LONDON — The stunning collapse of President Bashar Assad’s regime in Syria presents “a moment of historic opportunity,” President Joe Biden wrote on X on Sunday, as rebel fighters and Damascenes celebrated the end of their 14-year war against the authoritarian government.

But, the president added, “It is also a moment of risk and uncertainty.”

The uprising that began with a protest march in the southern city of Daraa in 2011 ended with celebratory gunfire in Damascus in 2024.

The surprise rebel offensive that surged out of northwestern Idlib province last month showed the regime in Damascus to be hollow. Its backers in Moscow, Tehran and Beirut were unable or unwilling to respond, perhaps because their attention and resources having been sapped by wars in Ukraine, Lebanon, Gaza and elsewhere.

The story of the fall of Damascus was — arguably — written in Donetsk and Dahiya. The coup de grâce, though, was inherently Syrian.

The offensive that toppled Assad was led by Hayat Tahrir al-Sham, an Islamist group with roots in al-Qaeda. The group is listed as a terrorist organization in the U.S. and European Union.

White House National Security Adviser Jake Sullivan said over the weekend that the group’s background “is a concern,” noting that elements of the group are affiliated with organizations “that have American blood on their hands.”

HTS leader Abu Mohammed al-Jolani has become the most recognizable face of the Syrian opposition. Speaking at Damascus’ 8th-century Umayyad Mosque on Sunday, Jolani said the opposition victory is “historic for the region” and that “Syria is being purified.”

It remains unclear whether and how Jolani — who is increasingly using his real name of Ahmed al-Sharaa, rather than his nom de guerre — will be able to exert control over the disparate groupings of rebel forces drawn from around the country.

What does the US think?

American officials are concerned that the power vacuum will allow ISIS to reconstitute. The U.S. launched 75 strikes against ISIS targets in central Syria on Sunday in a move that Defense Secretary Lloyd Austin said was designed “to keep the pressure on ISIS.”

“As this unfolds, there’s a potential that elements in the area, such as ISIS, could try to take advantage of this opportunity and regain capability,” Austin warned.

The future shape of U.S.-Syrian relations will depend on the composition and direction of the next government in Damascus.

The White House may be somewhat pleased by Jolani’s speech at the Umayyad Mosque on Sunday, in which he lamented how Syria became “a playground for Iranian ambitions.”

“We will remain vigilant,” Biden said on Sunday after Damascus fell. “Make no mistake, some of the rebel groups that took down Assad have their own grim record of terrorism and human rights abuses.” The president, however, added that the groups are “saying the right things now.”

“But as they take on greater responsibility, we will assess not just their words, but their actions,” Biden added.

Thomas S. Warrick — a former deputy assistant secretary for counterterrorism policy in the Department of Homeland Security — said the U.S. “has a huge stake in what comes next,” even if it was not directly involved in Assad’s ousting.

A more stable Syria “that frees itself from Iranian and Russian dependence” could, Warrick wrote, allow millions of refugees to return home, end its role as a Hezbollah conduit to threaten Israel and perhaps even join the Abraham Accords at some point in the future.

“All these unthinkable things are now possible,” Warrick — now a nonresident senior fellow at the Atlantic Council’s Scowcroft Middle East Security Initiative — wrote. “But this will not happen spontaneously, without outside help and support. Postwar planning for Syria needs to go into high gear.”

The incoming Trump administration will need to chart a policy approach to Syria. Some will be skeptical of success, but others — Warrick wrote — will note that “weakening Iranian influence, supporting Israel’s security, and peace in Lebanon are, collectively, one of the biggest wins that a Trump administration could hope to achieve.”

A Syrian power struggle

In a recent interview with CNN, Jolani said that Syrians should not fear HTS’s brand of Islamism. “People who fear Islamic governance either have seen incorrect implementations of it or do not understand it properly,” he said.

As the rebels reached Damascus, Assadist Prime Minister Mohammed Ghazi al-Jalali said the outgoing regime would “extend its hand” to the opposition and assist with the transition of power. Jolani said on Sunday that Jalali will remain in his post to supervise state bodies during the transition.

Jolani and his HTS will have competition for influence in the new Syria.

