In this May 11 file photo, a girl sleeps in her father's arms in western Mosul, Iraq, after fleeing from fighting between Iraqi forces and the Islamic State group. (AP photo)

By The Associated Press

BAGHDAD — Iraq’s fight to retake Mosul from the Islamic State group has been the largest and the longest operation against the extremists in the nearly three years since they overran a third of the country. Mosul is Iraq’s second-largest city after the capital Baghdad and was a key logistical and economic hub for IS when the extremists’ footprint spanned much of Iraq’s north and into neighboring Syria. Iraq’s prime minister had originally pledged Mosul would be retaken by the end of 2016, but it quickly became clear IS planned to draw out their inevitable defeat, leaving destruction and human suffering in their wake.

Below is a look at what makes the remaining battle so difficult:

TERRAIN

Mosul is a large city comprised of dense built-up neighborhoods, ancient, congested districts and agricultural suburbs. The U.S.-led coalition described the battle for Mosul as “some of the toughest urban fighting in decades.” IS snipers fire down on advancing Iraqi troops from inside bedrooms, perched on rooftops and from the minarets of mosques. Barricades erected by IS have turned residential blocks into mazes and the extremist fighters have used civilian garages to conceal massive, armored car bombs.

Initially, Iraqi forces punched too deep into Mosul too fast and suffered heavy casualties from the IS fighters who knew the terrain and had years to prepare defenses. When Iraqi forces slowed their advances to just a few hundred meters a day and coordinated moves across multiple fronts, IS defenses thinned and Iraqi forces were able to secure more victories and reduce military casualties.

CIVILIANS

When the operation to retake Mosul was launched last October, the United Nations estimated more than a million civilians were still living in the city. Unlike past urban battles against IS, in Mosul Iraq’s government asked civilians to remain in their homes in order to avoid massive numbers of displaced families requiring camps and other assistance.

Iraqi commanders said the presence of civilians inside Mosul during the fight has repeatedly slowed the pace of operations as they are unable to largely rely on airstrikes and artillery to quickly clear territory ahead of their ground forces. The U.S.-led coalition has repeatedly praised Iraqi forces for showing respect for human life in the Mosul fight, but there have been instances of high civilian casualties due to the use of artillery and airstrikes. One of the worst incidents came on March 17, where the Pentagon determined a U.S. airstrike set off secondary explosives laid by IS; the ensuing blast killed more than 100 civilians sheltering in a home in western Mosul.