CAMP SWIFT, Iraq — Thirteen years ago, Chase Snow's father was among the American troops who moved into the Iraqi city of Mosul during the U.S.-led invasion that toppled Saddam Hussein. Now Snow, a U.S. Army specialist, is deployed in Iraq to help in the fight to retake the city from the Islamic State group.

The assault on Mosul, Iraq's second-largest city, is bringing American forces into their most significant role in Iraq in years, in terms of numbers and presence on the front lines.

The lead-up to the assault has brought some U.S. forces into combat against the militants. Special forces carry out raids alongside Iraqi troops inside territory held by the Islamic State group around Mosul.

And now as Iraqi forces prepare for the operation to retake the city, those raids have increased in frequency, according to a coalition official, who spoke on condition of anonymity as he was not authorized to brief the media.

The U.S. also sent Apache helicopters to aid in the Mosul fight, a step that was not taken when Iraqi forces retook the western cities of Ramadi and Fallujah.

The number of U.S. troops in Iraq has steadily grown over the past two years to about 6,000 service members, up from almost none following the 2011 withdrawal from Iraq.

The latest group, numbering nearly 600, began to deploy in September to Qayara air base, the facility 30 miles south of Mosul that is to be the main staging ground for the assault on the city. Trucks have been rolling across the base for weeks with supplies and equipment, preparing it so coalition warplanes will be able to operate there.

“You've got to look at Mosul as the crown jewel right now,” said Maj. Gen. Gary Volesky, the head of U.S ground forces in Iraq, regarding the buildup of forces. The deployments have “all been targeted to assist in the Mosul attack.”

Besides the hundreds of special forces, most of the American personnel operate back from the front lines, coordinating coalition airstrikes, tracking Iraqi ground troops, sharing intelligence and helping plan operations.

Snow, from Nashville, Tenn., with the 101st Airborne Division, is advising Iraqi officers carrying out the Mosul operation. His father was also with the 101st in Mosul in 2003.

Now on Snow's Iraq deployment, he carries the same American flag his father kept with him on all of his tours and his father's good luck charm: a St. Michael prayer card.

“I know my father never thought I would be coming to Iraq,” Snow said.

The U.S. presence at bases closer to Mosul in the lead-up to the operation is “essential” to the advise-and-assist mission, said U.S. Army Col. Brett Sylvia, the commanding officer at Camp Swift, a small coalition base about 45 miles southeast of Mosul.

“If you're not there, then you don't have a voice,” Sylvia said, standing in front of the bank of televisions and desktop monitors that he says constitutes the forward edge of the battle for his men.

As of last week, there were 4,565 U.S. troops in Iraq, according to the Pentagon. That doesn't include another 1,500 troops considered there “on temporary duty,” whose number changes daily, according to the U.S. officials.

U.S. troop levels in Iraq peaked at 157,800 during the 2008 surge under then-President George W. Bush, according to the Pentagon.

More than 140,000 U.S. troops were in Iraq when President Barack Obama took office in 2009.

Obama drew down the forces until the complete withdrawal of late 2011 removed all combat troops from the country, leaving behind only a few hundred U.S. trainers, mainly civilians, to assist Iraqi security forces.

U.S. forces began returning after the Islamic State group overran Mosul in the summer of 2014 and blitzed across much of northern, central and western Iraq, joining it to territory it holds in Syria.

Weeks later, Obama announced the start of the air campaign against the Islamic State. At the time, he underlined that he will not allow the U.S. “to be dragged into fighting another war in Iraq.”

But the U.S. role has steadily grown as Iraqi and Kurdish forces continue to rely heavily on coalition air power and support in taking back territory the militant group overran in 2014.

Over the past year, three American service members have been killed by Islamic State militants in Iraq.

Iraqi commanders say despite months of training, their men are still almost entirely dependent on coalition air power and intelligence to retake territory.

“If we didn't have airstrikes we wouldn't be able to advance,” Iraqi Army Capt. Riad Ghafil with the Nineveh Operation Command admitted.