Battle of Mosul: How Iraqi forces defeated the Islamic State

Iraqi Prime Minister Haider al-Abadi declared the city of Mosul liberated from Islamic State control on Sunday, a watershed moment in the U.S.-backed Iraqi military campaign against the extremist group. The battle, which stretched nine months, was marked by fierce urban combat, the discovery of Islamic State atrocities and the use of small drones, “Mad Max”-style suicide vehicles and other tactics that prompted U.S. and Iraqi forces to adapt their operations as they fought.

The primarily Sunni city, with an original population of more than 2.5 million, was the biggest prize claimed by the Islamic State in its violent, chaotic sweep across northern and western Iraq in 2014. Islamic State leader Abu Bakr al-Baghdadi declared a caliphate from Mosul’s Great Mosque of al-Nuri, which the Islamic State blew up June 22 as Iraqi forces closed in.

The fight to retake the city in effect turned into two major battles: one for the eastern half of the city and one for the more tightly congested western one, with the two sides separated by the Tigris River.

The Iraqi and Kurdish peshmerga forces launched their offensive on the eastern half of the city on Oct. 17, 2016, pushing in from Mosul’s north, east and south under heavy resistance. The peshmerga moved out from their long-held defensive positions near Mosul's periphery and helped establish a cordon near the city's suburbs. From there, Iraqi forces began pushing into the city's eastern reaches.

By Nov. 1, Iraq’s elite counterterrorism service had captured a strategically important television station in eastern Mosul and the coalition had a strong foothold in the city. It was the first time the elite Iraqi forces, known as the CTS, had distinguished themselves in the campaign, though it would not be the last.

The U.S.-trained CTS unit was once a hard-hitting raid force used to go after al-Qaeda targets at the height of the Iraq War. In the years since the Islamic State's rise, however, the unit slowly transformed into a more conventional force. By the end of the Mosul campaign, 40 percent of the CTS force had been killed or wounded, according to Pentagon documents, having been used time and time again to clear some of the city's most dangerous neighborhoods.

Iraqi forces captured Mosul University from the Islamic State in January, taking away another militant base of operations.

By Jan. 24, Iraqi forces had captured the entire eastern side of the city, while civilians in the city suffered through shortages of clean water and food. The more difficult battle, as expected, was the fight for western Mosul – with its narrow streets and tightly packed Old City.

There, the Islamic State was especially dug in and Iraqi forces -- having nearly destroyed the five bridges separating the eastern and western sides, were forced to ford the Tigris River downstream of the city and approach the opposing bank from the South. Using lessons learned from the defense of east Mosul, the Islamic State had prepared complex fortifications, including trenches dozens of feet deep around the periphery of the western side to slow the Iraqis’ advance.

In February, the offensive for western Mosul began. Iraqi forces, led by federal police units, drove into the underbelly of the city, capturing the long-defunct Mosul International Airport before colliding with a ring of stalwart Islamic State defenses near al-Nuri mosque and the Old City.

As the southern front stabilized, turning much of that part of that city section into no-man’s land replete with frequent car bombs and skirmishes, Iraqi army units swung to the west and advanced. The move established a new axis, allowing Iraqi counterterrorism forces to push into a number of of neighborhoods in the western part of the city.

By late April, the Iraqi military declared it had captured western Mosul’s largest neighborhood, al-Tanek, while fighting its way toward the Old City itself. By May, the Islamic State held less than five square miles of territory, but tens of thousands of civilians were still caught in the middle of fighting. That was perhaps most notable May 25, when the Pentagon said that a coalition airstrike had inadvertently killed more than 100 civilians because the explosion detonated a cache of Islamic State explosives, leveling an entire building. The fight in western Mosul saw civilian-casualty claims skyrocket as conflict monitors struggled to keep pace with the volume of U.S.-led airstrikes and the frequent use of mortars and artillery that often landed indiscriminately among the city's trapped residents.

A new offensive in early May, spearheaded by Iraqi army and counterterrorism troops, effectively surrounded the Old City and a handful of nearby neighborhoods. In early June, Iraqi forces converged on the Old City. On June 21, with Iraqi special-operations troops closing in, U.S. and Iraqi officials said the Islamic State had destroyed al-Nuri mosque, a landmark that had stood in Mosul for more than 700 years.

The terrorist group blamed a coalition airstrike on the mosque’s destruction, while footage taken by nearby Iraqi units suggests that it was destroyed by explosives planted within. By early July, the Islamic State held only a few hundred square yards along the Tigris, where hundreds of militants and thousands of civilians still remained.

The Iraqis’ bloody advance into western Mosul was unique for a number of reasons. For one, Iraqi troops were heavily dependent on U.S.-led airstrikes to move into the city. The strikes kept casualties manageable but also turned the offensive into a grind. Iraqi troops would take a block at a time, often staying within the confines of their armored vehicles trading shots with the militants.

As the Iraqi troops engaged their enemy, they would feed information back to U.S.-led command centers nearby that would begin turning the information into targets for aircraft circling overhead. If the strikes dislodged the Islamic State fighters, the Iraqis would advance, if they did not, the shooting would continue until another round of air support could be called in. In a number of instances, the Islamic State took advantage of this methodic approach for clearing the city, moving under the cover of poor weather to launch small-scale counterattacks on the Iraqi forces. While the raids never amounted to a serious loss in territory, it gave the militants breathing room as they prepared their next round of defenses.

The advances were further slowed by the number of civilians trapped in the city. Each new push into Islamic State-held portions of the city would release a torrent of refugees that would snake their way back from the front lines during the fighting. In the Old City, this problem was further exacerbated by the density of the neighborhood and its winding streets and tiny alleyways. Here, fighting was at close quarters as militants and government troops often exchanged fire within hand-grenade range.

Canadian army Brig. Gen. D.J. Anderson, a senior officer in the U.S.-led coalition, predicted Thursday that after the complete fall of Mosul, operations against the Islamic State will continue in other parts of Iraq.

“The inevitable and imminent fall of Mosul does not mean that Daesh is defeated,” he told reporters in the Pentagon in a conference call from Baghdad, citing the city of Tal Afar and areas of western Anbar province as examples. (Daesh is an Arab acronym for the Islamic State.) “The sequencing . . . of course, will be up to the government of Iraq.”