Here is my strategic analysis:
The Sunnis occupy the middle of Iraq, and the 'Sunni Crescent' runs along the Tigris and Euphrates rivers toward Syria. This area is the lynchpin to controlling Iraq because from there they can strike to the north and hit the Kurds, and they can move south to hit Iraq's littoral zone, which is Shiite. The critical middle is defensible due primarily to the waterways and swamps that run between them. The British recognized this, and bolstered the Sunnis in the 1920s and onward, leveraging this naval superiority to ensure that the south could never landlock the Sunnis. They were effective because they understood how to divide and rule this tribal part of the world, and with their occasional involvement, maintain a semblance of balance of power between the parties.
Iran is Shiite, and if they exert control over Iraq's littoral zone, they will landlock the Sunnis like the Kurds are today. Kuwait was established as a buffer zone to keep Iran (Persia at the time) from doing just that - Kuwait and Saudi Arabia are Sunni, and the UAE is made up of originally Persian Sunnis. So, that this strip of land south of Basra is the counterpoise to Kuwait, leaving a vulnerable Shiite population in a Sunni controlled buffer to Iran's otherwise total control of Iraq's littoral zone. Iran's move into Iraq, even at their government's invitation, puts the Sunnis in a position where they will need to fight to the death or be forced to move their entire country's goods through their enemies' lands if they want access to the sea and trade (which would not come cheap).
Interestingly, ISIS' stated objective of creating and Islamic state in Iraq and Syria is a sort of religious highjacking of the Baath party's aim to create a secular state in Iraq and Syria in the late 1930s through the 1950. The Baath party was Saddam's party (Iraq branch), and is currently Assad's party in Syria. The Baathists wanted secularism and unity, but once each party took power in Iraq and Syria, they splintered but cooperated. Their aim was to bring democracy to their countries, which they did in middle eastern style (one party elections). So, they were the closest thing to democracy and secularism that part of the world had seen up to that point since the 'Young Turks' transformed Turkey after its defeat in WW1. We bombed the hell out of them for it.
Interestingly, the difference betwen Sunni and Shiite basically comes down to this: The Shiites believe that the leadership of the Muslim world should trace its bloodline to Mohammed, and the Sunnis believe that it should be elected by the Imams (who are for lack of a better analogy Bishops or Patriarchs).
So, the Sunni ISIS is currently the most democratic institution in Iraq, but since they are fucking crazy, we will have to bomb them.
This is nothing more that an army that could not cope with Assad, retreating to a secondary position, where they will be surrounded and butchered. They want to go to heaven, and the Kurds, Shiites, Iranians, Russians, and Western forces will oblige them from all sides. Happy happy all around, since we can now potentially bury the hatchet with Russia and Iran if a western coalition fights alongside them.
Who knows, once it all settles down, maybe Russia will even leave Ukraine alone.
That said, the cost of knocking them out will be to piss off the Sunni world, which is more ardent than the Shiite, and controls the vast majority of the oil in the region.