is permitted to fully and suddenly emerge, all actions, all words, and all recriminations would come too late. Trusting in the sanity and restraint of Saddam Hussein is not a strategy, and it is not an option."
- president George Bush, State of the Union, 2003
This explains the rationale of preemption, and also, incidentally, shows that Bush was not defining Iraq as an "imminent threat" when he was making the case.
A lot of people have been shocked that we did not wait until Saddam took the first shot before we attacked him. That's fine in cowboy movies, when the only person who loses by waiting too long is one cowboy, and all the townsfolk shake their heads and go home to supper. In any case the bad guy in the movies always misses with the first shot*. When terror groups and thug nations take the "first shot," thousands of people die and cities burn. We waited on bin Laden and al Qaeda for until they took the first shot. I think that was enough.
If it was up to me, the reactors in Iran and North Korea would already be target zones.
*In the original Star Wars movie, Han Solo blew Greedo away while Greedo was still threatening to kill him. George Lucas, in later versions, surrendered to political correctness and changed it so Greedo took a "first shot." Greedo, true to Bad Guy Tradition, missed his shot from three feet away. Solo then shot him according to the PC rules of engagement. Had there been any realism at all in the scene, it would have ended with Chewbacca scraping Solo's brains off the wall.