Correction: An earlier version of this story incorrectly said the rebels’ base was in the Syrian province of Deir al-Zour. It is actually located in Homs province.

The New Syrian Army completed the U.S. training course in Jordan, infiltrated into Syria and last March seized a pinprick of territory from the Islamic State, at the remote Tanaf border crossing with Iraq in the far southeast of the country. (TWP)

The New Syrian Army completed the U.S. training course in Jordan, infiltrated into Syria and last March seized a pinprick of territory from the Islamic State, at the remote Tanaf border crossing with Iraq in the far southeast of the country. (TWP)

Throughout the fiasco of the Pentagon’s $500 million effort to train and equip a force of Syrian rebels to take on the Islamic State, one small group endured.

The New Syrian Army completed the U.S. training course in Jordan, infiltrated into Syria and then, in March, without fanfare or publicity, seized a pinprick of territory from the militants at the remote Tanaf border crossing with Iraq in the far southeast corner of the Syrian province of Homs.

There they have remained, holding their ground without deserting, defecting or getting kidnapped, unlike many of the other similarly trained rebels whose mishaps prompted the temporary suspension of the program last year.

Even this modest success is now in jeopardy, however, following an Islamic State suicide attack this month. An armored vehicle barreled into the rebels’ base shortly before dawn on May 7, killing a number of them, said Lt. Col. Mohammed Tallaa, a Syrian officer who defected and is the group’s commander.

[The many missteps in the Pentagon’s failed struggle to train Syrian militants]

Tallaa would not say on the record the precise number of casualties — and the number of rebels at the base — for fear of further endangering the rebels who are left. But he said the attack came as a heavy blow to a force that was already small and suffering from a lack of weaponry and equipment that he said had been promised but not delivered.

Those who survived are now questioning whether they want to remain at all in their sparsely defended desert outpost to await further attacks, Tallaa said in an interview near the southern Turkish town of Reyhanli.

“I’m not saying the Americans let us down, but there is dereliction of duty. They are not doing what they could,” he said. “We don’t want the Americans to disrespect the lives of our men.”

A U.S. military spokesman said warplanes responded to a plea for help when the base was bombed, but did not arrive in time because the attack happened so fast. A number of airstrikes have since been carried out against Islamic State positions in the area and new supplies of weaponry have been delivered, said Col. Steve Warren, the spokesman. He said the U.S. military believes the group will survive.

“They still have Tanaf, they have been resupplied, and we think they can hold,” he said. “We think they have enough firepower, and we are providing support with airstrikes as available.”

[The hidden hand behind the Islamic State militants? Saddam Hussein’s.]

The suicide bombing has further exposed, however, the shortcomings and mistakes that have bedeviled from the outset the U.S. endeavor to build a force of Syrians capable of taking on the Islamic State. The $500 million Pentagon program conceived by President Obama two years ago got off to a slow start, with the training beginning only last spring. Months later it was suspended, after the first group of trainees was kidnapped by Jabhat al-Nusra and the second defected, handing over some of their weapons to the al-Qaeda affiliate in the process.

In March, the training was restored, with the less ambitious goal of working with existing rebel groups in northern Syria’s Aleppo province. Those groups are now battling for their survival against Islamic State fighters advancing in the area around the town of Azaz near the border with Turkey.

In the meantime, however, the Pentagon has forged ahead with a different alliance, with the Kurdish People’s Protection Units (YPG), which is seen as a rival or worse by many Arab rebel groups. The YPG is responsible for almost all of the territorial gains made so far against the Islamic State in Syria, and its close coordination with the U.S. military is a source of deep resentment among other Syrian rebels. An effort to rebrand the YPG as a coalition with Arab rebels called the Syrian Democratic Forces has brought only a small number of Arabs into the force so far.

[Top Islamic State official suggests the militants are feeling the heat]

As the United States accelerates plans to advance deeper into the Islamic State’s Arab strongholds, including a U.S.-backed push by a mostly Kurdish force toward the Islamic State’s self-proclaimed capital of Raqqa, the need for an Arab force that can win the loyalties of those who are living under militant control is growing more urgent, said Bassam Barabandi, a Syrian diplomat from the Deir al-Zour area who defected and lives in Washington.

“This is not a Kurdish fight, it’s an Arab fight — and most of the Arab Sunnis want to be part of this fight,” he said. “The only vehicle that exists is the New Syrian Army, and I have heard from many people that they want to be part of this army.”

The New Syrian Army has the potential to win support, he said, because its members are drawn from the nearby province of Deir al-Zour, which borders Iraq and contains most of Syria’s oil. Most recruits are from the remnants of a Free Syrian Army unit called the Authenticity and Development Front, which already was known in the province before it was defeated by the Islamic State during its sweep through Syria and Iraq in 2014.

“At the moment, the New Syrian Army is very small, but it would be very easy to empower it and create numbers,” Barabandi said.

Recruitment has all along proved the program’s biggest challenge, however. A requirement that applicants sign a document pledging to fight only the Islamic State and not the government of President Bashar al-Assad deterred many from signing up at all. Others who showed up for the training in Jordan were put off by a lengthy wait that sometimes lasted months as U.S. officials delved into the background of every rebel who applied. Many gave up and went back to Syria before they had been approved, said a former U.S. official familiar with the train-and-equip effort, who spoke on the condition of anonymity to be frank.

“We deserve a lot of criticism on this program,” he said. “One problem was that the vetting took way too long and the other was to force these people to commit themselves to fighting Daesh.” (Daesh is an Arabic acronym for the Islamic State.)

Meanwhile, the New Syrian Army has been left feeling abandoned and vulnerable in its isolated base, in a remote and forbidding area of empty desert miles from any major towns. It has carried out a number of operations against the Islamic State, always with U.S. support, U.S. officials and rebels say. But it never received the weapons and equipment it had hoped for, the rebels assert.

[‘It was a children’s soccer game. Of course he knew he was going to kill children.’]

It was always assumed that more rebels would be joining it, but the suspension of the program meant that none did. The original group of 50 trained by the U.S. military did later increase their numbers by recruiting and training extra fighters themselves, but the force remained far smaller than had been anticipated.

Antitank weapons, which have been used by Kurdish and Iraqi forces elsewhere with spectacular success to blow up Islamic State suicide bombs before they reach their target, also were not available when the bomber struck. The former U.S. official said the intent had always been to provide those trained under the program with antitank weapons, but Tallaa and another commander with the group said they did not materialize. Nor did equipment such as bulldozers that might have been used to build better fortifications, he said.

New Syrian Army fighters saw the suicide bomber hurtling through the desert toward them when the vehicle was two miles away, Tallaa said. They opened fire with everything they possessed, but the bullets, mortar rounds

and rocket-propelled grenades bounced harmlessly off the ­Russian-made armored vehicle, which had been reinforced with extra plates. Though they called for airstrikes, planes arrived only after the car bomb hit the base.

“If the program continues like this, there will never be an army big enough to fight ISIS,” Tallaa said. “And more than that, we will lose the guys we already have.”

The U.S. military also failed during the recruitment effort to understand some of the dynamics of the Syrian rebel movement, making poor choices when it selected commanders for the units being trained, said Barabandi, who cited Tallaa as one of the problematic commanders.

But there is no question that the suicide bombing has badly shaken faith in the U.S. commitment to supporting the force, he said. “This generated a lot of mistrust,” he said.

Zakaria Zakaria in Istanbul and Reyhanli contributed to this report.

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