WATCH: “There’s an ISIS Fighter in This House”

Described by some military commanders as the deadliest urban combat since World War II, the nine-month battle to drive ISIS out of Mosul, with thousands of civilians caught in the crossfire, was brutal and grueling.

Director Olivier Sarbil was on the ground filming the fight as it unfolded street-by-street and house-by-house. The resulting documentary, Mosul, is an up-close look inside a campaign that would leave at least 20,000 people dead and wounded before the Iraqi army declared victory in July.

Premiering tonight on FRONTLINE, the film begins with seven chilling words — “There’s an ISIS fighter in this house” — as an elite squad of Iraqi special forces soldiers tracks down a suspected member of the terror group.

From there, the intensity of the fight never lets up. Some of the soldiers say they thrive on it.

“For the ISIS guys I kill, I feel very happy I killed them,” Hussein, a sniper, says in the above excerpt. “It’s fun. It’s not like a war, but a wedding party.”

Hussein is one of four soldiers whom the film follows as they grapple with the complexities of war. In the dramatic opening excerpt above, you’ll also meet Jamal, a wise-cracking sergeant; Amjad, a young recruit whose wife is pregnant; and Anmar, a college graduate seeking revenge after his father was the victim of a suicide attack.

“Fighting is not just about holding a gun and shooting,” Anmar says. “It’s mental warfare.”

In vivid detail, Mosul chronicles what happens to these four soldiers over nine months of “mental warfare,” as the reality of fighting an elusive and vicious enemy, losing fellow soldiers to ISIS snipers and bombs, and trying to determine who’s a civilian and who’s a threat takes its toll.

A qualifier for this year’s Documentary Short Subject Academy Awards, Mosul is a stunning look at the high cost of the Iraqi Army’s victory over ISIS in the city, large parts of which have been destroyed, with hundreds of thousands of civilians still displaced.

Watch Mosul beginning Wednesday, October 18 at 10 p.m. EST/9 p.m. CST on PBS stations (check local listings) and online. FRONTLINE’s two-part October 18 hour will also include a special report that goes Inside Yemen.