There are two elements to keep in mind in assessing the sudden Sunni surge in Iraq. First, the Sunni-populated part of Iraq is virtually bereft of natural resources. The Kurdish and Shia parts in the northeast and south respectively are not. This is largely what renders partition problematic.
Secondly, the Sunni advance that began with the spectacular bloodless seizure of Mosul and moved south is not all the doing of the jihadist Islamic State of Iraq and Syria (ISIS). There are other Sunni groups in the mix of fighters: Sunni tribal elements, Baathist remnants of Saddam Hussein's army, etc.
The U.S. must be careful not to be considered as coming to the rescue of the Shiite Maliki regime in Baghdad, as this could have repercussions on the wider Muslim world, which is 85 percent Sunni.
President Obama is sending 300 Special Forces advisers to assist the Iraqi army but fortunately has ruled out the sending of ground combat troops. The provision of advisers is as far as the Administration should go. Were air strikes to follow, this would be a sign of overt military intervention by the U.S. on the side of the Shiite regime. It is a step too far for the U.S. to take.
The idea of forming an inclusive regime in Baghdad, composed of representatives of the majority Shiites and the minority Sunnis and Kurds is an excellent one, as it then might provide credibility for attacks against ISIS. But it may not work, given the level of distrust on all sides, and the no-quarter mentality in that part of the world. Ultimately it will be up to the Iraqis themselves to end this imbroglio.