HARI SREENIVASAN, PBS NEWSHOUR WEEKEND ANCHOR:

Just over three years ago, ISIS leader Abu Bakr al-Baghdadi ascended the pulpit of Mosul's grand mosque and proclaimed an ISIS caliphate, or Islamic state, stretching from Iraq to Syria. Now, that mosque lies in ruins, and ISIS's stronghold in Syria, the city of Raqqa, is under siege by American-backed militias.

"Reuters" reporter Stephen Kalin is in Mosul and joins me now via Skype.

So, how significant was this moment?