I think all the discussion about revisionism and WMD's circumvents and neglects the more important point: How did we screw up so badly in Iraq?
Yes, there are and were legitimate reasons for going in (I don't consider WMD's to be in the top 2, see On Iraq), but how did the US allow such a weak military plan to carried out? We had no postwar planning, not enough soldiers, very little international support, and thus set the stage for the situation we are in, where we have, despite being on the right path, thusfar done more, judging by numbers, not liberties, harm than good.
To make it worse, this is all in stark contrast to Gulf War I, where everything went smoothly in accordance with the Powell Doctrine, which was entirely neglected in GWII.
Now the very concepts of nation building, regime change, and aggressive warfare stands the chance of being maligned and shunned in the American political atmosphere for decades to come. And not because they are bad concepts, but because they were badly implemented.
Whose fault is this? Personally, I say primarily the executive branch for being hugely incompetent, secondarily congress for trusting the executive branch, and finally the American people for their unwillingness to sacrifice (thus put more effort into this war).
More importantly, how do we fix it? We funnel all the dissatisfaction with the war in Iraq towards productive ends, ensuring the competence of our warmakers in the future, instead of trying to hinder them in their mission today. The senate's recent action may be a step towards ensuring that competence. Or it may be a step toward authenticating withdrawal. Seeing our nation's and congress's (in)ability to sacrifice, the latter is looking more likely than the former.