There is already photographic evidence of a massive dismantling of a facility of some sort before last year's invasion. These photos were published on the front page of the New York Times. Whether or not that particular building was producing weapons of mass destruction, it shows that Saddam Hussein saw the need to get rid of some things before they got captured.
Nations do not wait for iron-clad proof when there are lethal threats. The massive Manhattan Project that produced the first atomic bomb was begun when the United States was at peace because of reports that Hitler's scientists were working on such a weapon.
We had no proof -- and, after Germany surrendered, it turned out that Hitler's atomic bomb project was nowhere near the stage that we feared. But we couldn't take that chance.
Thomas Sowell understands the importance of specifying a loss function. (Hat tip: Newmark's Door.)