The fact that Iraq is a clusterfuck doesn't demonstrate that preemptive war is wrong any more than WWII demonstrated that wars using Sherman tanks are right. It's the wrong unit of analysis.I'm not sure what the point in having this debate is. On the one side, you've got the people who predicted this war would be a disaster. This includes not just the left, but many conservatives and military leaders. Did every single pundit get every single facet of this disaster right? I certainly didn't -- I expected the fall of Baghdad to be a bloodier affair, and was surprised at the initial light casualties of the invasion. Nevertheless, I was fantastically more correct about this war than, say, George Bush or Donald Rumsfeld.
Crucially, I expected the Iraqis to resist a foreign occupying army that didn't understand or respect them, and for that resistance to eventually turn violent. That's all you needed to know beforehand. Invading armies are resisted by the invaded. I don't know why this is so difficult to understand. This doesn't make me a rocket scientist, it makes me someone who didn't sign up for the Iraq Lobotomy Drive of 2003.
I'm not the icon of anti-war opinion in 2003, of course. The more appropriate unit of analysis is the opinion of anti-war sentiment in general. The people who believed US forces faced a long occupation of a hostile populace were correct, and totally different from the people who believed that the US would be greeted as liberators. Here, a rational society would privilege the opinions and perspectives of the anti-war people, because (duh) they have been proven correct by events.
On the other side, you've got the Bush administration and it's water-carriers (too many of whom are nominally on "the left") who have been absolutely wrong about absolutely everything since this war began. I'm seriously worried about the day that Bush declares that the sun rises in the east. Why? Because on that day the Earth's rotation will clearly reverse itself, as "the Bush Administration is wrong about everything" seems to be a physical law of the universe.
Given that Martin Luther King day just passed, let me pose a question: MLK was notoriously sexist, refusing to grant women a serious role in his organization. On the issue of gender equality, MLK was clearly wrong. Does that narrow failure mean that his movement was wrong, and should be ignored? That they deserve no credit for dismantling the US apartheid system? Does a failure to be correct in every detail mean that he can be safely ignored by history?
Well, if you're a liberal hawk, yes, that's exactly what it means.