Barcham Zana knows her enemy. It is the Islamic State, which she calls “darkness.”

Islamic State militants killed two cousins, she said. For her, the group is not an abstract threat.

The 20-year-old spoke through a Syrian interpreter at a rudimentary training camp surrounded by golden waves of wheat in northern Syria. Zana and other young fighters had just finished target practice with AK-47s on a firing range encircled by earthen berms. The nearest front line fighting was about 50 miles away.

Zana is a member of the YPJ, a predominantly Kurdish women’s militia. She and her commander, Nujin Dirik, say they are dedicated to the cause. But they also reflect the depth of ethnic and other complexities facing the U.S. military as it seeks to develop a coherent and competent network of local Syria forces to defeat IS.

A small group of American military advisers works here with Zana and other Syrian volunteers — mostly Arab men — who have taken up arms against what they see as a scourge on their country and a threat to their families. One called IS an “illness.” Several said they were fed up with the group and eager to destroy it.

The Americans said the number of Arab volunteers has surged this spring, following a series of battlefield gains against IS, including the retaking of al-Shaddadeh in Hassakeh province. That success triggered a recruiting boom, with more local Arabs seeking to join than could be accommodated, the Americans said.

One U.S. adviser called the recruits “raw ... literally civilians coming off the streets.” The adviser could not be quoted by name under ground rules set for reporters who visited the camp Saturday with Army Gen. Joseph Votel, the head of U.S. Central Command. The camp’s location also could not be disclosed.

A female YPJ trainee who gave her name as Athima said she feels a moral imperative.

“It’s my duty to stop this fighting,” she said, referring to the Islamic State.

Dirik says she is fighting for something bigger: a Syrian Kurdistan, known locally as Rojava, which already is a semi-autonomous region of northeastern Syria comprised of three self-governing cantons. Judy Ossi, who runs a humanitarian coordination office for her canton, Jazira, said the focus on war is obscuring the suffering of thousands of displaced Syrians and Iraqi refugees, whose numbers she said already are overwhelming and are likely to grow.

The women of the YPJ revealed no fear of combat. Zana said she initially was afraid of the Islamic State, knowing of their brutal tactics, including beheadings. But once she joined the YPJ and began training, her fears disappeared, she said.

That strength-in-numbers theme applies as well to the broader U.S. strategy in Syria. The Americans hope they can pull together enough local fighters to capture the main prize in northern Syria: Raqqa, the Islamic State’s self-declared capital. Raqqa already is largely isolated, with signs that IS fears an imminent offensive.

The U.S. has organized the fighters into an umbrella group it calls the Syrian Democratic Forces, or SDF. It is comprised mostly of Syrian Kurds, numbering at least 25,000 fighters, with a smaller element of Syrian Arabs numbering perhaps 5,000 to 6,000.

One U.S. adviser here estimated the SDF will need perhaps 6,000 to 10,000 fighters for a Raqqa offensive. The U.S. strategy relies on training, organizing and advising local fighters for such combat, rather than committing American forces.

That has proven to be a slow approach, subject to much criticism in Congress. It is based on a belief that the locals are best able to sustain a lasting defeat of the militant group.

Votel, who inherited the strategy when he took over Central Command in April from the plan’s main military architect, Gen. Lloyd Austin, said in an interview that he believes it is working.

The key, he said, is tailoring U.S. support for the local Arab, Kurd and other local fighters so that they can do things their own way, “not trying to replicate how we would do things.”

“It may not be the exact way we would do something in the American army or a Western military force, but the approach they take works for them. That’s what’s important,” he said.