The WMD thing was an issue seriously questioned by some of his own intelligence people, but Bush chose to order invasion of Iraq. I think evidence of his convoluted motivation was the subsequent invention of the so-called “Bush Doctrine.”
The general notion that Saddam had WMDs wasn’t seriously questioned by US intelligence. There was some dispute over the particular instances that the administration used to make its case to the public that Saddam had WMDs, but the overall notion wasn’t in dispute. We knew Saddam had stockpiles of chemical weapons and an active biological weapons program as recently as the mid-1990s when he kicked out the UN inspectors. There was no evidence that he had since gotten rid of them, so the natural conclusion of the intelligence services, both ours and just about everyone else’s too, was that he still had them.