The country IS going to divide into three. Even Petreus thinks so (he just won;'t tell anyone officially). So what we have for US forces is essentially what we have in Bosnia, a PERMANENT troop presence to keep the faction apart. And in the case of Iraq, to also keep an eye on Iran with and to project force and influence into the Caspian-Aral sea natural resource district.
it will not divide into three parts. Turkey and Iran will not allow the kurds a homeland - simple as that...