The foreign policy community debate has led down some pretty interesting avenues, one of them being an interrogation of the idea that the United States has “vital interests”. The short answer is no, and that policy arguments made on the basis of “vital interests” are almost always non-sensical, and are often destructive. There are two ways to take apart the “national interest”; the first is to challenge the national bit, and the second the interest bit. The national bit is easy enough to understand, as it should be apparent that on most foreign policy questions we aren’t “all in it together”. Different groups have different interests, and benefit unequally from various foreign policy acts.
On the “interest” part, the best discussion of the term, in my view, continues to be Arnold Wolfers 1952 Political Science Quarterly article “National Security as an Ambiguous Symbol.” Responding to the efforts of nascent Cold Warriors to justify policy X or purchase Y in the pursuit of national security, Wolfers absolutely demolished the notion that the term could have any fixed, useful meaning independent of an assessment of prior values and a cost-benefit calculus of value trade-offs. It’s a remarkably important article; if you don’t have access to JSTOR, this link might work.
That said, I’m actually not sure how far the interrogation of the “national interest” concept gets us in terms of Iraq. While O’Hanlon and Pollack may have made mention of the national interest in some media fora, for the most part both of them made concrete (and wrong) arguments about how the invasion would forward some particular interest, thus avoiding the nebulous national interest justification. Indeed, I’m pretty sure that Pollack even included the furtherance of multilateral institutions as part of the reason for invading Iraq, thus suggesting that international law has a value that should be included in the US interest calculus. Some arguments for invading Iraq were quite explicit on this point, suggesting that the invasion was the only way to “save” international law and the United Nations, which was on the verge of failure because of the spiteful French.
On the whole, in fact, liberal hawks (and even some conservatives) made much more rhetorical use of international law and a sophisticated understanding of the national interest than did some opponents of the invasion. In the international relations community, “national interest” is a concept most often used by realists, who while recognizing the problems with the term still find it analytically useful. Realists, however, were among the firmest opponents of the Iraq War, which was especially notable given the fact that realists tend not to care a whit for international law or humanitarian issues.
What this all amounts to, I think, is that while the use of “national interest” as political rhetoric is full of problems, challenging the concept doesn’t do much for us in the context of the Iraq War. Proponents of the war tended to make wrong, but sophisticated, arguments that invoked particular values rather than nebulous “interest”, while at least some opponents (realists in the academic community, especially) held to the least sophisticated conception of national interest, but still opposed the war.