There's another issue that also doesn't come up much: there's a certain fraction of the population which wasn't in favor of the Iraq war, and they like saying "I told you so" but in fact they didn't. Most of their argument was meaningless slogans. Iraq wasn't about blood for oil. And there wasn't an intrinsic moral problem with it.
I'd be careful about that kind of thinking. When you get something wrong, dismissing the people who got it right as low status and wrongheaded is a convenient way to avoid updating on the evidence. A bit under half the US population opposed the Iraq war, including many prominent people (Senators, opinion writers, policy analysts, etc.), but you seem to be focusing on a small fraction of opponents ("no blood for oil", war is intrinsically wrong) who mostly opposed the Afghanistan war as well, which puts them in much narrower company.
Arguments made against the Iraq war at the time (many of which are at least hinted at in this short piece by Dahlia Lithwick) included:
But the simplest case is just that war is a destructive, costly thing, so if you're going to get involved in one you'd better have a damn good reason and you'd better get it right. The war needs to accomplish something at least as big as its cost, which means the bigger the war the higher the standard. Iraq didn't meet that standard at the time, still less once it turned out that there were no nuclear/chemical/biological weapons. Most of the arguments were variations on that theme, demonstrating the high costs of this particular war or poking holes in the various reasons given for the war.
Noah Millman wrote:
Link (which includes additional good retrospectives) thanks to Ampersand.
This article may have more political content than is suitable for LW-- if you'd rather discuss it elsewhere, I've linked it at my blog. I've posted about it here because it's an excellent example of updating and of recognizing motivated cognition even if well after the fact.