Who are the Syrian rebels that have toppled Assad and taken Damascus?
HTS, which has roots in al-Qaeda, is considered a terrorist group by the U.S.
After a lightning 10-day offensive, rebel forces in Syria captured its capital, Damascus, and toppled the regime of Syrian President Bashar al-Assad.
The rebel military operations command for the Islamist Hayat Tahrir al-Sham group, or HTS, claimed on Sunday the president was no longer in the capital, writing: "We declare the city of Damascus free of the tyrant Bashar al-Assad." Russia said Sunday that Assad had left the country.
The insurgent group claimed credit for taking over four Syrian cities in 24 hours -- Homs, Daraa, Queinetra and Sweida -- in a series of rapid advances by opposition fighters that had largely been met with little resistance from government forces.
The collapse of Assad's government ended a 24-year reign, the president having succeeded his father Hafez al-Assad in 2000. The Assad family had ruled Syria since 1971.
Assad oversaw Syria's slide into a brutal civil war in 2011. His security forces sought to crush a mass protest movement demanding democratic reforms as the Arab Spring buffeted the region. The standoff devolved into a bloody civil war that split the nation on political, ethnic and religious lines.
Here's what we know about HTS:
What is HTS?
The rebel offensive over the past 10 days has been waged by HTS and a collection of Turkish-backed Syrian militias known as the Syrian National Army.
HTS, which has its roots in al-Qaeda, is considered a terrorist organization by the United States.
Who is Abu Mohammad al-Jolani, leader of HTS?
HTS is led by Abu Mohammad al-Jolani, who in recent days has been trying to distance himself from his jihadist past.
Jolani was born in Saudi Arabia to Syrian parents from the Israeli-controlled Golan Heights and was raised in Damascus.
Jolani said in an interview with PBS in 2021 that he fought for al-Qaeda in Iraq during the American occupation. Jolani said he was arrested by American forces and held for more than five years in several detention facilities, including the infamous Abu Ghraib and Camp Bucca prisons.
Jolani said his ideology has since evolved. HTS claims it has severed ties with al-Qaeda in recent years and sought to remake itself by focusing on promoting civilian government and military action, according to The Associated Press.
Are Syrian rebels ISIS?
Amid the outbreak of the country's civil war in 2011, the chaos allowed ISIS to rise in the Iraq-Syria border region and seize swaths of territory in the Levant region. The conflict also became a proxy battleground, drawing in major world powers including the U.S., Russia, Iran, Israel and the Gulf states.
Assad retained nominal control of much of the country with Russian, Iranian and Hezbollah assistance. But last month's surprise rebel offensive revealed the weakness of the regime, as fighters surged out of the rebel-held Idlib province in the northwest of the country and quickly seized multiple major cities on their way to Damascus.
What do the Syrian rebels want, and what are they planning for Syria?
If HTS gains control of Syrian government institutions, it's unknown how they will seek to govern.
"Will they revert back more to that when it was affiliated with al-Qaeda?" Javed Ali, associate professor at the University of Michigan's Ford School of Public Policy, said during an appearance on ABC News Live.
"Or will it look more like the Taliban in Afghanistan -- which is Islamist, conservative, but not, for the most part, intent on threatening its neighbors or having its country be used as a launchpad for attacks against the West."
ABC News' Victoria Beaule, Rashid Haddou, Morgan Winsor, Martha Raddatz, Jack Moore, Kirit Radia and Patrick Reevell contributed to this report.