This is an interesting article, as is this one, which the first article links to.
The two major points that stand out for me are that the number of foreign fighters in the insurgency are small (4-10%), and that most of the insurgency are Sunni Arabs, who don't want a Shiite-led government, but not Baathists with a desire to return Saddam to power.
This effectively removes two arguments that the pro-warriors often use; that the "insurgency" is really composed of foreigners and that therefore we are on the side of the Iraqis against these foreign interveners; and that we are fighting "Saddamites" who are trying to return the Baath Party to power; and thus we need not worry that there is much popular support for, or identification with, the insurgents.
What we are actually dealing with, it appears, are people from an ethnic group (the Sunni Arabs) that stands to lose in the Shiite-Arab-dominated Iraq we are creating. The Sunni Arabs do not, I should point out, stand to lose because they have some aversion in principle to democracy, but because a rival ethnic group (i.e. the Shiite Arabs, in coalition with the semi-autonomous Kurds) is likely to be put in power. And as Matt Yglesias points out, the Shiites is not necessarily less cruel or corrupt just because they were the ones out of power and are "on our side."
The point is that what we are fighting is not so much a battle between good and evil, but a battle between different ethnic groups in conflict, each seeking the system that gives their group the most power. That is why I make references to "Kurdish death squads," including in a post for which I have been criticized by other blogs and in my comments. I don't think that the Kurds or their peshmerga are evil or that the Sunni Arab Insurgency are romantic freedom fighters. I am just concerned that we are in an ethnic war where one side will have to totally crush (i.e. severly oppress, drive out, or exterminate) the other in order to end it. I am also worried that for all of our rhetoric about "promoting democracy" or "promoting human rights," we are going to wind up fighting this war the only way it can be fought successfully, that is, fighting it dirtily; and if we are going to do this I think it is important that we be forthright about it and understand what we are getting into. (I am also concerned that, whether we moderate our strategy to become ruthless enough to win or not, we will lose a whole lot of soldiers in the crossfire once a full-scale civil war develops).
Some of the hawks have been trying to avoid discussing this type of concern by pretending that the insurgency is mainly foreign inspired or composed mostly of foreigners and Baathist "dead-enders" who have no natural base of support within the country. (This also conveniently provides a pretext for expanding the war, as I pointed out here). Therefore, Iraqis of all stripes (except for the 20,000 or so who are insurgents) will cheer as we eliminate the evil terrorists, and there is no danger of civil war. Of course, if the insurgency is mainly home-grown it means that civil war is a real danger, and that taking down Syria and/or Iran will, at best, be marginally helpful to our campaign in Iraq (there is also a good possibility that doing so will make things in Iraq worse, if, say, Syria or Iran totally collapses and thousands decide to retaliate by streaming into Iraq).
In short, we are fighting a complex war, and a mostly home-grown insurgency, with an embryonic civil war going on in the background. There are no easy solutions and the lines between good and evil groups are fuzzier than we would like to think. Thought this war was a slam-dunk case of "liberation?" Thought that Iraq was going to turn into a model liberal democracy? Welcome to the real world, folks. (If, of course, you are not still clinging to your delusions).
That is all.