Over on the wonderful normblog, Norman Geras argues passionately and persuasively in support of the invasion of Iraq, an unusual viewpoint for someone on the left. You should definitely read him if you think your own views would be seriously challenged. I doubt that I do him any justice if I summarise his position as being this: that the overthrow of a murderous fascist dictator was justification enough, and that other possibly ulterior motives by those invading were of little import set against that. To those who point out that there are plenty of other murderous fascist dictators in the world and we're not in the business of overthrowing them, he says that our failure to do the right thing elsewhere doesn't alter the fact that we did so here.
I am less clear though whether he considers that we had a moral responsibility to undertake effective 'post invasion planning' (well that's the term politicians seem to favour), and whether our failure to do so weakened the morality of invading. If the end result of our invasion is that Iraq is left in a state of civil war until another murderous fascist dictator makes his way to the top of the heap, any moral satisfaction we derive from the overthrow of the last one will be pretty meaningless.
I think that Jim Holt's lrb article is pertinent here too; he argues that US interests lie in there being continuing instability in Iraq, as this provides them with their best opportunity to secure Iraq's oil supplies. If he's right, then Britain's failure to consider the US motive for invading was wrong, because it failed to determine the (immoral) outcome they sought and might already have achieved.
My own position is that we were wrong to invade, not because there was no justification whatever for the overthrow of Saddam but because we didn't begin to consider or prepare for the entirely foreseeable outcome, let alone accept responsibility for it. The lies about WMD matter too despite what Norman says. They matter because it was the claim that Iraq represented an immediate threat to us that was the justification for invading without due consideration of the consequence.
More importantly though, I think that arguing the rights and wrongs of invading is little more than debating history right now; we are where we are and Iraq is a mess. We need politicians who take on board our present responsibility and have a clear strategy; at this point there is none and no prospect of one either, unless it's the hope that sooner or later the problem will go away if we keep our heads down long enough. Politicians who have taken us into a war (however justified) using lies and evasions are unlikely to be the ones to bring us out of it with honour.