Since at least 1991, we know that Iraq had in its possession at least 500 metric tons of yellowcake - and it was not prohibited from having it, and it wasn't doing anything with it. Why would an effort to get more either (a) be credible or (b) lead anyone to think it augered some jumpstart in their nuke program?
That is the reason that some CIA analysts considered the Niger claim not credible - because Iraq already had so much that they had previously bought from Niger. ...Though that hardly undermines the WMD argument.
We know that in 2003 we collected approx 500 tons of yellowcake from Iraq, after the war.
When did they acquire all of it, from where, and why, if they had no nuclear ambitions?
The real question was their ambitions, their intentions in the absence of too many answers to too many questions that remained from 1991 on.
The difference is simplied very easily by the demonstration of the nuclear/WMD disarmament programs of South Africa and the Ukraine. The international communinity had a high level of confidence in the transparent, open, cooperative, responsive, unobstructive actions of the governments of South African and the Ukraine. Anyone, any nation could do, as easily, what they did - if they wanted to. It was not difficult.
All it required was a real desire to do it and prove, demonstrate, assure everyone they were.
Sadaam played a game; saying he had nothing to hide and then doing everything imaginable to make everyone believe he was hiding things and then making everyone guess which you should believe, what he said or what he did.
He did not provide the grounds for trust that S. Africa and the Ukraine so readily provided.
We were not going to go another 12 years, wondering if we guessed right, as we began the WOT.
People forget that we were not required, by the UN resolution 1441, to find WMDs in Iraq. Sadaam was required to give everyone reasonable assurance that he did not have them. He didn't.
Recall that, post Gulf War, Iraq was under a weapons inspection regime. The IAEA knew where the 500 T of yellowcake were stored and had placed their seal on each container. It couldn't be tampered with without the knowledge and approval of the IAEA.
A fresh supply of smuggled yellowcake would, however, be available for exploitation in any way that the Hussein regime deemed fit.