Islamic State’s (IS) greatest conquest was its bold June 2014 seizure of Mosul, Iraq’s second largest city and home to approximately two million predominately Sunni inhabitants. For almost three years, IS dug in to defend this strategic stronghold and the site of the declaration of the IS khilafah (caliphate) by Abu Bakr al-Baghdadi, the self-styled “Caliph Ibrahim” (al-Jazeera, July 6, 2014). When a U.S.-backed coalition of jostling Kurdish Peshmerga, Iranian-backed Shia militias and Iraqi Security Forces made up of 114,000 troops launched “Operation We are Coming Nineveh” on October 16, 2016, they knew they were in for a bloody slog to dislodge IS fighters who had “worm-holed” the city, creating tunnels through buildings and building extensive defensive barricades.  They were not mistaken in this assumption, and for nine months the allies battled their way first through modern east Mosul, then through the warrens of older west Mosul on the opposite side of the Tigris.
The fighting cost thousands of lives on both sides and saw IS deploy suicide bombers, off-the-shelf drones armed with IEDs, snipers and human shields. However, with the tactical use of U.S. artillery in the form of HIMAR (satellite-guided rockets with small warheads) and “air artillery” guided to their targets by local and American Joint Terminal Attack Controllers (JTACs), the allies fought their way to the historic al-Nuri Mosque, where al-Baghdadi had declared his caliphate three years earlier. On July 10, Haider al-Abadi, the Iraqi prime minister, declared victory over the “false Caliphate,” and the Iraqi half of al-Baghdadi’s theocracy all but collapsed, even as the terrorist group mutated back to its original insurgent roots to continue its reign of terror.
The Prelude to Mosul
On June 6 2014, IS launched a raid that was initially intended to be a jihadist version of the 2003 American “thunder run” into Baghdad. Their goal was to strike numerous targets with massive suicide bombings as a bold statement against the Shia government of then Prime Minister Nouri al-Maliki (al-Arabiya, June 12, 2014). But when the much larger Shia-dominated government force defending this Sunni town collapsed and fled with very little resistance, the surprised IS fighters seized control of the city, their greatest conquest thus far.
It was obvious that the United States and its allies would not allow IS to retain control of such a strategic and symbolic prize, but Mosul would be the largest city yet that the U.S.-led coalition attempted to wrest from the so-called caliph’s grasp, and it took nearly three years to prepare. As the Iraqi army approached from the south and Kurdish militias advanced from the north and east, satellites, drones and spies made it apparent that IS had not been idle. Following a holy template set by the Prophet Mohammed’s successful defense of Medina in the Battle of the Trench in 627 AD , the outnumbered jihadists dug trenches and underground tunnels, blasted holes through the walls of buildings, established sniper posts on rooftops, piled cars in streets as barricades and turned Mosul into a massive defensive bastion
The fanatical defenders realized that the Americans would play a key role in the impending attack to overcome such obstacles. The United States had been shocked by IS’s bold 2014 conquest of Mosul, and President Barack Obama (who had been elected, to a considerable extent, off the back of his promise to end the blood drip of American lives in what he called the “war of choice” in Iraq) decided to officially intervene later that summer. Thus, the proxy campaign to “degrade and destroy” IS, ultimately dubbed Operation Inherent Resolve, was born.
Obama ordered Green Berets, who served as spotters on the ground, to embed with the Kurdish, PKK-linked Peoples Protection Units (YPG) in Kobane and Hasakah in northern Syria. They were also stationed with Iraqi Security Forces (ISF) and Kurdish Peshmerga fighters in Iraq to act in a “training and assisting role” as “force multipliers.” The president’s surrogate approach to war aimed to destroy the IS caliphate without costing American soldiers’ lives. Obama was well aware of the widespread unpopularity of President George Bush’s Operation Iraqi Freedom, which had cost America approximately 4,500 lives and over a trillion dollars, and he intended to use local forces to do the fighting in the back alleys of jihadist-occupied cities like Fallujah, Ramadi and ultimately Mosul.
Events would show that Obama’s “stand off” approach to waging war in the Middle East was effective. Ultimately, IS would be pushed back from controlling an area larger than Britain, with eight million inhabitants under its control, all the way to Raqqa, an inconsequential backwater town in the middle of the Syrian desert where the terrorist group is currently making its fiery last stand. Once Obama ordered supplies to be dropped to outgunned Kurds in the northern Syrian town of Kobane in the fall of 2014, the “ever-expanding caliphate” that IS proclaimed after its inexorable conquest of one third of Iraq and Syria began to recede (Rudaw, October 20, 2014). Obama also supported the Kurdish defenders with precision-guided bombings and put considerable pressure on a reluctant Turkish government to allow reinforcements in the form of heavily armed Kurdish Peshmerga forces from northern Iraq to cross to Kobane through Turkey (Sabah, October 21, 2014).
