I said in an earlier post that arguing about the rights and wrongs of invading Iraq amounts to little more than debating history; the important issue is what we should be doing now. We're where we are, and we can't turn the clock back.
But history is also important, not for political point scoring, but because we (hopefully) learn from it. And if mistakes were made in the way we decided to go to war we need to identify them. The current threat the (very unpleasant) government in Iran poses means there is a real risk that we will face a similar scenario; our actions at that point need to be informed by an acceptance of the tragic consequences of the Iraq war and a recognition of our moral culpability for them.
We do have a moral responsibility to do more than stand by in the face of evil; but our Iraq venture has been a disaster that we mustn't repeat.