Its membership drew on Syrians who gave up on the protest movement, as well as Islamists released from prison as part of what many saw as Mr. Assad’s strategy to undermine secular activists. Some members had fought in Iraq and Afghanistan.

The group established its bona fides as an anti-Assad military force, but it has remained grounded in militant Sunni Islam. A recent promotional video quotes Abdullah Azzam, Bin Laden’s mentor in Afghanistan. And Hassan Aboud, the group’s first leader, has called for the establishment of an Islamic government in Syria.

“Democracy is people ruling each other based on their own judgments,” Mr. Aboud said in an interview with Al Jazeera in 2013. “We have a godly system made by God for his creation and his worshipers, and God charged us to construct it on earth and to raise the word of God.”

“As Muslims, this is our right,” he added.

Last year, a mysterious blast targeted a meeting of Ahrar al-Sham’s leaders, killing Mr. Aboud and dozens of others and leading many to expect that the group would fade away. But its remaining members reorganized, giving it an institutional strength lacking in other groups, said Ahmed Qara Ali, the group’s spokesman.

It has since grown into one of Syria’s largest rebel groups, with fighters across the country, offices for aid and political affairs and control of a border crossing with Turkey. It is a key member of the Army of Conquest, a powerful rebel alliance fighting the government in the northwest. This month, it represented the opposition in direct negotiations with Iran over the fate of three besieged communities.

Analysts and the group’s members say it differs substantially from the Nusra Front.

It is led by Syrians, has few foreign fighters and opposes the breakup of the country. It has not started campaigns to impose strict religious mores and maintains good relations with other rebels. Its leaders have pledged to protect minorities, though some still call them derogatory names.

They have also pledged to fight the Islamic State, calling the self-declared caliphate a perversion of their religion.