By Dominic Evans
BAGHDAD (Reuters) - An array of Iraqi forces, backed by a broad international coalition, is closing in on the Islamic State stronghold of Mosul, united in their determination to crush the jihadists but driven by starkly opposed agendas.
They include government troops seeking to reunify a broken country, peshmerga fighters aspiring to an independent Kurdistan, and Iranian-backed Shi'ite militias projecting Tehran's power across the Middle East.
Local factions are also taking up arms to defend their interests around Mosul. Yazidis and Assyrian Christians, both persecuted by Islamic State for their faiths, have paramilitary forces of their own, and Sunni tribes are gearing up to fight.
Turkey says its troops must have a role in a region it sees as critical to its security, and thousands of U.S. personnel - on the ground in Iraq or flying air sorties - find themselves on the same side as fighters loyal to the Supreme Leader of Iran.
While the shared goal of destroying Islamic State in Iraq should hold this unlikely coalition together until the jihadists are defeated, divisions run deep and raise the possibility of power struggles, sectarian strife or cross-border intervention.
Here is a summary of the forces advancing on Mosul, where the Iraqi military estimates around 5,000 to 6,000 Islamic State militants are controlling a city of about 1.5 million people.
The combined force of Iraq troops, security forces, peshmerga, Shi'ite and Sunni militia is likely to be around 100,000, according to a tally by Hisham al-Hashemi, who advises the Iraqi government on Islamic State affairs. Washington also has close to 5,000 military personnel in Iraq.
IRAQI ARMY - The army is fighting for its credibility as much as for Mosul. Two years ago the U.S.-trained and equipped force melted away in Mosul in the face of an assault by a few hundred Islamic State fighters, a humiliating retreat which handed the jihadists an arsenal of U.S.-supplied weaponry.
Prime Minister Haider al-Abadi later said the government had discovered 50,000 "ghost soldiers" on the payroll who took their salaries and paid off senior officers to avoid military service.
Rebuilt and retrained by Washington, the army has deployed an estimated 20,000 regular troops, made up of three divisions - the 15th and 16th infantry divisions and the 9th armoured division. They are advancing on Mosul from the north, the eastern Nineveh plains and the south along the Tigris River valley.
SPECIAL FORCES - Elite Counter Terrorism Services have been at the forefront of battles to retake cities such as Falluja and Ramadi from Islamic State in the last year. Two CTS divisions, or around 15,000 troops, are involved in the Mosul campaign.
They have made the most rapid progress, advancing from the east and entering the city's eastern districts this week.
The army and special forces are expected to be tasked with taking over Mosul, leaving Shi'ite and Sunni militias and Kurdish forces outside to avoid sectarian tensions inside the mainly Sunni Muslim city.
FEDERAL POLICE - Three police divisions, including a rapid response force, are advancing with army units from the south, totalling about 20,000. The police report to the Interior Ministry which has been largely controlled by pro-Iranian Shi'ite parties since the overthrow of Saddam Hussein in 2003.
PESHMERGA - Peshmerga forces have been advancing from Kurdish-ruled areas north and east of Mosul. Hashemi put their strength at around 20,000 - an estimate which a senior peshmerga official said reflects the attacking forces only, with a similar number holding territory further back. A senior U.S. officer estimated their strength at a more modest 10,000.
HASHID SHAABI - The Hashid Shaabi (Popular Mobilisation) forces, are made up mainly of Iranian-backed Arab Shi'ite militias, but also include fighters from other backgrounds including Yazidis and Assyrians (see below).
They have launched an offensive to take the town of Tal Afar, west of Mosul, which would cut off any route for Islamic State fighters to retreat back into Syria or reinforce their defence of Mosul.
There are three divisions of Hashid Shaabi fighters, totalling around 20,000.
SUNNI TRIBES - Around 15,000 Sunni tribal fighters from the Mosul region, many of them displaced by Islamic State, have received basic weapons and training. They are not taking part in the battle but have been told to hold territory once it is captured by the army. Some of the fighters say Sunnis should be allowed to rule themselves after Mosul is retaken.
YAZIDIS - Yazidi and Kurdish fighters drove Islamic State out of the town of Sinjar, west of Mosul, last year, and fought off an Islamic State counter-attack in the area after the start of the Mosul campaign.
Persecuted, killed and forced into sexual slavery during Islamic State's rule in Sinjar, the Yazidis seek a form of self-rule. As part of the Popular Mobilisation forces, their militias are partly backed by the state. One of them, the YBS or Sinjar Resistance Units, is 2,700 strong but says less than half get salaries from Baghdad.
ASSYRIAN CHRISTIANS - Have formed a small paramilitary force of 300 fighters they say will protect Christian towns and villages in the Mosul region.
UNITED STATES AND WESTERN FORCES - The United States has around 5,000 military personnel in Iraq, around 1,000 of them at the Qayyara air base south of Mosul which forms their main staging post for the offensive. Their weaponry includes artillery rockets with a range of 70 km.
Troops from France, Britain, Canada and other Western nations are also deployed in support missions, and a U.S.-led air coalition has been striking Islamic State targets in Iraq for more than two years.
TURKEY - Neighbouring Turkey has troops inside Iraq training peshmerga and Sunni forces at a base northeast of Mosul - despite protests from Abadi's government over their presence.
President Tayyip Erdogan says his country should have a role in the Mosul operations. He has spoken about reinforcing the border with Iraq and has warned that Turkey would respond if the Shi'te militias "unleash terror" in Tal Afar, whose population have strong historical links to Turkey.