I think it was slightly more complex because Iraq was refusing UN inspections and because they did have a history of using illegal chemical weapons during the Iran-Iraq War.
Interviews with Hussein (after he knew he was facing death) revealed that they refused UN inspections because he was more concerned that Iran (their neighbor and enemy) would see their vulnerabilities, which he perceived as a more immediate threat than the US.
I don't know why the US was convinced that he had WMD in the first place though. Hopefully there was more to it than just out of control hype that was fed by Iraq refusing inspections.
It's a but mirky there. The use of those weapons in an international conflict against Iran was illegal under the Geneva Protocol, but not the possession thereof. In fact Iraq bought the precursors on the commercial market, mostly from the Netherlands.
And even more murkily, their use in internal operations was not illegal at that time ( prior to the Chemical Weapons Convention ).
Note that Iraq did resist inspections and didn't fully cooperate, but inspectors did get in and inspected a lot, and this was ongoing when the Bush administration brought it to a halt with the invasion.
It seems obvious that they were motivated to make a case for invading Iraq. But that doesn't make it immediately obvious to me that they completely made it up or that they weren't acting off of other intelligence.
Since we're on the subject of speculation, if I had to put my tin foil hat on today I'd say that the invasion of Iraq had more to do with Iran (and their allies) than Hussein.
All I can say to that is: lolwut. If it was obvious to me that the WMD story was nonsense, surely the top minds of the United Kingdom could have figured it out.