In my well-documented view Saddam had to go because (1) our presence in Saudi Arabia was an Al Qaeda recruiting tool; which is intimately tied to (2) our 12 year war against Saddam had to end with victory, if UN resolutions were to have any substance (and his sanctions evading routines, including Oil For Food, were working); (3) tyrannies are the grounds that breed terrorists, and that is especially true in the Middle East; (4) rogue states want WMD– they are the most-likely supplier of WMD to a terror organization, and post-9/11 we had to show tyrants we mean it about stopping WMD proliferation; (5) Iraq, with its water, source of capital (oil), and comparatively well educated and motivated populace is the prime place to affect democratic change in the politically dysfunctional Middle East.
SPELLING OUT YOUR OBJECTIVE FUNCTION. Chemical weapon shells have been turning up in Iraq for some time. That information has been recently released. Austin Bay, in reporting the news, provides a cogent defense of the cassus belli.