I believe the main reason was Sadams ability to facilitate proliferation of WMDs to terrorist (state sponsored). A third party strike in which he could have plausible deniability in the eyes of the UN and Europe. If nukes were not around I would not of worried about Sadam as much. We will be struck by such a weapon IMO whether the Iraqi invasion occurred or not. At least Iraq will not be accountable now.
Socialism rewards mediocrity. Communism enforces it.