But they did not suspect the teenagers pushing a broken-down sedan past the front gate. Then a boy who looked no more than 14 blew up the car and himself, unleashing an assault that killed or wounded nearly 30 rebel fighters and ultimately put the entire city of Minbej under the control of the most extremist jihadi group in the Syrian conflict.
“They call us godless. They attack us from the front, they attack us from the back,” said Sheikh Hassan, the leader of the Free Syrian Army brigade that came under attack.
In doing so, it is simultaneously battling the Syrian and Iraqi governments and Sunni rebels it considers insufficiently committed to Islam. Having seized vast areas of Iraqi territory and several large and strategic cities, including the country’s second-largest, Mosul, it controls territory larger than many countries and now rivals, and perhaps overshadows, Al Qaeda as the world’s most powerful and active jihadist group.
The fighting in Minbej took place six months ago, but the methods the Islamists used so effectively in northern Syria helped set the stage for their blitzkrieg in Mosul, Tikrit and other key Iraqi cities this week.
Detailed descriptions from Sheikh Hassan and his men, along with several other rebels who have been fighting the jihadists for the last six months, paint an unsettling portrait of the formidable jihadist movement.
The group is a magnet for militants from around the world. On videos, Twitter and other media, the group showcases fighters from Chechnya, Germany, Britain and the United States.
Its members are better paid, better trained and better armed even than the national armies of Syria and Iraq, Sheikh Hassan said.
Many of the recruits are drawn by its extreme ideology. But others are lured by the high salaries as well as the group’s ability to consolidate power, according to former members, civilians who have lived under its rule in northern Syria and moderate rebels.
Other rebel groups often squabble with one another while fighting the government. But the Islamic State of Iraq and Syria has stayed cohesive while avoiding clashes with the military of Mr. Assad, who seems content to give the group a wide berth while destroying less fundamentalist rebel groups.
In areas that fall under their control, the jihadists work carefully to entrench their rule. They have attracted the most attention with their draconian enforcement of a fundamentalist interpretation of Islamic Shariah law, including the execution of Christians and Muslims deemed kufar, or infidels.
But the group is not only following a stone-age script. It rapidly establishes control of local resources and uses them to extend and strengthen its grip.
It has taken over oil fields in eastern Syria, for example, and according to several rebel commanders and aid workers, has resumed pumping. It has also secured revenue by selling electricity to the government from captured power plants. In Iraq on Wednesday, the militants seized control of Baiji, the site of Iraq’s largest oil refinery and power plant.
In Minbej, the jihadists initially left bakeries and humanitarian aid groups alone, taking over their operations once they had established military control of the city. The group takes a cut of all humanitarian aid and commerce that passes through areas under its control.
One of the first militia leaders to resist the Islamic State of Iraq and Syria, Abu Towfik from the Nouredin Zinky Brigade, said that its sophisticated tactics made its fighters hard to dislodge. Since last year, the militant group has fought with tanks captured from the Iraqi military.
Given that tenacity, Abu Towfik said, they will be hard to drive out of the territory they now occupy in northern Syria and Iraq. “I am afraid as time goes on they will spread their extreme ideology and we’ll have a regional war,” he added.
At a meeting of rebel commanders at a Gaziantep Hotel cafe, Abou Sfouk, head of the rebel Free Syrian Army’s Palestine Brigade, brought a prized captive: a former jihadist named Mustafa.
At the beginning of the uprising, Mustafa had fought with Abou Sfouk’s brigade, but he joined the Islamist group in early 2013, when it entered Syria from Iraq, because it offered to triple his salary, starting him at $400 a month.
“Wherever we took territory, we would declare people apostates and confiscate their property,” Mustafa said. “We took cars and money from Christians, and from Muslims we didn’t like.”
Mustafa, a trained bulldozer mechanic, became the “emir of the motor pool.” But he eventually came under suspicion when it became known that he had once served under the kufar, or infidel, rebel army.
After a summary trial before one of the group’s Islamic courts, Mustafa was sentenced to death. A friend helped him escape, and he sought protection with his old brigade commander.
“I would never trust him again,” said his old commander, Abou Sfouk. “But he has useful military information.”
The defector has revealed the locations of Islamist prisons and the identities of the group’s commanders. Many of the top leaders and front-line soldiers come from abroad, but more than half of the membership is made up of Syrian and Iraqi tribesmen, people well known to their relatives and former neighbors now fighting against them.
“We are moderate Muslims,” Sheikh Hassan said. “We will fight anyone who covers themselves in Islam and tries to talk in the name of our religion.”
A graduate of Koranic studies from Damascus University, Sheikh Hassan considers his own credentials impeccable. He learned to fight as a foreign volunteer with Iraqi resistance fighters attacking American soldiers a decade ago.
Now, he said, he is desperate for more American help as he wages a war against jihadists with whom he once shared a struggle. “There is a hole between us,” he said with a shrug. “We will have to kill them. But we’re humane. We won’t cut their throats; we will shoot them.”