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I've surmised this question was posed on a now-deleted thread.
It's an old one.
The simple and short answer is, it was impossible. No less an authority than Scott Ritter explained it to me in early 2003, before the US invasion. Under the UN disarmament program of which he was the field commander until 1998, likely all Iraqi WMDs and at least 95% of the capacity for manufacturing them were destroyed, confirmably, by 1998. Ritter's team documented how Iraq acquired or manufactured the weapons and how they were disposed. Since the stockpiles had been substantial and the program complex and secret, some munitions or chemicals might have slipped through their net more or less by chance, but it was questionable whether even the Iraqi military could still find these minor stores. The story is told at length in Ritter's 2003 film, "In Shifting Sands."
The major powers and the UN all knew that Iraq had no WMDs -- more importantly, that its capacity to make WMDs had been eliminated, so that any stores that might have been missed were no threat. But acknowledging this meant the end of the UN sanctions program. In Aug. 1998 Ritter's boss Butler called upon him instead to back a Clinton plan to claim Iraq was still in violation, issue an ultimatum, and commence bombing. Ritter resigned a few weeks before the "Desert Fox" bombings of Dec. 1998.
Unlike with the domestic sites of 9/11, even after an invasion the US could not control the Iraqi crime scene. The US could never invade Iraq against widespread opposition among the major powers and then credibly claim to have found WMDs, unless these were presented to UN and international inspection. At that point, fakery would be impossible. As Ritter explained, starting from scratch in 2003, you cannot plant Iraqi-made poison gas of the 1994 vintage and credibly age it nine years in a fashion that will fool the UN chemists.
Ritter was prophetic. He had contacted every office in Congress with an ironclad case that no WMDs existed and that the invasion propaganda was demonstrably false but unfortunately was unable to sway the Iraq war vote (and also went completely unacknowledged after he was proven right on every point). To our group in 2003 he predicted that after the invasion, we would hear repeated announcements that WMD were in fact found, until most people would come to believe it even after each and every discovery was refuted and retracted. This is what happened.
The even shorter answer is itself question: Was discovering WMD a necessity for invading Iraq?
No.
Could you have predicted beforehand that it wouldn't have been, that the flow of events after an invasion would cause the question to recede and give rise to other justifications for the invasion? Yes.
"Well we're in there now, we have to stay in!"
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