Map of the territorial control during the 2016 Mosul offensive, as of August 2016
Mosul is Iraq's second most populous city. It
fell to 800–1,500 ISIL militants in June 2014, because of the largely
Sunni population's deep distrust of the primarily
Shia Iraqi government, and its corrupt armed forces.
 It was in the
Great Mosque in Mosul that ISIL leader
Abu Bakr al-Baghdadi declared the beginning of ISIL's self-proclaimed "caliphate" which spans Iraq and Syria.
 The original population of 2.5 million has fallen to approximately 1.5 million after two years of ISIL rule. The city was once extremely diverse, with ethnic minorities including
Shabak people, all of whom have suffered and continue to suffer considerably under the (majority Sunni Arab) Islamic State.
 Mosul remains the last stronghold of ISIL in Iraq,
 and the anticipated offensive to reclaim it was promoted as the "mother of all battles".
Preparations for the battle
In the weeks leading up to the ground offensive, the US-led
CJTF – OIR coalition bombed ISIL targets, and the Iraqi Army made gradual advances on the city.
Royal Air Force's
Tornados targeted "
artillery pieces and
mortar positions" in the 72 hours before the ground assault began.
Leaflets dropped on the city by the Iraqi military advised young male residents to "rise up" against ISIL when the battle began.
 To prepare defenses against the assault, ISIL operatives dug 4 m2 holes around the city, which they planned to fill with burning oil to reduce visibility
 and slow advances.
 They also built hundreds of elaborate tunnels in the villages surrounding Mosul, rigged with explosives and booby-traps, and laid
improvised explosive devices (IEDs) and mines along the roads.
 There was considerable concern that ISIL might employ chemical weapons against soldiers and civilians.
According to Iraqi sources, the assault towards Mosul was being waged from Al-Khazer axis (east of Mosul),
Mosul Dam (northern axis),
Baashiqa axis (eastern axis), Al-Qayyarah axis (southern axis), and Talul el-Baj- Al-Khadr axis (southwestern axis).
Forces involved in the offensive
About 3,000–5,000 ISIL fighters were estimated to be in Mosul city, according to the United States Department of Defense.
 Other estimates ranged as low as 2,000 and high as 12,000 ISIL fighters.
Mosul Eye estimated approximately 8,000–9,000 fighters loyal to ISIL, with "[h]alf of them... highly trained, and the rest... either teenagers or not well trained. About ten percent of the fighters are foreign (Arabs and non-Arabs). The rest are Iraqis. Most are from Nineveh’s townships and districts."
 Prior to the start of the battle, in late September 2016, it was estimated that around 20,000 ISIL fighters were living in Mosul,
 many of whom later fled the city to Syria and Ar-Raqqah, when Iraqi forces began to besiege Mosul.
The Iraqi-led coalition was initially estimated by
CNN to have 94,000 members,
 but this number was later revised upward to 108,500;
 54,000 to 60,000
Iraqi security forces (ISF) soldiers, 16,000
Popular Mobilization Forces (PMF) fighters (also referred to as PMU), and 40,000
Peshmerga (including approximately 200 Iranian Kurdish female fighters from the
Kurdistan Freedom Party (PAK)
 are deployed in the battle.
Among the PMF units, the
Nineveh Plain Protection Units composed of
Assyrians are among the paramilitary forces in the government coalition.
 Shia militias, including several brigades of the paramilitary organization Hashd al-Shaabi, the
Kata'ib Hezbollah, the
League of the Righteous, the
Badr Organization, Saraya Ashura, Saraya Khorasani,
Kata'ib al-Imam Ali,
Harakat Hezbollah al-Nujaba and
Turkmen Brigades also took part.
Ezidi community of the Sinjar region contributed the
Sinjar Resistance Units (YBŞ) and
Êzîdxan Women's Units (YJÊ),
 which are operating in concert with Sunni Arab
Shammar tribal militias and
People's Defence Forces (HPG) of the
Kurdistan Workers' Party (PKK).
Assyrian forces involved in the planned offensive includes the
Nineveh Plain Forces (NPF) and
Dwekh Nawsha, who are allied to the Peshmerga.
soldiers prepare to conduct a combined arms live-fire exercise with an Italian instructor near
, on 12 October 2016.
international coalition of 60 nations, led by the United States, is supporting Iraq's war against ISIL, providing logistical and air support, intelligence, and advice.
 The international coalition forces are headquartered 60 kilometres (37 mi) south of Mosul at
Qayyarah Airfield West (or Q-West) in
Qayyarah, which was retaken from ISIL in June.
 About 560 U.S. troops from the
101st Airborne Division were deployed to Q-West for the battle, including command and control elements, a security detachment, an airfield operations team, and logistics and communications specialists.
 The U.S. deployed
HIMARS rocket launchers and
M777 howitzers, manned by the 101st's 2nd Brigade Combat Team and the Golf Company, 526th Brigade Support Battalion. The French army deployed four
CAESAR howitzers and 150 to 200 soldiers at Qayyarah, with 600 more French troops announced at the end of September.
 An additional 150 French soldiers are in
Erbil, east of Mosul, training Peshmerga.
 The aircraft carrier
Charles de Gaulle, with a squadron of 24
Rafale M jets, was deployed from
Toulon to the Syrian coast to support the operation against ISIL through airstrikes and reconnaissance missions; 12 other Rafale jets are operating out of
French Air Force bases in Jordan and the
United Arab Emirates (UAE).
Australian special forces soldiers and 210
Canadian Special Operations Forces Command (CANSOFCOM) soldiers were also deployed to assist the Peshmerga. In addition, the
21 Electronic Warfare Regiment was also reported to be in the area, working to intercept and relay ISIL communications, while a Role 2
Canadian Army field hospital with 60 personnel has been set up to treat Peshmerga casualties.
An Iraqi soldier during a course on chemical, biological, radiological and nuclear defense training at
. Coalition forces have expressed fears ISIL may use chemical weapons during the Battle of Mosul.
Ba'ath loyalists group, known to be led by
Saddam Hussein's former vice president
Izzat Ibrahim al-Douri, issued a statement before the start of operations calling for the people of the city to start an uprising against ISIL and announced that they will fight the "terrorist organization."