Just adding to the mix that SH not only had, but had used, chemical WMD in the past. And I seem to recall he was bragging about having similar at the time. Nobody at the time could be sure he didn't have a well-hidden stash. It certainly wasn't beyond the realms of possibility, nor solely the opinions of Blair and Bush, which is why the UN sent weapons inspectors to have a look.
One of the principal arguments was to let Blix have more time for a thorough search before going to war. Blair convinced the House against giving more time using the threat that an imminent attack was possible, which came from the sexed up dossier and turned out to be false.
There is a lot of rewriting of history about how everyone on the march and indeed in the whole country/world knew the WMD story was a complete lie. I sometimes wonder how people would look back on that period if a few cylinders of chemical weapons had been found. It would still, probably, have been about oil but the flimsy pretext would have had more solid foundations.