Bagdad had been sacked and re-sacked dozens of times over many centuries by various factions of Islam.
ISIS does not have the strength to take Bagdad right now. However, a year from now? Who knows. In the meantime, they could make life in Bagdad completely insane with day after day of foreign religious fanatics willing to die for Allah blowing themselves up, killing hundreds if not thousands of people all while ISIS prepares to mount the final push from fallujah and ramadi. If Bagdad were to ever fall, it would be over, except for the mop up operations. I don’t know if that’s possible, but they could certainly paralyze the whole country.
One last thing, the real power behind ISIS are not the religious fanatics, they are idiot pawns. The true power behind the throne so to speak are the former Fadayeen and before that former Baathists of Sadaam. Their goals is to take back Iraq, not set up a caliphate. They want to install the next Sadaam and go back to their positions of power within the Baathist administration.
This is the same battle we were fighting in 2005 - 2007.