An operation is underway in Mosul, Iraq by the government to recapture the city from the Islamic State in Iraq and Syria (ISIS).
It is expected to be the biggest battle in the country since the 2003 US-led invasion which toppled Saddam Hussein.
Here's what you should know about Mosul and the offensive:
1. Mosul is the largest Iraqi city to fall to ISIS
It is the capital of the Nineveh governorate in north-western Iraq and home to Iraq's biggest dam. The city with a population of about two million fell to ISIS in 2014. About 800 militants took over it, facing little resistance from an army widely accused of corruption. Iraq and its Western allies were stunned. The militants were so confident at the time that their leader Abu Bakr al-Baghdadi calmly walked into the Great Mosque of al-Nuri in Mosul in broad daylight and declared a caliphate in Iraq and neighbouring Syria. He also declared himself the leader of the world's Muslims.
2. The city is strategically and symbolically vital
Mosul has given the 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.
If recaptured, the city could eventually welcome back hundreds of thousands of people who were displaced. It is also where hundreds of enslaved Yazidi women and children are thought to be held.
Mosul is also an ancient Assyrian city and a vital centre of antiquities and historical sites threatened by ISIS. The city's heritage makes it central to Iraqi identity and hopes that the country might again move forward as a multicultural society.
In a country dominated by Shiite political groups, Mosul is the biggest Sunni-dominated city. It was also the centre of the biggest Christian population before the ISIS invasion.
In addition, the ISIS chemical weapons operation is based in the city, and the group got a huge boost from raiding American armories there. Mosul commands critical road networks and physical resources, and it was also where the group rebuilt in secrecy when it was known as Al-Qaeda in Iraq.
3. The offensive is a critical test for US strategy against ISIS
The Pentagon's strategy relies on local forces and keeps as many Americans as possible away from combat.
After the militant group overran large parts of Iraq and Syria in early 2014, the US formed a coalition that launched an air campaign to strike the Sunni extremists and to train local forces to do the fighting. The coalition is made up of 65 nations, about a dozen of which have conducted air strikes in Iraq or Syria. The rest provide auxiliary or logistical support.
In Iraq, there are about 8,500 coalition troops. Most of them - about 4,900 - are American, but other nations including Italy, Britain and France are helping to train local Iraqi forces. The focus has been to train, arm and motivate local forces to fight the ISIS. The coalition has trained more than 45,000 Iraqi troops and launched more than 10,000 precision strikes in Iraq - including more than 70 in the Mosul area this month.
Despite the coalition efforts, it will fall to the Iraqis to do the actual fighting in Mosul.
4. Operation involves a staggering 80,000 troops, supported by US-led coalition
The troops include Iraqi and Kurdish Peshmerga forces, Sunni tribal fighters as well as coalition forces, which advanced from different directions into Mosul. The size of the operation is far larger than previous assaults. The fight for Ramadi earlier this year involved roughly 10,000 Iraqi troops, while the battle for Fallujah, just months later, had around 15,000.
The advancing forces are buoyed by Western air support, artillery, intelligence, advisers and troops that will help call in airstrikes, known as forward air controllers.
- Air Support: The coalition partners are providing a wide range of aircraft, from F-22 fighters to hulking Cold War-era B-52 bombers. Aside from jets, the US military also has Apache helicopter gunships and a cadre of reconnaissance aircraft, including unmanned and heavily armed drones. Requests by Iraqi troops for Western air support are usually relayed to an Iraqi liaison at a joint command centre in Baghdad. From there, the Americans would route the appropriate aircraft overhead.
- Artillery: These include howitzers and guided rockets known as HIMARS which have been integral in earlier operations against ISIS. US artillery detachments supporting the Mosul campaign are located in Makhmour, to the south-east, and Qayyarah Airfield directly to the south.
- Advisers: Advisers run the gamut between Special Operations forces such as Army Green Berets, to National Guard soldiers teaching an Iraqi unit how to build a bridge. For Mosul, US and other Western advisers will likely advance in tandem with whatever Iraqi units they are paired with.
SOURCE: REUTERS, AGENCE FRANCE-PRESSE, WASHINGTON POST, NEW YORK TIMES