Of course the various intelligence services around the world believed that Saddam might have WMD's- it's what they wanted to believe, and more importantly, what the hawks in the Clinton admin wanted them to believe. So a continuous stream of speculation was represented as fact, leading up to the Bush Admin's invasion, when, guess what? There weren't any... Which wasn't really much of a surprise to those who had deliberately fluffed up their reports to reinforce the prevailing popular sentiment... Of course, they had hedged those reports heavily, using phrases like might, could, indications, possibilities, dual use, and my fave, "unaccounted for".
Hans Blix claimed that the weapons inspectors' work would be completed in a matter of months, just weeks before the invasion, and their report would have put the whole thing to rest- no evidence of WMD's or related programs. That couldn't be allowed to happen by the Bush Admin's logic, so they invaded rather than waited...
After 9/11, and the Neocon admin's ruthless campaign of fearmongering and agitprop, the invasion of Iraq was a done deal, something they'd sought from the end of GW1. But now they had an excuse, a lever, a rationale previously nonexistent. GWB said "Find me a way" early in his Admin, and they did. 9/11 became the rallying cry, the justification for anything and everything the wanted, whether these things had anything to do with each other or not...
It doesn't make sense, but it doesn't have to- it's what the boss wanted, and that was enough.