DAQUQ, Iraq — The so-called Islamic State has recruited copious cannon fodder from around the world, along with quite a few ferocious fighters. But its toughest opponents on the ground, the Kurds of Iraq and Syria, are attracting Western ex-soldiers for their ranks who are determined to see the self-proclaimed “caliphate” not only “degraded,” as Washington puts it, but destroyed.

At a Kurdish Peshmerga base on the fluid battle lines outside the ethnically and religiously mixed Iraqi city of Kirkuk, three American fighters sat down with The Daily Beast. We were less than half a mile from the black flags of ISIS, as the would-be Islamic State is widely known, and the soldiers asked that I not give too many details about their identities. They worry that their families could become special targets for a fanatical fighting force whose battlefields, like its targets, seem limitless.

Dressed in a Peshmerga uniform, Jeremy is a compact, affable 28-year-old-guy from Mississippi who fought with U.S. forces in Afghanistan and Iraq. He’s been fighting alongside the Pesh for the last six months.

Leo is a tall and direct 38-year-old Texan who worked security for private military contractors in Iraq and Afghanistan.

Mel’s background also is in military security contracting and he says he served for a while with an army from a European country, but he won’t specify which. Mel’s a little eccentric. At 41, the Colorado native sports a pair of carefully pointed canine teeth—fangs, in fact— and a goatee that gives off a strong goth-metal vibe.

For two months Leo and Mel have been with the Peshmerga, the erstwhile guerrilla army that now makes up the autonomous armed forces of Iraq’s Kurdish Regional Government, and both are dressed in the gray flannel shirts and cargo pants often associated with private security contractors, but they and Jeremy all claim to be volunteers who are not receiving any kind of salary.

As we sit in the comfortable field office of Peshmerga Maj. Gen. Karwan Asaad, with Kurdish TV playing on a flat screen in the background, the hazy battle lines feel bizarrely distant despite a network of frontline dugouts only a few hundred yards away. But the Americans are anything but complacent.

“ISIS are tough, real tough,” Jeremy says with his Mississippi twang. With fog settling in, he says it’s prime conditions for ISIS to make a move. It’s a different kind of warfare from what he saw when he was with the U.S. occupation forces in Iraq and Afghanistan. He sees ISIS not so much as an insurgency as an invasion force. “It’s very different fighting a group that’s trying to take over,” he says.

The three men say their main assignments are guarding high-ranking Kurdish military officials and transporting jihadist prisoners in Peshmerga custody. It’s work Mel and Leo became well accustomed to when hired as contractors in earlier American wars. Here, Mel says he’s transported ISIS prisoners that come from Chechnya, Ireland, France, Germany, the UK, The U.S. and Canada, but maintains he is barred from speaking with them and has no idea what happens once they are handed over to Kurdish guards.

The three say, without specifics, they have received U.S. assurances they won’t be prosecuted when returning home, but that to be sure requires dealing with a lot of government clearances and maintaining a low profile. According to Jeremy, a lot of his ex-Army buddies are itching to get to Iraq and join the anti-ISIS fight, but he says many have been blocked because they make those plans public on social media.

The three say they have no interest in internal Kurdish politics and that even their sympathies for the Kurdish national struggle are secondary to their goal of contributing to the defeat of ISIS. They doubt the capabilities or commitment of the Iraqi Army and see the Kurds as the first defense against the spread of an American enemy.

Leo believes that if ISIS isn’t defeated, he could end up fighting its militants on battlefields around the world, and he is seriously disappointed in the way the Obama administration has handled the rise of the would-be caliphate. He says the failure of U.S. policy is a central reason he felt the need to join the Pesh.

Jeremy says he was uncomfortable sitting at home and watching the news of ISIS beheadings, mass killings and enslavements and felt obligated to use his military training and skills to support those fighting the jihadists.

For Mel, it was a matter of feeling disheartened by the large numbers of foreigners joining ISIS. He became convinced he had to join the Kurds.

None of these soldiers is interested in delving farther back in history to ponder the role the George W. Bush administration’s invasion and occupation of Iraq played creating the environment that allowed ISIS to emerge. When I ask Jeremy if guilt about the legacy of U.S. forces in Iraq was part of his decision to come back, he turned beet red. His eyes welled up with water. He didn’t want to answer. But Leo chimed in, saying that a longer American troop presence could have somehow left things different. Jeremy regained composure and repeated Leo’s claim word for word, but it sounded more like he was trying to reassure himself.

Mel insists the American fighters’ motivations are driven by the values of the American Constitution and they’re not going to interfere with coalition interests. “We are Americans, 100 percent,” he says emphatically.

Although these three see their fight as closely aligned with the aims of U.S. interests and values in the Middle East, foreigners taking up arms alongside the Kurds seem to span a very wide political spectrum, from leftists following in the tradition of the international brigades that went to fight fascism in the Spanish Civil War to Christians bent on their own version of a holy crusade.

Ageed Kalary, a frontline commander of a unit of guerilla forces in the leftist Kurdistan Workers Party (PKK), stationed in the village of Matara, told The Daily Beast that until recently his fighters had the assistance of a former soldier with Canada’s military. The PKK has been central in repelling ISIS but is labeled a terrorist organization in most Western countries for the tactics it employed in its 29-year war for Kurdish self-determination in Turkey, a NATO member.

In January there were reports that an Australian union leader and Labor Party president in the country’s Northern Territory had disappeared to join Syrian Kurdish YPG fighters defending the long-besieged town of Kobani. The group is a sister organization to the PKK.

On the other end of the political spectrum is the head of an NGO that’s essentially a militia for hire, Matthew Vandyke. The creator of "Point and Shoot," a documentary about his motorcycle journey across the Middle East during the Arab revolution and his participation in the Libyan uprising of 2011, he now appears to have veered toward Christian holy war. Heading a group called The Sons of Liberty International, which claims to provide, globally, military support for “oppressed populations to liberate themselves,” he recently tweeted that he is trying to raise a “Christian army” to fight the Islamic State.

Vast political differences aren’t the only major distinctions among Western fighters joining the forces arrayed against ISIS.

Jeremy, Leo and Mel portray themselves—and really do seem to see themselves—as volunteers motivated by a need to support a historically victimized people leading a fight against a ruthless entity that uses Islamic scripture to justify biblical slaughter. But there are more than a few foreign gunmen, these three tell me, who treat this war like a business.

“There are people who have come over here to form clandestine military groupings,” says Leo, who found one of the first hurdles he faced was avoiding recruitment by mercenaries. Mel says he met far more foreigners trying to make a buck out of the war than those that came to fight ISIS out of conviction. “It’s mostly mercenaries or people coming over here to build a security company,” he says, describing the emerging market for start-up militias.

What real impact do any of these people have on the fighting? That remains to be seen. But as these three Americans view things, ISIS has created an international obligation for those with military skills to join the battle. And, like the jihadists, they see their involvement as just the beginning in a long struggle with no borders and no clear end in sight.