CAIRO — In the ramshackle town of Atareb, a Free Syrian Army bastion 15 miles north of Aleppo, Maj. Anas Abu Zaid said he has looted Russian rockets, American-supplied anti-tank missiles and other firepower to hold off the forces of Syrian President Bashar Assad.

He says it’s time to move on.

“We were facing airstrikes on a daily basis, but now some civilians are coming back to Atareb,” said Maj. Abu Zaid. “We are working to put in place civil governance for the town and even rebuilding some houses.”

His optimism reflects an energy that has infused the once-faltering rebel force in the wake of missile attacks that President Trump ordered on a Syrian air force base this month following Mr. Assad’s suspected use of chemical weapons on civilians.

Analysts say it doesn’t take a lot to tip the balance from one side or another in Syria’s grinding conflict, which is why the U.S. missile strike, limited as it was, had such an impact, said Alberto Fernandez, a retired State Department counterterrorism officer who is the go-to authority on capabilities and limitations of the multiple rebel groups in Syria.

Add to that the fact that the much-derided U.S. effort to train the Free Syrian Army fighters is starting to pay dividends on the battlefield, boosted by substantial financial aid from wealthy Persian Gulf emirates, Mr. Fernandez said, “A war that has been going on so long is basically a war of attrition and exhaustion, and all parties are being worn down.”

Mr. Fernandez, now president of the Washington-based Middle East Media Research Institute, said, “Those that remain from each part, unit or entity are the fittest, the most clever, the most savage and the most capable. So the question is: Who is going to be the last man standing?”

“Too often, [the FSA has] been written off, and they shouldn’t be,” he said. “On the other hand, they have been limited — like everyone else — in what they have been able to do, so far.”

Charles Lister, a senior fellow at the Middle East Institute, wrote in a lengthy study that the Brookings Institution released in November that the FSA was far better than its reputation suggests and has evolved into an effective fighting force while retaining a base of popular support that few of its rivals can match.

“By late 2016, the FSA had come to represent an expansive, socially and symbolically powerful but complex umbrella movement, composed of dozens of semi-autonomous armed opposition groups that are united by the original moderate ideals of Syria’s revolution,” Mr. Lister concluded.

He called the FSA “the cornerstone of Syria’s moderate opposition component.”

“For the U.S. and allied countries seeking an eventual solution to the crisis in Syria, the FSA’s military preeminence does not necessarily have to be the sole objective, but sustaining its ability to represent opposition communities is of crucial importance given its mainstream positions,” Mr. Lister said.

Maj. Abu Zaid was one of the Free Syrian Army officers selected by the Pentagon in 2015 for a U.S. program to boost moderate forces after previous training programs faltered.

In February, that effort reaped results when, with help from the Turks, Free Syrian Army forces took over almost 1,250 miles of territory from the Islamic State group on Syria’s northern border.

“The Americans made it clear that the regime was not the world’s priority, and the issue was defined as terrorism,” said the major, who added that Mr. Assad’s behavior since then has proved that the U.S. training was worth the cost. “With the Khan Sheikhoun chemical attack, Assad reminded them he was the biggest terrorist.”

Mr. Assad’s forces were responsible for more than 90 percent of the 207,000 civilian casualties in Syria from March 2001 through February 2017, according to the Violations Documentation Center, a monitoring group working with human rights activists inside and outside Syria.

Assad’s weaknesses

Free Syrian Army fighters insisted that the chemical attack on Khan Sheikhoun revealed Mr. Assad’s fundamental weaknesses while highlighting their own stamina as a fighting force.

“His only way to defeat the people is by punishing civilians with these weapons to put pressure on them to make local truces, forcing them to leave,” said Maj. Issam Al Reis, a 41-year-old spokesman of the Free Syrian Army’s southern front near the Jordanian border. Pro-Assad forces “don’t have enough manpower to defend their front lines.”

Despite reports in the second half of last year that Mr. Assad’s forces, backed by Russian and Iranian support, had scored some major victories, facts on the ground support the rebels’ confidence.

Analysts at Omran Center for Strategic Studies, an independent think tank in the United Arab Emirates, said that despite Russian and Iranian backing, the Free Syrian Army controls almost 17,700 square miles inside the country, compared with less than 14,000 square miles in 2015.

Northeast of Damascus, Free Syrian Army forces briefly occupied the towns of Qaboun and Barzeh. The wins were ultimately reversed by the regime and Russian airstrikes, but they were a surprise to those who had written off the rebel group as irrelevant to Syria’s future after their defeat in Aleppo late last year.

“Thanks to the Russian brutality, we tended to think a month or two ago that Assad had prevailed and that he can do whatever he likes,” said Mordechai Kedar, a Syria specialist at the Begin-Sadat Center for Strategic Studies at Bar Ilan University near Tel Aviv. “I would not repeat that assessment today.”

As the civil war continues, the insurgents’ success should help them garner more aid from the West, said Fahad Almarsy, a former Free Army spokesman who now leads a loosely affiliated political organization in Paris called the National Salvation Front.

“The United States and Israel can target [Lebanese] Hezbollah and Iranian forces propping up Assad in and around Damascus and help the Free Army advance and clear Syrian territory of foreign fighters,” he said.

While most of the Islamic State’s losses in its Syrian base stem from Kurdish Syrian Democratic Forces, who now control 20 percent of Syria, the group’s links to Kurdish separatists in Turkey bar them from becoming close U.S. allies, said Ayman Abdul Nour, an early opponent of Mr. Assad and a leader of Syria’s exiled Christian community.

“The Free Syrian Army is now positioned as America’s best bet if Washington wants to see a unified or at least a federal Syria,” Mr. Abdul Nour said in a telephone interview from Dubai, United Arab Emirates.

The rebels said they intend to keep up the pressure on Mr. Assad. Their “Victory Army” in west-central Syria recently turned their guns on the regime’s Hama Military Airport, using Russian missiles to destroy a Russian-made fighter jet. Like the American missile strike, which destroyed six Mig-23s at the Al-Shayrat Air Force base, the attack was designed to downgrade the size and shorten the reach of Mr. Assad’s air force.

Refugees from regime-controlled areas, meanwhile, are joining rebel enclaves committed to Mr. Assad’s downfall.

“The people suffer exhaustion from the war, but they are still loyal to the Free Army,” said Kamal Bahbough, a 36-year-old physician in the besieged town of Al Rastan, about 14 miles north of Homs. “The Free Syrian Army soldiers are the sons of this region.”

• Gilgamesh Nabeel reported from Istanbul.

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