After months of intensive preparations, the advance toward Mosul has begun. Mosul is in northern Iraq on the Tigris River, which flows south through the city toward the town of Qayyarah, the site of a strategic air base. The city, surrounded by the Nineveh plains on the north and east and flanked by the Badush and Atshan mountain ranges to the west and northwest, is near several key roadways. Highway 1 runs through the desert north toward Mosul before proceeding west between the Badush and Atshan mountains. Highway 80 heads toward the city through the Nineveh plains from the south and east of the Tigris, and Highway 2 leads west from Arbil to Mosul before heading north to Dohuk. Before the Islamic State seized it in 2014, Mosul was Iraq’s second-most populous city. The effort to recapture it will be the largest urban operation yet for Iraqi forces.

Currently, the city is tucked in a salient surrounded on three sides. Kurdish peshmerga fighters are positioned 15 to 20 kilometers (roughly 9 to 12 miles) to the north and east of Mosul, and they hold all of the mountainous ground that overlooks the Nineveh plains. The front lines, filled out with the Iraqi security forces who have been making their way up from the south, lie about 50 kilometers south across an arid and depopulated region. To the west is the last remaining link between Mosul and the rest of the territory the Islamic State controls. Since a Kurdish offensive in November 2015 cut key roads heading west — including the main highway between Mosul and Raqqa — the Islamic State’s supply lines to Mosul now rely on secondary roads through the mountains.

The battle will take place in phases, separated by considerable pauses. In the first phase, the advance to Mosul, the coalition will have two principal objectives: to tighten the perimeter around Mosul, placing the city under siege, and to start preparations for the breach of the city. From their elevated positions, the Kurds will try to advance down through the Nineveh plains and toward Mosul. In doing so, they will not only tighten the perimeter around the city but also divert the Islamic State’s attention and resources and open alternative lines of advance into Mosul. The bulk of Iraq’s manpower will move toward Mosul from positions in the south along highways 1 and 80 on either bank of the Tigris. Advances toward the Adaya, Atsan and Zambar mountains are also likely, putting pressure on the Islamic State’s last supply lines to Mosul. Secondary operations in the north could also figure prominently in the overall battle, forcing the Islamic State to dilute its forces and send reinforcements to other areas, depriving Mosul of additional units.

Iraqi security forces will also have to decide whether Mosul should be fully or partially encircled in the first phase. In previous battles to seize key urban areas from the Islamic State in Iraq and Syria, advancing forces have made a deliberate choice to leave an escape route open. The reasoning behind this is that fighting the Islamic State in the open is far preferable to fighting the group in the confines of a city. Not only is urban fighting more difficult, but it is also destructive, something the coalition would rather avoid as much as possible in Mosul, given its large population and historical importance. At the same time, the Islamic State has used the gap not to withdraw but to bring in reinforcements. In the initial advance to Mosul, the coalition will likely leave an escape route open. If the Islamic State does not withdraw, however, the Iraqi security forces and peshmerga will be quick to close it.

But as with any difficult and complicated battle, much could go wrong in the expansive operation. Stratfor will be tracking the battle closely as it evolves and moves toward Mosul.