This is the next phase of our rape of Iraq. At the outset I would note that PARTITIONING a sovereign state is a violation of the United Nations Charter and a warcrime. We'll say no more about that though since the United States clearly doesn't recognize any international laws or treaties as binding upon itself.
1) We failed to set up a unified client state. The Shias are closer to Iran than they will be to us. The Sunnis can't accept losing top dog status and will fight to deny control to the Shias. The Shias themselves have helped initiate partition by ethnically cleansing major cities of Baghdad and Basra as well as provinces.
2) The Saudi Royal family are adamantly demanding that we support the Sunnis in the western province, the same folk who did the most to kill us after our invasion. We are complying with Saudi wishes, as predicted by Sy Hersh's reporting on the "Sunni Realignment" of our M.E. strategies. They (the Saudis) demand a buffer state between them and the soon-to-be-aligned-openly-with-Iran Shia state of Iraq. The Sunnis of Anbar regard Iraq as lost now, but will never stop resisting any attempt to impose control over their home province from the Shia led central government.
3) The Shia dominated national government in Baghdad can never allow Anbar province to go its own way. The Kurds are long gone down the road of secession, but Anbar is too important.
As you can see in this map, the eastern reaches of Anbar province push right to the suburbs of Baghdad. No state on earth would allow its territory to split away so close to its capital. A Western nation invasion force would be parked literally on its doorstep and could surround the capital in a few minutes. A couple hours later and that invasion force can drive to the far eastern border of Iraq and will have split the country in two right in its middle. There's such a force sitting in Anbar right now. Today's US occupation force in Anbar will be tomorrow's re-invasion spearhead. (This is what the cryptic expression "residual force" means in the bleatings of our candidates). Letting Anbar go its own way would be an intolerable security threat to any country on Earth, and no country on Earth would let it go without a fight to the end.
And in three steps we have assembled all the ingredients needed for a civil war--a classic, textbook civil war, not a maybe it is civil war, and maybe it's not, depending upon interpretations.
Thanks to the ever evolving meddling of the United States, this conflict is becoming a sectional as well as a sectarian blood letting.