Sorry for not answering your question from before, I must have missed it.
"The question is when and why that decision was taken.
WMD was the reason given, prompted by 9/11, I expect you would say.
Well, 9/11 brought a sea change in many things, but was that the reason for toppling Sad-damned ?"
WMD (again) were not the only reason given for toppling Saddam. In fact, it has been US policy since 1998 to remove Saddam from power due to his connections with terrorists. His administration is widely believed to be a part of the first attempt to destroy the WTC in '93, and has been involved in various other terrorist acts since and before then. There is a multitude of information regarding these facts, here is an article that is a good start in examining these events-