ANTAKYA, Turkey — A rebel commander named Mohamed Zataar sat on a living room couch in the ancient Turkish city of Antakya one recent night, taking a short break from the war across the border with Syria some 15 miles down the road. He was eager to return. “There is a new battle starting,” he said, staring at the door. Instead Zataar, who leads a battalion of moderate rebels called Wolves of the Valley, decided to call his enemy from his iPhone.

He dialed the number for the shadowy jihadi known as Abu Ayman al-Iraqi, one of the most notorious men on the chaotic battlefields of northern Syria. Abu Ayman doesn’t fight for the Syrian regime. He’s a leader in the Islamic State of Iraq and Syria, or ISIS, the al-Qaeda-inspired force that has upended the rebellion with its fanaticism and brutality — while also kidnapping Western journalists and raising global alarms that the foreign fighters who fill out its ranks will return to sow terror at home. Other rebel groups turned on ISIS at the start of the new year, sparking an internal war that men like Zataar, a former dealer of fake antiques who despises extremists, were happy to join. “We are fighting a war against terror,” Zataar said.

Someone answered on the other line, and Zataar asked to speak with Abu Ayman, whom he referred to as “sheikh.” Then he hung up, saying it wasn’t uncommon for the two men to speak. An hour later, Abu Ayman called back.

The long conversation that followed — full of threats and insults, religious debates and petty disputes — opened a rare window into an internal conflict that has changed the landscape of Syria’s civil war, as homegrown fighters try to wrest back the rebellion from the radicals who have increasingly defined it both inside Syria and abroad. The two commanders jawed at each other for more than 90 minutes, with Zataar putting his phone on speaker so this reporter could listen and record. It showed the battle’s strange intimacy, and the new confidence of men like Zataar as they fight to reclaim their turf.

The call also offered a unique and candid look at the mindset of ISIS at its leadership level, showing the group’s fear and isolation inside Syria — and above all, a deep-rooted suspicion of U.S. influence that makes ISIS see rival rebels as agents of America and its allies, sent to destroy them, and leaving little room for compromise.

“The West has understood the game. They won’t send anyone whose name is William or Benjamin. They’ll send people named Ahmed and Mohamed and Abdullah,” Abu Ayman told Zataar in his distinct Iraqi accent. “The American soldier is expensive, so they will use people from among us — and this is the truth of the battle, they will use you — in the fight against us.”

Like other ISIS leaders, Abu Ayman keeps a shadowy persona, releasing no audio or video statements and revealing nothing about his true identity. His nom de guerre simply means “Ayman’s dad, from Iraq.” He is the so-called emir, or prince, of the Latakia province, one of a handful of regional leaders who answer to the group’s top commander, Abu Bakr al-Baghdadi. The most senior figures in ISIS are thought to be Iraqi — and veterans of the insurgency against U.S. forces. Some rebels speculate that Abu Ayman sits as high as third in the organization’s Syria ranks. But beyond that, there’s only his reputation.

Among rebels and activists, Abu Ayman had “maybe the worst reputation” of any ISIS commander during the group’s surge in Syria last year, said Noah Bonsey, a Beirut-based analyst with the International Crisis Group. They accuse him of a special brand of savagery. He is said to have killed one well-liked moderate commander in cold blood last summer. Later, he allegedly tortured and executed another rival commander’s men, leaving their bodies by the roadside. When a local cleric was sent over to mediate, the story goes, Abu Ayman killed him too.

Abu Ayman’s notoriety is “symptomatic” of the group’s aggressive push for influence, Bonsey said, and its ruthlessness helped it overtake other rebel groups. It also imposed hardline Islamic law in the areas it controlled and kidnapped, tortured, and executed civilians.

“Abu Ayman, just for you to relax, I will let you know something: You are fighting people who don’t lie down before oppression,” Zataar said.

“Yeah?” Abu Ayman replied.

“And who will never allow anyone to subjugate one person from among the Syrian people,” Zataar said.

“To me, Islam is higher than the Syrian people,” Abu Ayman said.

“To me it’s the Syrian people, brother,” said Zataar.

