WATCH: Surviving an ISIS Truck Bomb in Mosul

As a child, Ghaith Abdul-Ahad spent time in the city of Mosul with his father, walking through the once-bustling outdoor markets of Iraq’s second largest city.

But by the time the Iraqi-born journalist returned to Mosul to film tonight’s new FRONTLINE documentary, everything had changed.

Battle for Iraq, produced in association with The Guardian, is Abdul-Ahad’s up-close look inside the brutal fight to retake Mosul from ISIS, which has controlled the city of more than 1 million people for more than two years.

Despite early progress by the Iraqi army — a largely Shia group backed by Kurdish forces and U.S. airstrikes — Abdul-Ahad discovers a complicated situation on the ground: there is deep-seated mistrust of the Iraqi Army by the majority-Sunni population, who are suffering heavy casualties. Before the ISIS takeover, the army was accused of sectarian abuses and illegal detentions.

“This is a battle happening between two enemies on a land inhabited by civilians,” Abdul-Ahad says.

The extent to which ordinary civilians are caught in the crossfire is made clear in one of the most harrowing scenes from the documentary, when Abdul-Ahad and producer Joshua Baker are traveling with members of Iraq’s elite special operations forces, known as the Golden Division.

A local family has agreed to let Abdul-Ahad, Baker and the Golden Division soldiers spend the night in their house. They are a mile back from the front line, and they think they’re safe.

But they’re wrong. Early the next morning, just feet from the house, a soldier spots a truck that’s been packed with explosives by ISIS — and tells Abdul-Ahad to run. The truck bomb detonates seconds later.

When they emerge from the rubble, Baker has a fractured spine. But it is the Iraqi civilians who bear the brunt of the attack — with homes destroyed, children injured and many feared dead under the debris.

Abdul-Ahad says that’s the day he realized the true complexity of the battle.

“No one knows how many civilians have been killed in this battle of Mosul,” he says, “not even the government of Iraq.”

Battle for Iraq, a two-part FRONTLINE special report, premieres Tues., Jan. 31 at 10 p.m. EST/9 p.m. CST on PBS (check your local listings) and online. Part one, Battle for Iraq, is in association with The Guardian. Part two, Hunting ISIS, goes deeper into Mosul, with a Golden Division unit that has been leading the fight against ISIS.