Whose fight in Iraq?

Charles Krauthammer thinks about the end game in Iraq. Right now, our troops are engaged in an offensive to maximize the opportunity of the Sunnis to participate in the election. If they do not (or if they reject the outcome of the election) the civil war that has already begun will be “institutionalized.” At that point, Krauthammer seems to say, we will need to turn most of the fighting over to the Shiites and the Kurds.
I’m not a blind adherent to the pottery store view of Iraq — “you broke it, you bought it.” And I certainly agree that we should not lead the fight in Iraq indefinitely. However, I think that, in Krauthammer’s institutionalized civil war scenario, our national interests would require more engagement than Krauthammer seems to envisage.