'Finish the Job': More Than 1 in 3 Western Anti-ISIS Volunteer Fighters Are American, Many Vets

— -- A new report estimates that over 100 Americans, almost all male and many military veterans, have joined up with militia groups in Iraq or Syria to fight ISIS, accounting for more than one-third of all anti-ISIS Western volunteer fighters.

The report, published overnight by the London-based Institute for Strategic Dialogue, is based on a database of 300 foreign fighters from Western nations in anti-ISIS groups found in public accounts -- from media reports to social media. The report found that Americans are more prevalent in the groups than those from any other Western nation, with the U.K. following. Most of the volunteers are serving in Kurdish militias in Iraq or Syria.

The average age of a Western anti-ISIS fighter is 32 -- a few years older than the estimated average age for foreign fighters streaming into ISIS's side -- though most of the Western anti-ISIS fighters are in their 20s, the report says. The term "anti-ISIS foreign fighters," as used in the report, also includes those who joined groups that are battling al Qaeda's Syrian affiliate.

The reasons for so many Americans to leave the safety of their homes and travel thousands of miles to fight ISIS in the desert vary, but the report notes that for many, they are similar to those that motivated so many young men and women to fight for ISIS: the search for a sense of belonging and meaning in their lives, or a dissatisfaction with the way the international community has responded to strife in the Middle East.

"The primary grievance relates to atrocities being committed against civilians, with many accusing world leaders of turning a blind eye to the ongoing suffering of those caught up in the conflict," the report says. While the report in that section is referring to anti-ISIS foreign fighters, a similar refrain is common in pro-ISIS propaganda, which accuses the U.S.-led coalition of routinely killing women and children and calls on recruits to join the fight to protect the "caliphate."

A 28-year-old former U.S. Army soldier, identified only as Brett, told ABC News' "Nightline" last year that he joined an anti-ISIS Christian group to help defend the defenseless.

Brett, who is referenced in the ISD report, told ABC News then, "People ask me, 'Why you?' I come back and say, 'Why not? Why just me? Where's everyone else at? ... Jesus says, you know, 'What you do unto the least of them, you do unto me.' I take that very seriously."

The motivations are a bit more complicated for some of the other U.S. military veterans who have made the dangerous journey. For those individuals, the report says, they may have had trouble adjusting to life in the civilian world after their service or felt more at home in a war zone.

Jack Murphy, a former Army Special Forces soldier who has written for the special operations news website SOFREP.com about the phenomenon of American anti-ISIS volunteers, told ABC News that many vets who volunteered served in non-combat roles previously and feel like they "missed the war."

"Professional military veterans among the foreign fighters are very rare. Many served in the military, but experience ranges widely," Murphy said. "Many never saw combat previously. Some were support guys -- all sorts of stuff."

There are also those who said they came back to "finish the job," the report says.

"This factor is inherently linked with not only frustrations with existing responses to the current crisis, but also the failure to consolidate the post-conflict reconstruction and stabilization that they contributed to in Iraq after 2003," it says. "For these fighters, the state of affairs in the region has led them to feel that the death or suffering of civilians and fellow military personnel has been largely in vain."

"All the American veterans that have died over there -- and paid their lives for that country so they could have democracy -- that just resonated in my mind," another American volunteer, Jordan Matson, told CNN, as cited in the report. "I couldn't live with myself letting that country fall, and all my brothers' lives be for nothing."

Then there are those just in it for the rush. The report says some Westerners are "motivated by a search for personal fulfillment, adventure and excitement, or even a base urge to fight. Murphy added more bluntly, "Some are downright psychopaths."

Murphy said that while an interesting phenomenon to study, the few hundred Western foreign fighters trying to take on ISIS are nonetheless "strategically irrelevant" when considering all the much larger military and militia forces involved in the fight against extremist groups in Iraq and Syria.

"Foreign volunteers, who are not crazy, can help and have a positive impact, but they are not that important to the war effort at the end of the day," he said.

The U.S. State Department has strongly discouraged Americans from traveling to Iraq or Syria, much less taking up arms there.