The defense of Kobane — the Kurds described it as their “Stalingrad” — was the first test of Obama’s “stand off” strategy, which relied on surrogate forces in the region to fight IS, instead of directly putting Americans in harm’s way (Rudaw, January 19, 2015). History would show it was an astounding success. By the time IS was repulsed from Kobane at the end of January 2015, after taking unsustainable loses, the jihadists officially acknowledged for the first time, via their Amaq news agency, that their fighters had been defeated.
The defeat of IS in Kobane proved to be a decisive turning point and, with the U.S. air force supporting their movements, the Syrian Democratic Forces (SDF), a Kurdish and Arab fighting unit dominated by the YPG, pushed IS out of an area in northeastern Syria about the size of Massachusetts. By mid-2016, the Kurds had created a proto-state known as “Rojava” that consisted of three separate cantons: Jazira, Kobane and Afrin (al-Monitor, March 1, 2016). American-backed SDF forces have, as of the time of writing, moved further south and conquered 90 percent of the IS capital of Raqqa and are poised to defeat the remaining 2,000 IS militants holed up there (SOHR, September 20; ARA, August 6).
Meanwhile in Iraq, Iraqi Security Forces, once again backed by the might of U.S. Central Command’s air armada, fought to the west of Baghdad and retook the heart of Sunni Iraq, namely Fallujah and Ramadi, the original base for IS. Iraqi forces loosely aligned to the Iranian-backed Popular Mobilization Unit (PMU) militias also moved northward and conquered another Sunni bastion, former Iraqi leader Saddam Hussein’s hometown of Tikrit, on April 1, 2015. Having deprived IS of its strongholds in the Sunni Triangle, it was time to focus on Mosul.
Breaching the Jihadists’ Lair
By October 2016, the pieces were in place for a coalition of Iraqi government forces, supported by U.S. special forces (primarily the Green Berets Fifth Group, as well as artillerymen and trainers), Shia militias from the PMU and Kurdish Peshmerga fighters coming from the east to move on Mosul.
The Tigris River bisects Mosul, and the Coalition forces began their assault on the city’s more modern eastern side. The ISF advanced from the south while the Kurdish forces moved in from the east in the rural villages and northern plains of Nineveh, where the jihadist defenders burnt tires to create clouds of drifting smoke that would conceal their positions from the ever-present Predator and Reaper drones and U.S. bombers. It was here that Coalition forces had their first encounter with one of the IS fighters’ greatest psychological and tactical weapons — suicide vehicle-borne improvised explosive devices (SVBIEDs).
Despite the losses they suffered in the outskirts of eastern Mosul from SVBIEDs and snipers, the Coalition forces moved forward inexorably and reached the city’s eastern gates in the Gogjali district by November 1, 2016. There they began their first probes into the city. Now the real battle would commence between a force of outgunned and encircled by IS militants numbering somewhere between 3,000 and 12,000, and an uneasy alliance of Arabs and Kurds bent on eradicating all traces of IS in Iraq. 
In November 2016, Coalition forces on the ground supported by U.S. B-1 Lancer, F-16 Fighting Eagle, F-18 Hornets and B-52 Stratofortress bombers began to tentatively probe IS positions. Here they encountered a complex set of defenses. IS defenders used everything from forcefully recruited child soldiers, known as the “Cubs of the Caliphate,” to booby-trapped houses with explosives capable of blowing up a whole platoon to halt the advance. In addition to these tactics, IS deployed off-the-shelf drones, which could spy on enemy movements and even drop improvised explosive devices. The jihadists also corralled thousands of terrified civilians to be used as human shields and deployed a vast array of seemingly endless suicide bombers, including those in cars that hurtled into enemy troops at an estimated rate of 14 per day (CTC Sentinel, April 14).
Support From Above
Although U.S. attention switched to Donald Trump’s presidential victory and his inauguration on January 20, 2017, the proxy war started by his predecessor three and half years earlier continued building momentum. The culminating battle of Mosul would be either a vindication of Obama’s strategy, or its greatest setback.
By this time, the troops on the ground were making significant progress, including the American “ghost soldiers” moving furtively through the lines of Coalition forces and calling in airstrikes. Despite official statements that U.S. ground troops were not on the frontlines, according to the Kurds, Green Berets were in the thick of things calling in close airstrikes that acted essentially as “airborne artillery.”  Time and again hard-pressed Kurdish Peshmerga, Iranian-backed PMU Shia militias and the ISF — including the elite Golden Division fighters, who wore skull masks, some inspired by the Marvel comic book character “The Punisher” — were able to rely on this air support in the form of JDAMs (satellite-guided munitions) dropped by fighter-bombers or, GPS-guided artillery strikes from High Mobility Artillery Rocket System (HIMARs).
