In a post seemingly designed to enrage academic realists, Atlantic Monthly blogger Ross Douthat says the war in Iraq tells us nothing about theories of foreign policy:
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.
The trouble here is that efforts to weigh the costs of war inevitably involve theories of how the world works. As my Professor Steve Van Evera likes to point out, foreign policy makers can use good or bad theories to guide their actions, but if they attempt the slightest foresight, they cannot have none. In other words, there is no such thing as foreign policy without foreign policy theory.
Beyond that, the idea that the war tells us nothing about the relative merits of realism and the brand of idealism we call neoconservatism is just silly.
Neoconservatives weighed the costs and benefits of the war using their theory of how the world works -- how democracy spreads, how states react to conquest, the efficacy of military force in social transformation, and so on. Realists weighed the costs and benefits differently because realism contains competing ideas about how the world works. Realists were right; neoconservatives were wrong.
Liberal internationalists, I'd submit, were confused about the war because their school of thought is largely another brand of idealism -- one that substitutes the question of whether a war involves what they want, which is multilateral cooperation, for more probing questions about whether it is wise.