Matthew Parris in the Spectator gives a reasonably good explanation of the pessimism felt by some Iraq war opponents. Note: he is a conservative. But this is the best piece I've seen laying out the case for condemnation. It does depend necessarily and heavily on retrospection. But I highly recommend it. And he's a good writer:
Stop blustering about how the choice can be defended — we know it can. Stop protesting that critics have been unfair — of course they have. Stop insisting that many false claims have been made — that is doubtless the case. Stop reciting the benefits that may yet flow — they may indeed. Cease the angry cry of ‘well, what would you have done?’ — critics may well not have the foggiest idea.
Accept that your side of the argument is stronger and better-intentioned than an unjust world has even begun to accept — for so it indubitably is. Take it as read that your critics are a shower of stupid, whingeing, unreasonable twisters — of course they are, my darling — and pause, shut your eyes and take a deep, quiet breath, and in a perfectly friendly and unaccusatory way ask yourself that eternal little question, ‘Would I do it again?’
The answer will pop from your lips before your intellect has time to register it. Of course you wouldn’t.
IMO you need to dilute Parris a little - he also vociferously opposed going even into Afghanistan where he at least could have accepted there was provocation. He didn't. In the heat of the Iraq debate, Parris put Tony Blair's support for the war down to mental illness and then sleep deprivation, respectively. Novel. But this piece definitely repays close reading, or re-reading.