New U.S.-Backed Alliance to Counter ISIS in Syria Falters

But 10 days of interviews and front-line visits across northern Syria with many of the forces in the alliance, called the Syrian Democratic Forces, made clear that so far it exists in name only, and that the political and logistical challenges it faces are daunting.

One Arab commander, sitting near the earthen wall that separates this deserted town in Syria from the Islamic State’s front line, bitterly recalled being chased from his Syrian hometown by the jihadists and said he would do anything to reclaim that territory. But then he detailed a list of things his forces needed: ammunition, radios, heavy weapons and more American airstrikes.

“This is the state of our fighters: trying to fight ISIS with simple means,” he said, pointing to a fighter in broken boots, tattered fatigues and a dirty sweatshirt that read “Skateboarding ruined my life.”

Beyond the early logistical factors, the new alliance faces what is perhaps a more serious challenge in the long term: Though it is intended to begin clawing back territory from the Islamic State in mostly Arab areas, nearly all of the group’s fighting power comes from ethnic Kurdish militias.

That demographic reality is likely to further alarm Turkey, a vital American ally that considers Kurdish autonomy near its southern border a security threat. It also limits the forces’ ability to strike the jihadists in predominantly Arab communities — Kurdish fighters have less motivation to fight for those areas, and could deeply anger residents by doing so.

“The backbone of these forces are the Kurdish groups because of their experience fighting ISIS and their numbers,” said Redur Xelil, a spokesman for Syria’s dominant Kurdish force, the Y.P.G. But he talked about how that could be a limiting factor in fighting for cities like Raqqa, the Islamic State’s headquarters in Syria: “We have to be realistic that the Y.P.G. can’t go by itself into Raqqa, or people will say, ‘What are you doing there?’ ”

A newly appointed spokesman for the alliance briefed reporters in Syria beneath a yellow banner bearing its name in Kurdish, Arabic and Assyrian. But the meeting took place inside a Kurdish militia facility because the alliance does not have its own bases yet, nor flags to put on its cars or a defined command structure, said the spokesman, Talal Sillu.

The combined force is to be commanded by a six-person military council, Mr. Sillu said. But he acknowledged that only one member had been selected so far — Mr. Sillu himself.

Last week, President Obama announced plans to deploy dozens of Special Operations troops to support the new alliance. And before that, American officials said 50 tons of ammunition had been airdropped for Arab fighters with the new group.

But already, things have not always gone as planned. Since the ammunition airdrop, American officials have privately acknowledged that the Arab units it was intended for did not have the logistical capability to move it. So, again, the Kurds were called to help.

An array of smaller groups have allied with the Kurds, including Arab and Turkmen rebels, Christian militias and Bedouin fighters loyal to a sheikh who considered the Libyan leader Col. Muammar el-Qaddafi a friend.

While these groups hate the Islamic State, most are small, and some have been repeatedly routed by the very jihadists the United States now hopes they will defeat.

While the Kurds have become used to securing territory, with uniformed forces and a clear chain of command, their Arab allies often leave teenagers with Kalashnikovs at checkpoints who stop and release cars at random, scaring drivers.

A commander of one Arab group lamented that while Kurdish commanders could simply order their fighters to move, he could only make suggestions and hope his men complied.

Some of the alliance’s forces have cooperated before, but relations are not always smooth. The Kurdish military strength in the area means that Kurds set the agenda, and many clearly look down on their Arab partners.

For their part, Arab rebel fighters say they worry about their partners’ close ties to the Kurdistan Workers’ Party, or P.K.K., which the United States, Turkey and others list as a terrorist organization. They also distrust the motives of the thousands of Kurdish fighters who have come to Syria from Iraq, Turkey and Iran.

“ISIS brings foreign fighters for an Islamic State, while they bring foreign fighters for a Kurdish project,” said one Arab commander with the Raqqa Revolutionaries Brigade who goes by the name Abu Hamza. “But if that is how they think, they’ll fail.”

At another position near Ein Eissa, a swaggering Kurdish commander listed his militia’s victories against the Islamic State before acknowledging that he — like many of his fighters — was not Syrian. He was from Iran, and unabashed about being another foreign fighter in Syria’s civil war.

“I came to bring democracy, while ISIS came to kill,” said the commander, Gali Cilo. “That is the difference.”

The roots of the Syrian Democratic Forces lie in Syria’s northeast corner, a long-neglected region where most of Syria’s Kurdish minority lives alongside other ethnic groups in impoverished towns scattered among wheat fields dotted with aging oil wells.

While world attention since the Syria conflict began has focused on fighting between the forces of President Bashar al-Assad, Sunni rebels and the Islamic State, the Kurds have taken advantage of the chaos to carve out an autonomous zone.

Much of that has been done over the last year, as the Y.P.G. — the Kurdish abbreviation for the People’s Protection Units, the dominant Kurdish force in Syria — has closely coordinated with the United States and its allies to seize land from the Islamic State in a long strip along the Turkish border.

Evidence of the Kurdish group’s dominance is obvious. The militia runs ubiquitous checkpoints; photos of its “martyrs” adorn billboards; and its fighters hold most of the more than 280-mile-long front line with the Islamic State. Parts of it have come to resemble an international border, with deep trenches and high berms running for miles, lined with bright lights to prevent jihadist infiltrators. The whole line is dotted with heavily sandbagged positions to protect against machine gun and mortar attacks by the jihadists.

A senior United States military official said the United States had encouraged the Kurdish militia to create an umbrella group that would make more sense to an international audience, and Kurdish leaders decided to call it the Syrian Democratic Forces.

But the name of a subgroup of Arab brigades called the Syrian Arab Coalition was “an American invention,” the senior official acknowledged. It had about 5,000 fighters, and roughly 20 percent of them said they would defend their land but would not go on the offensive against the Islamic State.

The dominant Kurdish force, the Y.P.G., meanwhile, is believed to have about 40,000 fighters — including thousands from neighboring countries and many linked to the outlawed Kurdistan Workers’ Party.