On Tuesday, Turkish-backed fighters were “violently attacking” in the vicinity of Kobani, said Farhad Shami, a spokesman for the U.S.-allied forces. Both he and an independent group monitoring the war said Turkish warplanes were assisting their allies on the ground airstrikes.

Turkey and the United States, allies in NATO, both welcomed the ouster of Syrian President Bashar al-Assad over the weekend. But one of Turkey’s central strategic goals in the region is to weaken Kurdish forces, putting it at odds with Washington.

The clashes on Tuesday signaled an expansion of the fighting between the two sides that in recent days centered on the Kurdish-controlled city of Manbij, not far from the Turkish border. Turkish-backed forces captured the city on Monday, according to the Syrian Observatory for Human Rights, and pushed north toward Kobani, less than 40 miles away.

Kobani was once a relatively obscure Kurdish town near the border with Turkey in northern Syria. But it rocketed to global prominence in 2014, when Islamic State fighters surrounded the town and Washington intervened to help fend them off.

The United States armed the region’s main Syrian Kurdish militia and carried out hundreds of airstrikes in what was lauded as a model of international cooperation. That was the beginning of a partnership between Washington and the militia, known then as the People’s Protection Units, or Y.P.G., that drew the United States into Syria. After a bitter battle, the Kurdish militia prevailed in Kobani in early 2015, then drove ISIS from other parts of Syria and forged ties with other militias.

Underscoring the relationship, General Michael E. Kurilla, chief of the U.S. military’s Central Command, which covers Asia and the Middle East, visited U.S. and Kurdish-led forces in Syria on Tuesday. “He received a firsthand assessment of force protection measures, the rapidly evolving situation, and ongoing efforts to prevent ISIS from exploiting the current situation,” Central Command said in a statement.

The Kurdish militia began as a Syrian offshoot of the Kurdistan Workers’ Party, or P.K.K., that has long fought for Kurdish autonomy in Turkey and is considered a terrorist organization by the Turkish government.

After prompting from the United States, in 2015 the Y.P.G. rebranded as the Syrian Democratic Forces — a mix of Kurdish, Arab and other fighters. It controls northeastern Syria, a region along the Turkish border, and a small contingent of U.S. troops is stationed there.

Analysts say that in recent days the Syrian Democratic Forces have been expanding the territory under their control by moving into areas vacated by the retreat of forces loyal to Mr. al-Assad. At the same time, they have been battling a faction of Turkish-backed fighters, the Syrian National Army, that is seeking to seize control of parts of northern Syria and drive out the Kurds.

The Syrian Observatory for Human Rights, which monitors the war, said that Turkish forces had shelled a key bridge west of Kobani and that there were unconfirmed reports of casualties.

The reports from both the Syrian Democratic Forces and the Syrian Observatory could not be independently verified, and there was no immediate comment from the Turkish or U.S. militaries. But on Sunday, the Pentagon said that Defense Secretary Lloyd J. Austin III spoke to his Turkish counterpart and “reaffirmed the importance of close coordination” between the countries to prevent any further escalation “as well as to avoid any risk to U.S. forces and partners.”

Even before the civil war, Sednaya was known for widespread torture and abuse. But during the conflict, it became a site of depravity and violence, used to commit some of the worst atrocities of Bashar al-Assad’s rule when he was president.

Human rights groups say tens of thousands of people were detained in Sednaya. They were tortured, beaten and deprived of food, water, medicine and basic sanitation. Thousands were executed in mass hangings after sham trials. One group estimated that more than 30,000 detainees were killed there.

In most cases, the families of prisoners were given no information about their fate.

Who was sent to Sednaya prison?

The prison, built in 1987 on a hill north of Damascus, was a military prison that housed political prisoners.

As The New York Times has reported, it was the most notorious prison in a brutal system that was the government’s main weapon against the civilian opposition. Amnesty International described Sednaya as a “human slaughterhouse.”

According to a report by a group representing prisoners, it was protected by hundreds of guards and soldiers and surrounded by a ring of minefields.

The prison held an estimated 1,500 inmates in 2007, but its population surged to as many as 20,000 people at once after Syria’s civil war began, according to a 2017 report by Amnesty International.

What was its history?

Before the civil war began, in 2011, a majority of Sednaya’s inmates were Islamists, who had been encouraged by the Syrian government to join an offshoot of Al Qaeda that was fighting the United States in Iraq. Once they returned home, Mr. al-Assad jailed them to prevent them from threatening his rule.

As antigovernment protests spread across Syria in early 2011, the government released many of those jihadists and began imprisoning thousands of protesters, activists, journalists, doctors, aid workers, students and other Syrians. Many were sent to Sednaya.

The prison was the last place detainees were often dumped after long periods in other detention centers.

What were the conditions?

The Amnesty report and a separate investigation by the United Nations found that the Syrian authorities had deliberately exterminated detainees at Sednaya after torturing and housing them in appalling conditions. The U.N. investigators determined that such acts could amount to crimes against humanity.

