The long-envisioned assault on Mosul, the hugely symbolic Iraqi city where Abu Bakr al-Baghdadi declared ISIS' self-styled "caliphate" more than two years ago.
Not only is Mosul Iraq's second largest city, it is also a key to ISIS' claim to have installed for the first time since the collapse of the Ottoman empire a genuine caliphate with millions of subjects and territory that once encompassed lands the size of the United Kingdom.