The Islamic State forced the world to take notice when the extremist group overran Mosul, the second largest city in Iraq, in June 2014.
Just months earlier, President Obama had described ISIS as "the JV team." But by August 2014, the U.S. was bombing ISIS in Iraq, and early Monday, the U.S. teamed up with the Iraqi army and other allies in a major offensive to recapture the northern Iraqi city.
If the Islamic State's capture of Mosul marked the high point of the group's expansion, its expulsion would mark a major setback for a group that's steadily been losing territory in Iraq and Syria.
Here are five key things to look for as the battle for Mosul plays out:
1. Mosul is the last Iraqi city held by ISIS: ISIS has already been driven out of other important Iraqi cities, such as Fallujah and Ramadi in the west. But it took weeks of intense street battles.
Based on this experience, the Iraqis are not planning to storm Mosul, and all signs point to a campaign expected to proceed at a measured pace. The main Iraqi forces are 20 to 30 miles outside the city, as NPR's Alice Fordham reports from Erbil.
But if defeated, ISIS will no longer control any large population centers in Iraq and will be reduced to a rural insurgency concentrated mostly in the west. Whatever happens in Iraq, though, the Islamic State will still have a strong presence in Syria, including the desert city of Raqqa, its de facto capital.
2. ISIS declared the caliphate in Mosul: ISIS leader Abu Bakr al-Baghdadi declared the caliphate, or Islamic state, at the Grand Mosque in Mosul shortly after the group took the city. The Islamic State was on a roll at the time, surging across Iraq as the country's army collapsed in one city after another.
The capture of Mosul demonstrated the confidence of ISIS at the time and the threat it posed to all of Iraq and the wider region. In contrast, losing Mosul now would be a huge blow strategically and symbolically for the group. ISIS would be in retreat and would have a much harder time selling the idea of a caliphate if it lost the city where it was declared.
3. A stepped up U.S. air campaign: The U.S. bombing campaign, now a little more than 2 years old, has been limited. One reason is the difficulty of identifying and striking small, isolated groups of ISIS fighters in densely populated urban areas.
Critics have called for the U.S. effort to be stepped up and that's widely expected to happen in a coordinated assault on Mosul that will include the Iraqi army moving up from the south and allied Kurdish fighters advancing from the north and the east of the city.
An Iraqi general told the AP that more than 25,000 Iraqi troops and allied militia fighters will be advancing on the city from all sides.
ISIS fighters will be greatly outnumbered, though exact figures are unknown. Estimates range from 3,000 to 8,000. However, the group has had two years to dig in.
"Residents of [Mosul] who are still there describe concrete defenses that have been erected, trenches and tunnels," Fordham says. "In other cities, we have seen them hang on with quite small numbers. They are famously prepared to fight to the death. We see suicide attackers, snipers, and they've had lots and lots time to lay improvised explosive devices."
4. Warnings of a major civilian exodus: Mosul had about 2 million residents before ISIS took over and is still estimated to have a million or more. The fighting is certain to drive out civilians, and the Iraqi government and aid groups are worried about a humanitarian crisis.
The Iraqi government was largely unprepared when civilians poured out of smaller cities. The Kurdish areas in northeastern Iraq are already packed with those who fled Mosul or other parts of the region over the past two years.
"What people here are terrified of is what's going to happen to the civilians inside the city," adds Fordham. "They have been urged to stay in their houses in leaflets that have been dropped from the sky by the Iraqi security forces. But we hear from inside that lots of people are secretly making plans to run away. If they do that, there may be no safe route out, and aid agencies could be overwhelmed."
5. "The day after" will be crucial: The battles for Iraqi cities have a Groundhog Day quality about them. Mosul is just one of several cities that Iraqi government forces and the U.S. military have recaptured time and again during the past 13 years of war.
Military victories were often squandered as Iraqi sectarian tensions bubbled to the surface afterward. In many cases, this happened as Iraq's mostly Shiite army and allied Shiite militias move into mostly Sunni cities — which is precisely what could happen in Mosul.
In announcing the offensive before dawn Monday, Iraqi Prime Minister Haider al-Abadi said only Iraqi government troops would enter the city. Sunni residents may harbor reservations about the army, but the prime minister was trying to reassure them that Shiite militias and Kurdish fighters will not be operating there.
"If the taking of Mosul goes well," notes Fordham, "it will do a lot to restore relations between people in Iraq and their security forces and government, which, especially in Mosul, have not been good for a long time."
Greg Myre is the international editor of NPR.org. Follow him on Twitter @gregmyre1.