The detainees were sexually assaulted, beaten on the genitals and forced to beat, rape or even kill one another, according to rights groups and a Times investigation. In 2017, the United States accused the Syrian government of using a crematory to hide mass murders in Sednaya, listing methods of physical torture such as beatings, stabbings, sexual assault, electric shocks and cutting off ears and genitals.

The few who won release, often through family connections or bribes, described detainees left to die of untreated wounds and illnesses in filthy, overcrowded cells. Prisoners were given just seconds to use latrines, so were often forced to relieve themselves in the cells, which lacked toilets. Meals usually consisted of a few mouthfuls of spoiled food. Many people developed serious infections, diseases and mental illnesses.

The conditions were similar at many prisons across the system. But at Sednaya, treatment could be especially sadistic, according to ex-inmates.

Prisoners were not allowed to look at the guards, talk or make any noise, even during torture. They could be punished by being denied water or forced to sleep naked, without blankets, in freezing cold.

Every morning, guards collected the bodies of those who had died overnight and took them to a military hospital, where their deaths were recorded as cases of heart or respiratory failure, according to the Amnesty report. Then, they were trucked to mass graves outside Damascus.

Loved ones outside the prison often never knew their fates.

A grim protocol

According to former officials cited in the Amnesty report, detainees at Sednaya were routinely tortured into giving confessions. Then they were taken to military field courts, where they were convicted after trials that lasted two or three minutes.

Every week and often twice a week, according to the report, guards pulled groups of up to 50 people from their cells, telling them they were being transferred to civilian prisons. Instead, they were blindfolded, beaten severely in the prison’s basement and then taken to another building, where they were hanged in the middle of the night, the report said.

Prison officials called the mass hangings “the party.”

From 2011 to 2015, Amnesty found, 5,000 to 13,000 people, most of them civilians, were put to death in this way. The group did not have direct evidence of executions after 2015, but because detainees were still being transferred to Sednaya and sham trials still being held, it was likely the executions continued.

What is happening now at the prison?

Some 2,000 prisoners emerged from Sednaya on Sunday, according to Fadel Abdul Ghany, the director of the Syrian Network for Human Rights, which has rigorously monitored Mr. al-Assad’s labyrinth of prisons. But the rest of the approximately 11,000 detainees who he said were being held there when the government was overthrown were nowhere to be found.

“Where are the remaining prisoners?” Mr. Abdul Ghany said. “They have been killed.”

Still, in the confusion, different groups had different estimates of the numbers, and many Syrians held out hope that their disappeared relatives could still be found. Journalists, armed fighters and civilians, including children, roamed the prison, looking for signs of them.

“Seizing the city is a joy — we are joyous,” said one rebel fighter, Mohammad Bakir. “But the real victory will be when I find my family.” He had not heard from his mother, brother and cousin since they disappeared in 2012 after protesting against the government.

Videos sent to The Times by a group of doctors visiting Sednaya showed crowding and the dire conditions inside. Numbered cells, each of which appear to have held a dozen or more people, were littered with debris, clothing and belongings.

The White Helmets, a volunteer civil defense organization in Syria, said it had helped to release about 20,000 to 25,000 people from Sednaya, but noted that thousands more prisoners remained unaccounted for.

The group sent specialized teams to Sednaya looking for secret cells that might hold more prisoners, based on reports that it has hidden elements. But around midnight, the group said it had not found evidence of hidden rooms.

The Association of Detainees & the Missing in Sednaya Prison said that it had obtained a document showing there had been about 4,300 detainees as of Oct. 28 and that approximately that number had already walked free. In a statement, it said there was “no truth to the presence of detainees trapped underground.”

On Monday in Aleppo, a vehicle dropped off one former prisoner from Sednaya, his face gaunt and his legs and body weakened by years of detention. Two relatives helped him stand. A small band of musicians beat drums to celebrate his survival.

The man was soon thronged by people holding their cellphones up to his face. They were showing him photographs of detainees, hoping he might have news.

Aryn Baker, Ephrat Livni and Muhammad Haj Kadour contributed reporting.

The alliance, Hayat Tahrir al-Sham, has been banned by Britain since 2017 because of its origins as an affiliate of Al Qaeda. It is designated a foreign terror organization by the State Department and by the United Nations.

Since the Islamist group’s rapid advance through Syria and the fall of Bashar al-Assad, questions have arisen as to whether Britain and others might consider removing its terrorist designation as the alliance attempts to form a transitional government.

In recent years the group has broken with Al Qaeda, and under its leader, Ahmed al-Shara, who is also known by the nom de guerre Abu Mohammad al-Jolani, it has sought greater international legitimacy and focused its ambitions on governing Syria.

During a visit to Saudi Arabia on Monday, Britain’s prime minister, Keir Starmer, said that it was “far too early” to consider a change of policy on the group’s status.

Britain’s foreign secretary, David Lammy, told lawmakers that the government would remain “cautious.” He noted that Hayat Tahrir al-Sham had offered “reassurances to minorities in Aleppo, Hama and Damascus” and had “committed to cooperating with the international community over monitoring chemical weapons.”

