How can you purge the Shiite militias from the Iraqi police? By prosecuting any policeman implicated in the murder of any Sunnis not accused of any crime.

And if you don't, what kind of police force do you have? Would it be OK with you if you were targeted for being Protestant, Catholic, atheist, French, English, etc.--and not for any crime you personally were accused of? Would you trust the police? And if you were armed would you be willing to give up your weapons? I wouldn't.