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Located 419 kilometers (260 miles) north of Baghdad, Mosul is close to the Syrian and Turkish border, which provides the ISIS operation with a strategic lifeline.
It's also close to some of Iraq's most important oil fields and an oil pipeline that runs into neighboring Turkey.
What is happening in Mosul?
More than two years ago, the Iraqi army surrendered control to ISIS after armed militants violently seized the oil-rich city. It is where the group's leader, Abu Bakr al-Baghdadi, in August 2014, declared for the first time that he had established an Islamic state or caliphate.
Earlier this week, Iraqi forces and their allies launched a major offensive
to reclaim the city.
The battle for Mosul is especially significant considering the terrorist group has already lost control over the formerly besieged Iraqi cities of Fallujah, Tikrit and Ramadi.
If the operation is successful, Mosul would be the last ISIS stronghold in Iraq to fall.
How do you pronounce Mosul?
CNN's Nima Elbagir explains that the city's name is pronounced "MO-sul." She reveals that, translated from Arabic, Mosul means "the connection point."
"It's a reference to its place historically as a settling point for so many civilizations: the Assyrians, Christian prophets and then, of course, the Islamic push," Elbagir says.
How many ISIS fighters are there in Mosul?
Exact numbers are tricky to establish. It is believed there are between 3,500 to 5,000 ISIS fighters in Mosul, according to US military officials. But the militant group's supporters put the number at 7,000.
Although they are significantly smaller in size, ISIS' defense forces still pose a serious threat
to the Iraqi-led offensive. Suicide bombs, IED booby traps and car bombs are common tactics employed by the group
to stall advancing forces. Its fighters move through a network of elaborate underground tunnels and trenches. They also set oil-filled moats ablaze, which creates choking black plumes of smoke.
How big is Mosul?
Mosul is Iraq's biggest metropolis outside of Baghdad, home to a diverse population.
Before ISIS took Mosul in June 2014, the city was home to more than 2.5 million people.
After being subject to widespread violence under the regime -- which included public beheadings and the enforcement of sharia law -- more than half the population fled.
Today, only 1 million residents remain.
Why is Mosul important to ISIS?
Mosul is strategically important. It's a key trading city, close to the Syrian and Turkish borders. Cutting off ISIS's access to those borders would significantly hinder the movement of its fighters, weapons and supplies -- not only in Iraq but in neighboring countries.
On top of that, Mosul is situated near some of Iraq's most important oil fields. Stopping the flow of oil to ISIS would dramatically impact one of its illicit financial sources.
How far is Irbil from Mosul?
Irbil is the Kurdish capital, located some 85 kilometers (approximately 53 miles) from Mosul.
Tens of thousands of people who fled Mosul on foot during the 2014 siege found a safe haven there.
Kurdish fighters, known as Peshmerga, are taking part in the operation to take back Mosul.
Who is fighting in Mosul?
The offensive has been a long time coming and Iraqi Prime Minister Haider al-Abadi and Kurdish President Masoud Barzani are insistent that it is an Iraqi-led coalition. As well as the Kurdish Peshmerga fighters, Shiite paramilitary forces are leading the offensive. They are backed by American allies.
Support is also being provided by detached coalition special forces and advisers and trainers further back. Altogether, an estimated 100,000 troops
will be involved in the operation -- greatly outnumbering their opponents.
When and how did ISIS take Mosul?
In June 2014, militants seized
Mosul's airport, TV stations, the governor's office and most of the city over the course of five days before ISIS chief Abu Bakr al-Baghdadi declared the establishment of a so-called caliphate, or Islamic state, at the Mosul Grand Mosque.
During the siege, ISIS fighters took control of a prison where they freed as many as 1,000 prisoners. Mutilated bodies of captured Iraqi security forces littered the streets. There were other reports of brutality in ISIS-occupied Mosul, including the killings of civilians fleeing their homes.
CNN's Lauren Said-Moorhouse contributed to this report