The Turkish-backed Syrian National Army also took part in the offensive, with fighting still ongoing between the SNA and the Kurdish Syrian Democratic Forces — who are supported by the U.S. — in the northeast of the country. Ankara enjoys significant control over the SNA and other groups and can be expected to seek influence over the future direction of its neighbor.

“The opposition is not a homogenous movement and there is a risk that internal fractures within the HTS-led umbrella movement — which may become more salient in the weeks and months to come — may lead to discord and threaten Syrian stability,” Burcu Ozcelik of the Royal United Services Institute think tank in the U.K. told ABC News.

“A new transitional Syrian administration will soon need to take on the task of state-building, including the rebuilding of a national Syrian security force and a constitution-building process, as the Syrian state has been painfully hallowed out by the Assad regime,” Ozcelik said.

The next government will also need to address the question of Russian presence in Syria. Russian forces retain control of Khmeimim Air Base and Tartus naval base on Syria’s Mediterranean coast, two key strategic facilities from which Moscow helped keep Assad in power.

“It is in Russia’s interest to seek to maintain access, but its ability to project power in and through Syria is now severely debilitated,” Ozcelik said.

“It will take time and negotiations with the new Syrian administration, a yet to be determined entity, before it is clear what Russia’s stakes in Syria will be,” Ozcelik added. “But this is now a radically transformed Syria, and Russia has no good options.”

Moscow is in touch with the opposition factions, Russia’s Foreign Ministry said in a Sunday statement. “All necessary measures are being taken to ensure the safety of our citizens in Syria,” it said. “Russian military bases in Syria are on high alert. There is currently no serious threat to their security.”

For Tehran, “there is no doubt that the fall of the Assad regime is a highly consequential defeat for Iran,” Ozcelik said. “Syria was the conduit for Iran’s systematic support for Hezbollah in Lebanon, this supply chain has now been cut off.

Iranian Foreign Minister Abbas Araghchi said Tehran is concerned about the “possibility of a renewed civil war or a sectarian war between different sects or the division of Syria and the collapse of Syria and its transformation into a haven for terrorists.”

Meanwhile, rebel-liberated territory is already being bombed by Israeli warplanes and occupied by Israeli soldiers. Israeli officials have said they intend to deny “extremist” elements access to the Assad regime’s advanced military capabilities.

Israeli Prime Minister Benjamin Netanyahu said the regime’s collapse “is a direct result of the blows we have inflicted on Iran and Hezbollah, the main supporters of the Assad regime.”

“The collapse of the Assad regime, the tyranny in Damascus, offers great opportunity but also is fraught with significant dangers,” he added.

Assad’s legacy

Assad fled the country for Russia in the early hours of Sunday, state-owned Russian media said, having resigned the presidency following negotiations with opposition factions, per a Russian Foreign Ministry statement.

Kremlin spokesperson Dmitry Peskov announced Monday that President Vladimir Putin would grant his longtime ally political asylum in the country.

Assad’s departure ended 24 years of his rule — and 50 years of Assad family rule. Posters and statues of Assad, his brothers and his father, Hafez Assad, were being torn down by jubilant crowds around the country.

More than a decade of civil war left at least 307,000 people dead by the end of 2022, per United Nations figures. The fighting forced around 12 million Syrians — more than half of the country’s 2011 population of around 22 million — from their homes, around 5.4 million of whom were still living abroad as of late 2022.

Assad’s regime fought bitterly to retain control of much of the country during the hot phase of the civil war. But his victory proved a pyrrhic one.

The northern city of Aleppo fell to the Idlib rebels on Nov. 29 — a shocking development that helped spark renewed rebel uprisings all across the country.

In Daraa — known as the “Cradle of the Syrian Revolution” for its role in the 2011 unrest — opposition groups rose anew and began their march on the capital.

“Damascus has been liberated and the tyrant Bashar Assad has been overthrown, and oppressed prisoners in regime prisons have been released,” a rebel spokesperson said at the state television headquarters in Damascus after opposition forces seized the building.

“We ask people and fighters to protect all property in Free Syria,” the spokesperson added. “Long live Syria free for all Syrians of all sects.”

ABC News’ Hami Hamedi, Ellie Kaufman, Luis Martinez and Lauren Minore contributed to this report.