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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.

The March 17 strike sparked calls from Iraqi and world leaders for greater protection of civilians. The U.N. called on the Iraqi government and its partners "to undertake an urgent review of tactics to ensure that the impact on civilians is reduced to an absolute minimum, in full accordance with international humanitarian law." Following the launch of the U.S. investigation into the incident in late March, Mosul advances ground to a near halt for weeks. The following month, the coalition dropped 38% fewer munitions on Mosul, according to London-based monitoring group Airwars.

COALITION ROLE and IRAQI FORCES

The U.S.-led coalition steadily increased its footprint in Iraq in the lead-up to the operation to retake Mosul. The U.S. fight against IS was initially described as one that would not involve "boots on the ground," and both U.S. commanders and politicians pledged that American forces would not be dragged into fighting another war in Iraq.

But U.S. troops are now stationed at a number of bases in the Mosul area and the Pentagon has acknowledged that hundreds of U.S. soldiers operate inside Mosul backing Iraqi ground troops in the fight.

However Iraqi forces are still taking the lead on the ground in Mosul and after more than seven months of grueling fighting, Iraq's best-trained fighters are depleted. Iraq's special forces suffered significant casualties in the fight for eastern Mosul and in the first weeks of the push on Mosul's west, Iraq's federal police relatively inexperienced in urban combat took a lead role in one of the city's most difficult districts.

After the advance led by the federal police stalled, the Iraqi army's ninth division an armored division not immediately suitable to fighting in urban environments was brought in to assist.

THE END GAME

Iraqi forces announced the beginning of the final push on the last IS strongholds in Mosul Saturday. Coinciding with the beginning of the holy month of Ramadan when Muslims fast during the daylight hours, advances have so far been slow. Iraq's military has described them as "cautious" and the coalition has warned that the most difficult battles in the Mosul operation could be ahead.

Mosul's Old City is an ancient district of narrow alleyways and tightly packed homes where the U.N. estimates more than 100,000 people are being held by IS. Iraqi forces dropped leaflets over the area Friday, telling civilians to flee in an effort to facilitate the military operations.

According to residents interviewed by The Associated Press, many families are trapped in their homes by IS with the doors welded shut; the extremists have also repeatedly targeted fleeing civilians with small arms fire and mortars. Aid groups have warned that a mass exodus of thousands of residents would likely be chaotic and deadly as the area lacks safe passageways forcing those who flee to cross front-line clashes.

Read more:
AP Explains: Iraq's slow grind to retake IS-held Mosul - Fox News