I think invading Iraq wasn't a bad move in the circumstances, given the suffering of the people. But (1) we ought to have done the job properly in 1990, (2) should have made the argument 'we're doing this on humanitarian grounds, because the suffering of these people is intolerable' rather than 'we've some intelligence about WMD', (3) proper plans should have been made to avoid a vacuum in governance/avoid the insurgency developing before the invasion started, (4) in general proper thought should be applied to military action to avoid repeating mistakes a la Vietnam.

My other, rather cynical justification for supporting the intervention in Iraq is that it's controlling George Bush (and the US in general). It's gone 'wrong' now, so the US will be less happy to intervene in the same way again, because of the personal/financial cost. I don't trust the US to get intervention right and I want them to be more cautious and thoughtful about it. Iraq has sort of accomplished this (ie. Congress passing bills in an attempt to get the troops brought home soon).