If we replace a war against Iraqis Sunnis with a war against the Shiites, we will not only have suffered a serious, self-inflicted operational defeat, we will endanger our whole position in Iraq, since our supply lines mostly run through Shiite country. I say such a defeat would be self-inflicted because Shiite attacks on Americans in Baghdad seem to be responses to American actions.
In dealing with the Shiites, we appear to be doing what spurred the growth of the Sunni insurgency, i.e., raids, air strikes and a "kill or capture" policy directed against local Shiite leaders. Not only does this lead to retaliation, it also fractures Muqtada al-Sadr's Mahdi Army as he tries to avoid fighting us. Such fracturing works against, not for, the potential re-creation of an Iraqi state.