Sporadic artillery fire was heard around Mosul early Monday, shortly after Iraqi Prime Minister Haidar al-Abadi announced that the military operation to recapture the city from the two-year occupation of the Islamic State had begun. Iraqi army and Kurdish forces were reported on the move to the south and east of the city. Along the front line, US-led coalition air strikes and heavy artillery fire sent up clouds of black smoke.US Secretary of Defense Ash Carter described it as "a decisive moment" in the campaign to deliver IS "a lasting defeat,” but warned it would be a "difficult fight." It was in Mosul that Abu Bakr al-Baghdad proclaimed his caliphate. DEBKAfile: The assault by the Iraqi army, Kurdish Peshmerga, US special operations and allied forces, was formally scheduled for launch on Oct. 19 amid negotiations to stave off a sectarian war in its aftermath and concerns for the remaining civilians. It is widely believed that many ISIS fighters fled the city ahead of the attack.