A decade after his first Iraq tour, former U.S. Marine Jamie Lane has returned to the battlefields of the Middle East to fight a still unvanquished enemy and wrestle with the demons of his past.

The 29-year old from Mt. Pleasant, Michigan served as a machine gunner from 2004 to 2008, mainly in the western Anbar province, where he saw fierce fighting against al-Qaida in Iraq. Now, as a private citizen suffering from post-traumatic stress disorder, he is back in the region to battle its successor, the Islamic State group.

"In order to aid my recovery from PTSD, I have taken it upon myself to fight on my terms, against an enemy I know is evil," said Lane, who joined Kurdish militiamen in Syria. "It is redemption, in a sense."

He is one of a small but growing number of Iraq war veterans who are making their way back to the Middle East, not as uniformed soldiers, but as individuals waging their own personal battles.

Many describe feeling a sense of unfinished business as they watched the Islamic State group rampage across the country last summer, seizing territory they had fought and bled for during the U.S.-led intervention. Some express remorse for taking part in that war, while others say they are driven by the same sense of moral obligation that brought them here in the first place, joining their fate to that of a deeply troubled country.

Scott Curley, another U.S veteran of the Iraq war, returned to join the Kurdish peshmerga fighters after Islamic State militants in Syria beheaded Peter Kassig, a former U.S. Army Ranger who had returned to the region to provide humanitarian aid to Syrian civilians.

"I'm just a man with a gun, but whatever little difference I can do," he said. 'There weren't many Western volunteers (with the peshmerga), so I figured I could help here."

A U.S.-led air campaign began targeting the Islamic State group in Iraq in August, helping Iraqi and Kurdish forces to halt the extremists' advance and begin rolling them back. The Pentagon plans to supply some $1.6 billion worth of arms and training to Kurdish and Iraqi forces.

But after more than a decade of inconclusive wars in Iraq and Afghanistan, the United States has no interest in sending ground troops back to the Middle East. And as the vets learned at great cost on their various deployments, territory cannot be won and held by airstrikes and arms deliveries alone.

"I'm not a mercenary or in love with killing people," said Bruce Windorski, a former Army Ranger and ex-police officer now training Kurdish fighters with the Syria-based People's Protection Units (YPG). The Green Bay, Wisconsin native says he would rather see a "random Westerner" fighting alongside the Kurds than another full-scale invasion.

"I wouldn't want our American servicemen and women to have to fight a third war in two decades," Windorski said. "I've lived through the loss of loved ones fighting on foreign soil. I have seen families with deployed loved ones. It's hell on everyone involved."

Most of the U.S. vets are fighting alongside Kurdish forces, who have invited foreigners with military experience to join their ranks. A Facebook page called "The Lions of Rojava" offers the chance to "send terrorists to hell and save humanity," and features portraits of smiling Kurdish female fighters.

Iraq's Kurdish forces, known as the peshmerga, once strictly banned non-Kurdish fighters from joining. But when Islamic State militants came within 30 kilometers (20 miles) of their regional capital, Irbil, last summer, they began bending the rules in an act of desperation.

"During wartime, necessity imposed itself," said peshmerga spokesman Halgurd Hekmat. "It's an extraordinary situation and it's not a secret that the peshmerga wasn't prepared for this battle," he said.

So far, the U.S. hasn't banned Americans from fighting with militias against the Islamic State group, though it considers the Turkey-based Kurdistan Workers' Party, commonly known as the PKK, a terrorist organization. The PKK has been fighting alongside the YPG in northern Iraq and northern Syria.

The American vets say the Kurds need all the help they can get, but that potential volunteers should come with military experience and knowledge of the region.

"Don't show up and look stupid not knowing what you're doing," said Matthew Van Dyke, 35, a Baltimore, Maryland native who fought alongside Libyan rebels in 2011 and now fights with the Kurds. "It will get you killed. It's more important to have knowledge of the region than to have military training."

Even for seasoned American soldiers, this battle is different. Gone are the sprawling, heavily guarded U.S. bases, the state-of-the-art surveillance and communications equipment, the helicopters that once ferried them across hostile territory and the direct air support they could once call upon during intense fighting. A number of foreign fighters have been killed, including an Australian man, a British man, and a German woman.

Lane, who had recently returned from the front line with the YPG, said "fighting with them is certain, 100 percent death."

"I am shaken by my last battle," he said. "The YPG is a very young, new military, and they need all the help they can get against the bastards in ISIS who are well-trained and well-equipped. It is a miracle we aren't all dead."

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Janssen reported from Irbil, Iraq. Associated Press writer Zeina Karam in Beirut contributed to this report.