It began before dawn: One thousand troops taking up position.

The goal? Push ISIS fighters back toward Mosul and farther from Irbil, the capital of Iraq's Kurdish region.

ISIS had already destroyed a bridge linking the two cities a month ago, hoping to prevent any opposing force from advancing on Mosul -- the country's second largest city.

But the Kurdish fighting force, known as the Peshmerga, found another way, taking a long route around, traveling on dirt roads through mountainous areas -- first north, then west, then back south, past the bridge.

They stopped near villages seized by ISIS, which refers to itself as the Islamic State. Most people had fled or been forced out of the area, the Peshmerga told CNN.

Only fighters for the terrorist group and its sympathizers remained. ISIS, also known as ISIL, has seized large swaths of land as part of its effort to create a caliphate -- an Islamic state -- that stretches from western Syria to eastern Iraq.

Once the Kurdish forces were in place, the firing began -- a series of loud explosions of mortar and artillery, says CNN producer John Fiegener, who along with CNN correspondent Anna Coren embedded with the Peshmerga.

"Then you hear the aerial bombs, and those are bigger. Then you hear the small arms -- the machine guns." That sound indicates the Peshmerga forces have reached the areas where ISIS fighters are holed up, he said.

It lasted hours. "You see a lot of fires, smoke coming out -- whether it's a fuel station or a gas tanker."

But ISIS, known for its ferocious acts of terror and military tactics, expected Pesherga forces to arrive at some point, so the militants were prepared with hidden bombs and snipers.

"ISIS had a tanker full of explosives that they tried to drive into the Peshmerga," Fiegener says. It caused a "really huge explosion," followed by a "very large cloud of dust."

The United States played an instrumental role in the operation Tuesday, with airstrikes that shattered some ISIS positions.

"At least two jets were flying the entire time that we were there," Fiegener said.

There were two U.S. airstrikes northwest of Irbil, according to the U.S. military. That's the same area where the Peshmerga operation was underway.

The strikes targeted an armored vehicle and ISIS fighting position, according to the military.

The Peshmerga estimated it faced about 100 ISIS fighters, killing between 20 and 30 of them. Six or seven members of the Kurdish forces were killed in explosions, a commander said.

The Peshmerga did not say whether they had captured any ISIS fighters.

The Kurdish forces took back enough terrain to include the bridge site and nearby villages.

(CNN)