The U.S.-backed Coalition of jostling and often hostile allies also made use of American-supplied M1-Abrams tanks, Humvees and TOW wire-guided anti-tank missiles — used against SVBIEDs — and light infantry weapons to punch through street after street to reach Mosul University on January 14, 2017. With the five bridges that link the two sides of Mosul destroyed by the Coalition, the IS fighters, made up mainly of foreign die-hards, could expect no reinforcements.
Although eastern Mosul was not officially declared liberated until January 24, 2017, four days after Obama’s departure from office, the victory was a parting gift to the Obama administration, which had loosened the rules of engagement for JTACs in its final months (Military, February 24). However, the victory was darkened by the fact that advancing troops often found civilian corpses hanging from lamp posts, burnt and beheaded bodies left by fleeing IS troops in mass graves and a landscape of homes that had been reduced to rubble, forcing approximately one million civilians to flee for their lives.
Despite the atrocities and the damage to the city, eastern Mosul was liberated from terrorist rule and IS was clearly in retreat by late January 2016. Compounding its defeat in Iraq, by the fall of 2016, the group had also lost its fallback wilayat (province) in Libya, based in the city of Sirte — its “Raqqa on the Sea” — to a coalition of anti-IS Libyan militias backed by U.S. special forces and air support.
While the battle for western Mosul would continue for six more months under the Trump administration, by the time President Obama handed over the three-and-a-half-year war, the departing president could take credit for the fact that 180 top IS leaders had been killed in 17,000 airstrikes. IS had lost 30 percent of its territory in Syria and 62 percent in Iraq, and had seen its oil production and export destroyed in a bombing campaign known as Operation Tidal Wave II.  The number of fighters in IS’ ranks had also been reduced from 31,000 to approximately 12,000.
Continuing with the battle plan that had already been carefully put in place by U.S. Central Command, the Coalition forces prepared for the final battle for Mosul across the Tigris River. Western Mosul was made up of a labyrinth of medieval-era infrastructure, narrow streets and densely packed neighborhoods. With the black flag of IS, the Rayat al Sawda (Banner of the Eagle), still defiantly flying over the famed al-Nuri Mosque, the Coalition made its way across the Tigris River on pontoon bridges and began to drive into the west.
The Conquest of West Mosul
Since October 2016, the Kurds had been advancing from Kurdistan into the Makhmur region northeast of Mosul. By February 14, 2017, when the move into western Mosul began, the Peshmerga offensive had already been brought to an end —they never actually entered Mosul city itself. The Kurds established defensive berms to protect their troops from SVBIEDs and effectively annexed the areas to the north and east. For all their hatred of IS, the Peshmerga had no desire to fight an urban battle for an Arab city that would ultimately lie outside what they considered to be their own state of Kurdistan. 
Meanwhile, in Mosul, IS reacted to the ever-present “air artillery” that supported Iraqi troops by increasing the use of thousands of human shields. That technique was in evidence in the Jidideh section of west Mosul on March 17, 2017. As Iraqi forces were advancing, two IS snipers began firing at them from atop a nearby building. Spotters on the ground contacted the Erbil-based command center and requested immediate air assistance. But when an American aircraft targeted the snipers with satellite-guided munitions, the entire building collapsed because IS had been using the building as an ammunition storage facility. More than 100 Iraqi civilians were killed in the attack on the booby-trapped building, and news of the incident provoked international outrage.
Despite this public relations setback, the Coalition forces continued the advance and, by June 2017, had encircled the famous al-Nuri Mosque. By this time, al-Baghdadi had gone into hiding somewhere between Syria and Mosul. Even so, it was clear by June 2017 that his claim to have established an “ever-expanding, ever-victorious” transnational theocracy was hollow. As his “state” on the Tigris collapsed, Iraqi troops took to bulldozing buildings with IS fighters holed up in them in order to avoid further casualties.
In a final act of spite and defiance, the surrounded jihadists trapped in one square mile of Old Mosul, stunned the world by blowing up the al-Nuri Mosque. The building had stood as a symbol of the 12th Century Seljuk atabeg (governor) Nur ad-Din Zengi’s success in repulsing Western crusaders. Ironically, Zengi had had the al-Nuri mosque built to celebrate his triumph over Western foes.
As the smoke cleared, on July 9, 2017, Prime Minister al-Abadi arrived in Mosul to proclaim the liberation of Mosul to his people (Twitter, July 9). The battle for IS’ greatest prize was officially over, but in Mosul, the victorious (largely Shia) Iraqi troops sought to avenge the deaths of the approximately 1,500 Shia Iraqi army troops gunned down during the IS jihadists’ massacre at Camp Speicher in Tikrit on June 12, 2014. They threw captured IS fighters off buildings or shot them in extrajudicial executions. Many of those summarily executed were Sunni insurgents who had been previously captured by the Iraqi authorities, but had subsequently been liberated by IS assaults on Iraqi prisons in the summer 2013 — this was part of a campaign that Baghdadi had dubbed “Breaking the Walls.”
