Iraq in Theory and Practice
I'm broadly sympathetic to the non-interventionist critique of the Yglesias thesis - namely, that Matt's book is trying to draw a bright line between Bush-style crusading neoconservatism and liberal internationalism, when they're actually both aspects of the same hawkish and interventionist spirit that has run through American foreign policy for generations now. To Michael Brendan Dougherty's points, though, I do think that the paleocon lens tends to obscure some very real distinctions between neocons and liberal internationalists: the two worldviews do have significant commonalities, but there are differences as well, which manifest themselves in the sort of interventions the two groups tend to end up championing. (You'll rarely hear, for instance, many liberal hawks waxing eloquent about how we must prepare for war with China.) Thus it isn't quite so outrageous as Dougherty suggests for Matt to present the invasion of Iraq as an "isolated freakout" on the part of the liberal foreign-policy establishment. Yes, there was some overlap between Clintonian hawks and PNAC signatories in the 1990s, and yes, the whole foreign policy establishment was technically committed to "regime change," but the neoconservatives were always vastly more interested than the liberals in the cause of toppling Saddam, and without the impetus of 9/11, things probably would have stayed that way. (Likewise, had Al Gore been President instead of George W. Bush, it's possible that the U.S. would have still invaded Iraq ... but I'm not sure it's all that likely.)
That being said, I do think that the ease with which many liberal hawks who would have been cool to the idea of invading Iraq circa 1999 went over to the interventionist position after 2001 suggests a deeper problem with Matt's attempt - or any attempt - to build systematic theories for international engagement: Namely, that unless you're a very stringent non-interventionist (or a pacifist), no matter what theory of foreign policy you choose, you'll always be able to find justification within the confines of that theory whenever a particular intervention seems like a good idea. In this vein, I sometimes think too much of the debate over the Iraq War has been bogged down by arguments over theory - by Christians arguing over whether just war tradition accommodates the invasion; by liberals arguing (sometimes with themselves) over whether it fits within the Truman paradigm, by everybody arguing about neoconservatism's place in American political history - when to my mind the chief lessons of the war have to do with issues of prudence and practicality, and more specifically with the question of when the costs of war, in lives and treasure, are worth the risk involved and the gains that might be won.
Put another way, I don't think the lessons of Iraq necessarily discredit liberal internationalism, or realism, or neoconservatism, or any of the many theories of U.S. engagement with the world that were invoked to justify support for the war. I don't come away from the events of the last five years convinced that we should never intervene abroad on purely humanitarian grounds, or that we should never go to war without an international body's authorization, or that the whole of American Middle East policy since 1991 (or 1945) has been discredited, or even that we should never launch wars of pre-emption. I come away from them convinced of a point that's simultaneously narrower in scope, but more universal in its application: That whatever theory we take as our guide to international affairs, we need to proceed with greater caution than America displayed in the aftermath of 9/11 about the efficacy of military force, and the costs and consequences of using it.