Oldfag here.
The WMDs were pretty credible at the time. A bunch of countries
>France
>Israel
>UK
All had intelligence that backed up WMDs. In addition to that, Iraq kept bluffing about having a nuke. They kicked out inspectors, and basically acted like they were going to fire a nuke any day now if America invaded.
In retrospect it was bullshit, but so was first marriage.