Mosul is the second largest city in the Middle Eastern nation of Iraq. It's also a stronghold of the ISIS terrorist group, which took over Mosul in 2014. Defeating ISIS here would be a major setback to the terrorist but it's not easy. There may be only a few thousand ISIS fighters left in Mosul, but they're using tunnels, roadside bombs, explosive traps and guerilla warfare in the fight.
They're up against force of tens of thousands Iraqi troops supported by Americans and allied airpower, plus, ethnic Kurdish fighters known as the Peshmerga. They're all working to push ISIS out.
The battle for Mosul has been going on since October. It was expected to take months and it is. The reasons why are clear in the struggle to take over one key part of the city.
NICK PATON WALSH, CNN SENIOR INTERNATIONAL CORRESPONDENT: Day four, and perhaps the biggest push yet from the north into the plains around Mosul. Trying to dislodge the determined and deranged remnants of ISIS, but the Peshmerga backed with staggering air power.
But now common sight of American special forces, who the Pentagon says are advising, not assaulting position in front of the attack. The work was slow, destructive. Begging the question, what becomes of the wreckage under new masters?
Suddenly, in the sky, a hail of bullets. They've spotted a drone. Trace rounds dance around it and finally take off its nose.
ISIS used them to spot targets for artillery, even drop small bombs. This one tumbles down. Its wreckage picked over. It's still unclear whose it is.
Yet progress down the road Khorsabad is agonizingly slow.
This is a source of so much of the fighting this morning, but still full of ISIS. And, in fact, we've heard that Peshmerga have listened to those militants on their radios this morning discussing how they should wait and only launch a counterattack once the Peshmerga are inside.