IRBIL, Iraq (AP) — Iraqi forces appear poised to launch their most complex anti-ISIS operation to date: retaking the country’s second-largest city of Mosul. While the country’s military has won a string of territorial victories that have pushed the Islamic State group out of more than half the territory the group once held, some Iraqi officials worry that the Mosul fight has been rushed and if the city is retaken without a plan to broker a peace, it could lead to more violence.

HOW WE GOT HERE

Mosul fell to the Islamic State group in June of 2014, when the extremist group blitzed across northern and western Iraq, overrunning nearly a third of the country and plunging Iraq into its most critical political and security crisis since the U.S.-led invasion in 2003.

The most recent string of territorial victories for Iraqi ground forces have been in the country’s west. Iraqi forces retook the city of Ramadi in late 2015, followed by a number of towns and villages along the Euphrates River valley and then Fallujah in June. This allowed Iraqi forces to weaken the group by cutting supply lines used to ferry fighters and supplies between territory held in Syria and Iraq.

TROOP BUILDUP

Iraqi forces began moving into Nineveh province to surround Mosul in July, when ground troops led by the country’s elite special forces retook Qayara air base south of the city. Thousands of Iraqi troops are now massing there ahead of the planned operation. Iraqi troops are also being deployed east of Mosul in the Khazer area, along with Kurdish Peshmerga forces, and to the north of the city near the Mosul Dam and Bashiqa areas.

THE PLAYERS

In addition to the Iraqi army, Kurdish Peshmerga forces, Iraqi special forces and Sunni tribal fighters, Shiite militia forces are also expected to play a role in the Mosul operation. The role of the Shiite militias has been particularly sensitive, as Nineveh is a majority Sunni province and Shiite militia forces have been accused of carrying out abuses against civilians in other operations in majority Sunni parts of Iraq.

A very small number of Turkish troops deployed for over a year in Iraqi territory at a base north of Mosul have caused a recent spike in tensions between Iraq and Turkey. Iraq has repeatedly called for the Turkish forces to withdraw, claiming they entered the country without the permission of the central government. It’s unclear if they intend to play a role in the operation to retake Mosul; Turkey’s president has said they cannot be excluded. Shiite militia fighters have said they are violating Iraqi sovereignty and have vowed to expel them.