Here are some facts about the battle for the strategically and symbolically vital city of Mosul.
Q Why is Mosul significant?
A The ancient Assyrian city, with its antiquities and industries, is the capital of Nineveh governorate. It was the largest Iraqi city to fall to the Islamic State in Iraq and Syria (ISIS) when 800 militants seized it in 2014, facing little resistance from an army accused of incompetence and corruption.
With a population of two million before the ISIS seizure, Mosul was Iraq's biggest Sunni-dominated city and home to Iraq's biggest Christian population.
Q Why is Mosul important to ISIS?
A It gave ISIS its best claim to legitimacy as an Islamic caliphate and it has been the militant group's most vital source of tax revenue and forced labour.
ISIS' chemical weapons operation is based in the city and the group got a huge boost from raiding American armouries there.
Also, Mosul commands critical road networks and physical resources, with oil wells nearby.
Q What resources are being put into winning it back for the Iraqi government?
A The huge operation involves a total of 80,000 troops, supported by the United States-led coalition of 65 nations. About half of them are Iraqi and Kurdish Peshmerga forces and Sunni tribal fighters advancing from different directions, supported by coalition artillery, intelligence, advisers and troops to help call in air strikes. By comparison, the fight for Ramadi earlier this year involved about 10,000 Iraqi troops and the battle for Fallujah involved around 15,000.
Coalition aircraft include F-22 fighters, Cold War-era B-52 bombers, Apache attack helicopters and a cadre of reconnaissance aircraft, including unmanned and heavily armed drones.
Coalition artillery has been airlifted to strategic locations around the city. This includes howitzers and guided rockets.
Q What will defeat in Mosul mean for ISIS?
A ISIS will have been forced out of almost all the territory seized during its 2014 blitzkrieg. Even if the group holds out in the eastern Syrian town of Raqqa, it will no longer hold swathes of territory - nor control the lives of millions of people in the heart of the Middle East.
The defeat of ISIS as a territorial power would dramatically rearrange the three-dimensional chessboard of the Syrian war. It could potentially benefit the regime of Syrian President Bashar al-Assad or rival Islamic militant groups, such as the Al-Qaeda-linked faction known as Al-Nusra Front.
Many analysts and officials predict that an angry and impoverished ISIS without territory could lead to a spike in retaliatory attacks, particularly in Europe.
REUTERS, AGENCE FRANCE-PRESSE, THE GUARDIAN, WASHINGTON POST, NYTIMES