Iraqi forces battled their way into Mosul's Old City on Sunday, starting a difficult fight for the last neighborhoods still in the hands of Islamic State.
As the sun rose, a massive barrage shrouded parts of the Old City in smoke. It was a prelude to the arrival of black Humvees from Iraq's Counter-Terrorism Service, an elite U.S.-trained force that was attacking from the district's western flank.
The jihadis have steadily lost ground over the eight months of the Mosul offensive. They are now contained in one square mile of what was once Islamic State's Iraqi capital, which the group took in a blitz offensive more than three years ago.
The group's defeat in Mosul is more than symbolic. Not only will it have lost all of its major urban centers in Iraq, but the loss also would signify the beginning of the end of its crumbling “caliphate.”
But the battle promises to be a hard one.
Iraqi forces will have to abandon their armored vehicles as they storm the Old City's narrow walkways, while struggling to distinguish friend from foe among the estimated 100,000 to 150,000 people still trapped in the area.
The district's structures are also thought to be frail, forcing military planners to avoid using heavy ordnance for fear of having buildings collapse onto civilians hiding inside.
Meanwhile, surrounded and with no chance to escape, the militants are expected to give no quarter in what is their last stand in the city.
The ferocity of their resistance could already be seen on Sunday.
Overhead, warplanes from the U.S.-led coalition lobbed bombs on Islamic State positions as troops stacked up for the initial thrust.
But the moment bulldozers began breaching Islamic State barricades at the Old City's outskirts, the jihadis countered with heavy machine gun fire. Then the telltale whistles of mortar shells could be heard, seconds before they landed on the Iraqi troops' rear lines with powerful crashes.
"It's raining mortars," joked one soldier after the fourth shell had fallen, the explosions rattling the blown-out windows of an abandoned flour mill.
Before noon, at a medical station two miles from the front line, the first casualties began to trickle in; all were soldiers so far.
Yet it is the civilians trapped inside who are expected to bear the brunt of the hostilities, aid organizations warn.
In recent days, Islamic State snipers have cut down hundreds of residents fleeing the carnage around them, even as terrified families struggle to dodge airstrikes and artillery fire.
"This will be a terrifying time for ... people still trapped in Mosul's Old City and now at risk of getting caught up in the fierce street fighting to come," Nora Love, acting Iraq director for the International Rescue Committee, said in a statement on Sunday.
"Both coalition and Iraqi forces must do everything in their power to keep civilians safe during these final stages of the battle for Mosul," she said.