In a smoky living room in a makeshift military headquarters, Brett, a former U.S. serviceman with tattoos of Jesus etched on his forearms, explained how he hopes to help keep the church bells of Iraq ringing.

“Jesus tells us what you do unto the least of them, you do unto me,” said the 28-year-old from Detroit who served an extended tour in Iraq in 2006 and 2007. He asked for his surname not to be published, to protect his family at home. “I couldn’t sit back and watch what was happening, women being raped and sold wholesale.”

So in December he traveled to northern Iraq, where he joined a growing band of foreigners leaving behind their lives in the West to fight with new Christian militias against the Islamic State extremist group. The leaders of those militias say they have been swamped with hundreds of requests from veterans and volunteers from around the world who want to join them.

The new arrivals add to a varied array of foreign fighters and donors drawn to the expanding conflict, which has had a brutal impact on Iraq’s minority sects and is threatening Christianity here at its roots. But while they say they welcome the gesture, Kurdish authorities in northern Iraq are wondering how to vet foreign recruits who are clamoring to sign up.

Brett’s group, Dwekh Nawsha, which means “self-sacrifice” in Aramaic, the ancient language spoken by Jesus, has only six Westerners among its 200 Iraqi Assyrian Christian fighters. But Emanuel Khoshaba Youkhana, the secretary general of the Assyrian Patriotic Party, which funds the group, says that more than 900 other foreigners have been in touch to find out how to join.

Some of the volunteers said they have come to fight for their religion — others just to fight. Brett calls himself the “King of Nineveh,” after the province of ancient Christian villages now occupied by the Islamic State. He lifted his shirt to show a tattoo on his back of Saint Michael, and Psalm 23 inked up his side.

The Iraqi Assyrians he serves alongside, not all of whom are impressed by the “King of Nineveh” persona, stress that this is not a crusade but a fight to return to their homes.

“We don’t want to fight a holy war for Christians,” said Marcus Naissan, 25, an Iraqi Assyrian fighter. “We fight for our land.”

He said that while the influx of foreigners brings a media spotlight and potential funding, it also brings concerns.

“We aren’t crusaders,” he said. “That’s how they make it look.”

Brett and others said they receive dozens of e-mails a day from potential recruits.

“This place will be flooded,” Brett said. “From Australia, Asia, literally everywhere. It’s overwhelming. It’s awesome.”

All recruits are interviewed before they come, to ensure their intentions are good, he said. Youkhana said checks are carried out, but he would not go into the details.

At the base in Dahuk, Louis, a former Marine from Houston, nervously drummed his fingers on his Kalashnikov as he explained that he was desperate to get to the front line to fight Islamic State militants.

“I was prepared for this to be a one-way ticket,” said the 24-year-old, who was discharged from the service just over a month ago after being diagnosed with post-traumatic stress disorder following a six-month tour in Afghanistan in 2012. “I just want to get out there.”

He decided to join the group after the Marines said he would not be deployed again.

“I couldn’t adjust to peacetime,” he said. “The fight for me wasn’t over. I felt I didn’t have a purpose anymore.”

While religion was not a particular motivation, he said Dwekh Nawsha’s fight against the Islamic State was an easy one to relate to.

Some foreigners have concerns about potential legal entanglements upon their return home. The Christian units are on the home turf of the Kurds, and a Kurdish paramilitary group — the People’s Protection Units, or YPG — that is battling the Islamic State in Syria and Iraq is affiliated with the Kurdistan Workers’ Party, designated a terrorist organization by the United States.

Masrour Barzani, the Kurdistan Regional Government’s national security chief, said foreign governments were being kept up to date on those traveling to join military groups.

“There are regulations we have to bear in mind and make sure everything happens according to the legal structure,” Barzani said. “We are thankful for any volunteer that is coming here [to] fight. But we don’t lack men — we lack weapons.”

Currently waiting for papers to confirm that their status is legal, most of the foreigners with Dwekh Nawsha have yet to see much in the way of combat.

Just two have the right paperwork to travel with their weapons, Youkhana said, leaving the others largely confined to their base in Dahuk.

Other recruits are being held back until paperwork is organized for those who have already arrived.

Tim Locks, a 38-year-old from Britain who ran a construction firm before arriving in Iraq two weeks ago, said the wait had been frustrating. “We are just waiting for the i’s to be dotted and the t’s to be crossed,” he said.

As the Islamic State takes hundreds of Christians hostage in Syria, the leaders here say they are still trying to work out how best to use outside assistance.

“It’s very good to have the Westerners coming,” said Capt. Khalid Hamzo, an officer with the official peshmerga forces. He said he recently obtained permission to take two foreigners out to the front lines to see whether they could help resolve an issue with a supply of mortars that were not exploding.

“But they couldn’t help without the Internet,” he said as he left the makeshift headquarters with a printout of a manual on 81mm mortars. “We need artillery experts.”

The Assyrian Christian fighters are largely kept away from the front lines. At Dwekh Nawsha’s forward operating base in the Christian village of Baqufa, they make up the second line of defense behind Kurdish peshmerga forces. The battle here is largely static, but mortar fire is exchanged daily.

For the Iraqi Christian fighters here, the battle is more personal.

Roni Najm, 21, can see his village from the telescope positioned on the building’s roof. The Islamic State’s black flag flutters above its church. “It just makes you cry,” he said.