There is yet another possible explanation, and that is that he convinced himself first - IOW, wanting to believe that there were, indeed, WMD.
Here, too, I don't actually believe this; I am only proposing alternative explanations.
>No, actually it makes perfect sense in a sort of Machiavellian way. The first priority was to remove Sadam (with a line on the oil). So, use fake WMD's to convince the American people to get behind the strike. Then pretend to search for WMD's (in fact, have the soldiers search regardless of your knowing they won't find anything) so people will believe that you believed they were there. Then, tell everybody that you really thought there were WMD's - I mean why would you search for them if you knew they weren't there, and finally say the war was really about bringing democracy to Iraq anyway, so what difference does it make whether there were WMD's or not.
Difference in opinions hath cost many millions of lives: for instance, whether flesh be bread, or bread be flesh; whether whistling be a vice or a virtue; whether it be better to kiss a post, or throw it into the fire... (from Gulliver's Travels)