It was the religiosity and brutality of ISIS that helped turn Syrians against the group, and when the blowback erupted in January, it came with surprising intensity. A broad cross-section of rebel factions has taken part in the war on ISIS, including hardline Islamists. But front-and-center is a new coalition of moderate rebels called the Syrian Revolutionaries Front, made up of commanders like Zataar — who considers himself one of the original rebels, fighting the Syrian regime from the start.

By his own admission, Zataar, 46, spent four years in prison for his business dealings, during which he continued his work and bought three new homes. He’d been free for a year when the uprising began and quickly formed his own fighting force — because, he said, “nobody can take the decision not to be involved in this revolution.” He had a brash and wind-blown air, wrapped in a black cardigan, his greying hair swept back into a thick, frizzy mane. He rested a big fist on the coffee table as he spoke, flipping a paring knife through his fingers.

“My dear Abu Ayman, listen to me,” Zataar said. “Let’s talk man-to-man.”

The two commanders clearly took the fight personally. Their forces had been engaged in bitter combat, and they bickered like hardened rivals, each with his own axes to grind, from stolen weapons to broken ceasefires. As Zataar spoke, two of his men leaned forward in their seats around the coffee table, silently nodding and pumping their fists — or when it was Abu Ayman’s turn, throwing up their arms in exasperation. The background noise suggested a similar scene on Abu Ayman’s side.

Zataar often seemed to be toying with his enemy, mocking him with his tone, trying to rile him. He skinned and ate oranges as the conversation dragged on.

Lamenting a broken truce in a village called al-Zanbaq, Zataar said: “You went into al-Zanbaq and took our horses, Abu Ayman, and went into our homes and took our clothes.”

“You listen to me — what horses?” Ayman said.

“Brother, I have genuine Arabic horses, and they took them,” Zataar said.

“Who took them?”

“You took them!”

“Do you see a hair of your horses here? We didn’t take them.”

“I got them back. No problem.”

“We didn’t take the horses!”

“Abu Ayman, just listen to me, you got into our houses, and checked them out, and took our women’s clothes.”

“This is not true!”

The two men then argued about the treatment of prisoners. Zataar accused Abu Ayman of executing one of his men — and of beating another, whom he referred to as Sheikh Ibrahim, and shaving his beard.

Abu Ayman discussed the matter with someone nearby. “The guy with me now knows Sheikh Ibrahim,” he said, and put the man on the line.

“May God be with you,” the man said.

“May God give you life,” Zataar replied.

“In the name of the one who raised the skies without foundations, I will say what I have seen with my own eyes,” the man said. He explained that while Sheikh Ibrahim had been beaten briefly by an ISIS fighter, “I immediately stopped him.”

“Are you Abu Mansour?” Zataar asked.

“I am Abu Mansour or another man, it is no difference.”

“Just listen to me, Abu Mansour. Our guys asked you to bring a Qur'an for them, but you brought a divergent version of the Qur'an.”

“God forbid.”

“Man, do not say God forbid,” Zataar said impishly. “I beg you just to be honest.”

“Oh Lord, this is a huge lie.”

“You brought them a Qur'an, and the guys discovered that there were mistakes in it.”

“Oh Lord, this is a huge lie. Oh Lord, this is a huge lie.”

“Didn’t Sheikh Ibrahim tell you that there were mistakes in the Qur'an?”

“We have foreign fighters — they may have brought Warsh’s version, which is different from Haf’s version. Our brothers from Morocco and Tunisia read Warsh’s version, if you know about versions.”

“I don’t know about versions,” Zataar said.

Later, Zataar offered Abu Ayman “a piece of advice from us to you: Kill any of us who you arrest, OK?”

“OK.”

“Even if you get me, I don’t have any problem with you killing me, because, Abu Ayman, we made everything clear, and in the war against you, we won.”

“Any one of us you see, kill them too, and if I’m the first to be caught, kill me,” Abu Ayman replied.

“Especially you, God willing, if we catch you, we won’t kill you.”

“Why?”

“I promise you.”

“Why won’t you kill me?”

“I will make you live among us for at least a year, so you will know who we really are.”

“A whole year — good.”

“And then I will release you to go wherever you want.”

“Good.”

“That’s a man’s promise, God willing.”

“Your generosity overwhelms me and makes me want more,” Abu Ayman sneered.