While IS still controls al-Qaim on the Syrian border, with the conquest of the Turkmen-dominated city of Tal Afar in the northwest and the liberation of Hawija in Kirkuk, the jihadist movement that began among disgruntled Sunnis after the 2003 invasion of Iraq has for the most part been suppressed.
The final battle to obliterate the Syrian half of the so-called IS caliphate now lies in the hands of advancing Kurdish and Arab forces of the SDF, who have been heavily backed by 500 U.S. special forces and an artillery unit from the 11th Marine Expeditionary Force.
According to the Pentagon, as of August 7, 2017, about 70 percent of IS’ territory in Iraq and 50 percent of its territory in Syria has been lost since August 2014, and the group’s dream of a jihadist empire in the Middle East has been all but crushed in the process. 
IS will doubtless continue to wage hit-and-run guerilla campaigns in Iraq and Syria of the sort that have already occurred in Mosul and most recently in the Shia south, where suicide bombers and gunmen killed 74 people near the town of Nasiriya at a checkpoint restaurant on September 14 (al-Arabiya, September 14). With Shias in control of Baghdad, home of the Abbasid caliphs that al-Baghdadi sought to emulate, and the Alawites in control of Damascus, the first Sunni caliphate of the medieval Ummayads, two of the Sunnis’ most important historical centers are in the hands of what the extremists call munafiqs (hypocrites) or apostates. These festering issues remain, as does the sense of Sunni disenfranchisement in post-U.S. invasion Iraq following the empowering of the Iranian-linked Shia “Safavids” (a term used by Sunni extremists that refers to a fanatical Turkmen Shia dynasty in Iran). The pressure cooker has been capped for now with the suppression of the Sunni jihadist state-building project in Iraq, as it was under General Petraeus with the 2007 troop surge. Hoever, just as the IS rise out of the ashes of Abu Musab al-Zarqawi’s suppressed al-Qaeda in Iraq (AQI) shows, the virulent form of Sunni jihadism he created has remarkable resilience and regenerative capabilities. The pot could once again boil over in the form of a new shamal (sandstorm) of violence like the one that emerged from the desert south of Mosul in June 2014.
Al-Baghdadi, after months of hiding, issued a “proof of life” recording on September 25, in which he bemoaned the loss of Mosul to the “crusader air force,” but stated: “The leaders of the Islamic State and its soldiers have realized that the path to … victory is to be patient and resist the infidels” (al-Jazeera, September 28).
IS is clearly mutating back to its original insurgency mode, and its resilient historical “re-enactors” take inspiration from the Quran and hadiths (Niqash, July 27). Its followers seek the fulfillment of prophecy and even accept that their “holy” movement will come close to extinction, before once again returning to power as predicted in the Quran. Most notably, they take heart in the words of the Prophet: “A victorious band of warriors from my followers shall continue to fight for the truth —despite being deserted and abandoned — they will be at the gates of Jerusalem and its surroundings, and they will be at the gates of Damascus and its surroundings.”
 It is worth noting that only Iraqi security forces would enter the city of Mosul. Peshmerga fighters were limited to seizing and securing areas on the outskirts, which they planned to garrison over the long term. Iranian-backed PMU militias focused more on isolating Mosul from the west to fill out the desert on the Syrian border, whereas non Iranian-backed PMU forces initially involved in breaching eastern Mosul were those raised from local micro-minorities such as Christians, Shabaks and Yazidis.
 Based on the authors’ monitoring of IS-related channels on the encrypted messaging app Telegram, the jihadists spoke frequently of reenacting this famous battle in Mosul.
 Estimates vary depending on the source. At a US Defense Department briefing on October 17, 2016, Peter Cook told reporters there were an estimated 3,000 IS fighters in Mosul, while outlets such as ARA News placed the estimate much higher at 12,000. https://www.c-span.org/video/?417070-1/peter-cook-briefs-reporters-military-operations-mosul; http://aranews.net/2016/06/12000-isis-militants-fighting-mosul/
 Author interview with Masrour Barzani, Kurdish intelligence minister (January 12, 2016)
 The numbers of airstrikes be found on the official Operation Inherent Resolve website, available here. Maps with the percentage of territory IS controls can be found on the Global Coalition Against Daesh website, available here.
 Author interview with Falah Mustafa Bakir, Kurdish foreign minister (January 15, 2016).
 See Pentagon statement (August 7, 2017). Available here.