Patrick Poole has some thoughts on the matter. The first two points (Please read all links in the excerpt below):
1) ISIS will not be claiming to the be the Islamic State, they will BE the Islamic StateHERE is the entire essay. Worth your time.
Symbolism doesn’t matter much to your average post-modern Westerner, but it still does in the Islamic world, and the capture of Baghdad will hold enormous value. For 500 years Baghdad was the seat of the Abbasid caliphate, and its fall to ISIS would allow the terrorist group to reclaim that mantle. Such an event will electrify the Middle East and beyond, with many Muslims holding firmly to the belief that the abolition of the Ottoman caliphate in 1924 by Ataturk was one of the key contributing factors in the decline of the Muslim world over the past century. No amount of State Department hashtags or tweets, or pronouncements by Sheikh Barack Obama and Imam John Kerry that there is nothing Islamic about the Islamic State, will be able to negate any claims by ISIS to be the revived caliphate.
2) The Great Reconciliation between jihadist groups will begin
Much of the Obama administration’s anti-ISIS efforts have been trying to leverage other “vetted moderate” groups in Syria against ISIS, with some “smart set” thinkers even advocating engaging “moderate Al-Qaeda” to that end. We are already seeing jihadist groups gravitating towards ISIS, such as the announcement this week by Pakistani Taliban leaders pledging their allegiance to the Islamic State. Other groups of younger jihadis are breaking away from Al-Qaeda franchises in North Africa and defecting to ISIS. Despite bitter rivalries between ISIS and other jihadist groups in Syria, namely Al-Qaeda’s official Syrian affiliate Jabhat al-Nusra, these other groups will be hard-pressed to deny ISIS’ caliphate claims if they do take Baghdad. In that part of the world, nothing succeeds like success. If Baghdad falls, jihadist groups, some of whom have been openly hostile or remained neutral, will quickly align behind ISIS. And the horrid sound coming out of Washington, D.C., will be of foreign policy paradigms imploding....
The words of William Butler Yeats immediately come to mind:
The darkness drops again but now I know
That twenty centuries of stony sleep
Were vexed to nightmare by a rocking cradle,
And what rough beast, its hour come round at last,
Slouches towards Bethlehem to be born?