Legally it's about proving Saddam's possession of WMD and about demonstrating that he is not serious about disarming.
Legally, it is not even about having a militarily useful/deployable WMD capability.
Morally, however it has to be about possession, capability AND (not and/or) intent. For this to be morally right, surely Saddam has to represent a credible threat, otherwise he'd be behind North Korea in the queue?
If Iraq has WMD but has no means of using them it's hard to justify the hurried timescale that Bush is pushing for.
If Iraq has deployable, useable WMD and is not serious about disarming, then the case for war is much stronger, though if he doesn't have an intention to use them Iraq is morally no different to the USA, the UK, India, Pakistan, or Israel, all of whom have WMD and have no intention of disarming.
Because Iraq has a proven record of aggression towards its neighbours, and because of the nature of the regime, a moral case can be made that it is in humanity's best interests that Saddam is forced to disarm. But if this is the case, then it must be up to the World (and not the US acting autonomously) to achieve this.