By this time, Mr. VanDyke, whose experience was popularized in a documentary seen by a number of these veterans, had started his own company, Sons of Liberty International, where today he trains Kurdish battalions in the northern Iraq city of Erbil.
He says he often finds himself discouraged, however, by the motivations of the former US military veterans who reach out to him.
“Some people never got to fight,” he says. “So they want a chance to do that.”
Other reasons are even more problematic. “Some people just enjoy war – I don’t want those people around me,” he adds. “I don’t want thrill-seekers.”
What appears to be missing in many of these folks, he says, “is a genuine sense of someone wanting to make the best contribution to the region.”
It is a complex combination of these motivations that drive Americans – upwards of 130 of them, by the estimates of US officials – to join the ranks of those fighting either against or for the Islamic State, also known as ISIS or ISIL.
The pro-IS contingent was brought into sharp relief this week when the US filed charges against former Air Force mechanic Tairod Nathan Webster Pugh, who served from 1986 to 1990. Now almost 50 years old, he is accused of trying to link up with the Islamic State.
He appears to have been drawn in, US prosecutors say, at least in part by IS propaganda videos found on his laptop.
By contrast, many foreign fighters who have opted to fight against IS say they are seeking a greater sense of purpose. Some US veterans add that they see in the anti-IS fight a clear vision of the enemy that they never found in combat.
Others want to complete combat work they feel was left unfinished or undone by the advance of IS in Iraq.
Wearing body armor and a traditional keffiyeh, the black-and-white scarf popularized by Palestinian leader Yasser Arafat, US Army veteran Jordan Matson teaches his fellow Kurdish fighters how to fist-bump.
He is no longer a US soldier, but has arrived in Iraq on his own, after getting in touch with Kurdish fighters via Facebook.
He was drawn by the prospect of protecting the Iraqi people from IS, he tells a French documentary film crew.“I want to stand up for the Kurds in this area,” he says.
After a two-year stint in the Army, which included no combat experience, Mr. Matson was among the first foreign fighters to have joined with Iraqi Kurdish forces against the Islamic State.
In the documentary, he arrives at the camp as the fighters are taking a break. “Stay here, it’s comfortable,” the Kurdish fighter tells Matson, who responds that he would rather be fighting ISIS.
“You really like to fight?” a Kurdish comrade asks him. “Jordan is impatient,” one of his fellow fighters notes.
Working at a food-processing plant in Wisconsin after leaving the military in 2007, he told CNN, "Civilian life just wasn't for me."
Another former service member, Marine infantryman Patrick Maxwell was honorably discharged in 2011 after fighting in some of the toughest regions of Iraq.
In writing about Mr. Maxwell’s possible motivations for joining battle, fellow Marine Thomas James Brennan posits in a New York Times blog post that perhaps it was because “he never fired his weapon, and I could understand his disappointment.”
Indeed, Maxwell reportedly said that he “felt robbed.”
Mr. Brennan cautions, however, that while he can understand that sentiment, there is a darker side to battle. “I carry the weight of the lives I’ve stolen, some of them innocent.”
Yet Brennan admits that there is an allure of battle, in recapturing the bonds he once felt with his fellow comrades in arms.
“I was jealous of him and it upsets me that I don’t fully understand why,” he writes. “My desire for war is something I believe I will always struggle with even though my longing for peace is much stronger.”
For his part, VanDyke recalls, too, the close bonds he formed in battle, and the belief in the cause that led him to it. An aficionado of action films growing up in Baltimore, he found something different on the front lines.
“Combat was a lot more stressful – the randomness of death was a real eye-opener,” he says.
“Whether one person lived or died didn’t necessarily depend on whether they did everything right or not,” he adds. “That was a surprise.”