It was the largest city in Iraq captured by ISIL fighters.
Now, it's the only one they have left - and not for much longer, if the Iraqi army has its way.
The battle has begun to recapture Mosul and Iraqi commanders say victory will mean the end of ISIL in Iraq.
Iraq's armed forces, along with Kurdish, Sunni Arab and Shia allies, have been preparing for months.
They have the backing of the United States, which describes the operation as a "defining moment".
Will the joint force be able to kick ISIL out of Iraq altogether? And how many civilians are in the firing line?
Presenter: Laura Kyle
Judit Neurink - Author of 'The War of ISIS: On the Road to the Caliphate'
William Patey - Former British Ambassador to Iraq
Hala Al-Sarraf - Founder, Iraq Health Access Organization
Source: Al Jazeera News