But he concluded: “We will judge HTS by their actions, monitoring closely how they and other parties to this conflict treat all civilians in areas they control.”

On Tuesday an official spokesman for Mr. Starmer said that the presence of Hayat Tahrir al-Sham on the list of terror organizations did not mean the government could not communicate with the group because of a law that restricts such contacts.

Britain’s antiterror legislation permits dialogue with banned organizations for specific reasons, such as to encourage them to enter into a peace process or to facilitate the delivery of humanitarian aid, he added.

Early on Monday, there had been some confusion within Britain’s government when another senior minister, Pat McFadden, said the government would consider removing the terror designation and that a decision could come quickly.

But the British government appears to want more time to assess the intentions of Mr. al-Shara and to judge whether some of the assurances he has given are matched by actions.

“The realities so far is that the H.T.S. and also the other armed groups have been sending good messages to the Syrian people,” Mr. Pedersen said, referring to Hayat Tahrir al-Sham, the militant group at the head of the lightning advance that ended half a century of Assad family rule.

Hayat Tahrir al-Sham and other armed groups controlling Damascus, the capital, have issued “reassuring statements” about forming a government of “unity and inclusiveness,” Mr. Pedersen said. But he warned of the dangers of renewed violence among the patchwork of armed groups operating across Syria, citing continuing clashes between groups in the northeast, and of the risks posed by what he called Israel’s “very troubling” military operations in the country.

The Israeli military seized control of a buffer zone between Syria and the Israeli-occupied Golan Heights over the weekend, and in the past two days its forces have carried out strikes on airfields, military warehouses and other targets around Damascus and the port city of Latakia.

“This needs to stop,” Mr. Pedersen said. As Syria’s armed groups attempt an orderly transition to a new government, it is “extremely important that we don’t see any action from any international actor that destroys the possibility for this transformation in Syria to take place,” he added.

Mr. Pedersen urged Syria’s armed groups to protect civilians and to move toward a government based on the broadest possible representation of the country’s many ethnic and religious communities.

“If we do this, if we unite the Syrian parties, we bring together the different Syrian communities, then this could be the real beginning of something new for Syria,” he said.

The United Nations has listed Hayat Tahrir al-Sham as a terrorist group for the last nine years based on its past association with Al Qaeda. Mr. Pedersen said that this was a complicating factor but added that he believed the formation of an inclusive government would lead the international community to reconsider the designation.

“We also have to be honest — we have to look at the facts and to see what has happened during the last nine years,” Mr. Pedersen said, citing the group’s messages of inclusiveness and “reassuring things on the ground” in the areas it controls.

But Sobhi Firwana, 29, said that he was concerned that the situation in Syria could prolong the war in Gaza and further delay efforts to reach a cease-fire, pointing to the media’s pivot of coverage to Syria and Israel’s bombing campaign there.

Even as Israel pummeled targets in Syria, saying it was destroying military and weapons facilities there, its forces have continued strikes in Gaza. On Monday night, 25 people were killed in an airstrike on a house in Beit Hanoun, a town in northern Gaza, according to Mahmoud Basal, a spokesman for the Palestinian Civil Defense emergency service.

The Israeli military, which has called for evacuations of much of north Gaza, did not immediately comment on the strike, but it has been conducting an offensive there to try to extinguish a Hamas resurgence. It maintains that it tries to avoid civilian casualties but that Hamas has embedded itself in residential areas.

The deadly attack “due to the events happening in Syria and the diversion of the media coverage, possibly did not echo,” Mr. Basal said in a statement on Tuesday.

Talks on a cease-fire in Gaza are quietly advancing behind the scenes, according to mediators and officials, but details remain murky and an agreement is not in hand. Mr. Firwana, a lawyer and law professor from Khan Younis, said he was concerned that the situation in Syria could further push back a deal because Israel was now focused on securing its borders from potential threats in Syria.

But Ahmed Khalil, a 29-year-old from central Gaza, said he thought that the fall of Bashar al-Assad, along with the deaths of Hezbollah leaders in Lebanon, sent a strong message to Iran.

Iran has sustained blow after blow at the hands of Israel and the ouster of Mr. al-Assad severely damaged Iran’s regional strategy, which cultivated a network of allies, including Hamas, Hezbollah and the Syrian government.

Mr. Khalil said he thought that Iran’s weakening influence could “hasten the process of negotiations for a cease-fire and a deal for prisoners in Gaza.” If a cease-fire is reached in Gaza, the bakery worker said he would celebrate like many in Syria have over the last week, by preparing and handing out local desserts.

Husam al-Fiqy, a 41-year-old from Nuseirat in central Gaza, said that he was indifferent to the details of what happened in Syria, but he felt a sense of relief seeing the Syrian people “finally free themselves from their oppressive leader.”

He gave no details about how the rebel group would pursue that course, or of any judicial process, which human rights experts say is essential to helping Syria move forward.

The group Mr. al-Shara leads, Hayat Tahrir al-Sham, had earlier announced an amnesty for the rank and file of the government of former president Bashar al-Assad, but Mr. al-Shara said this did not extend to senior officials. His comments came amid fear that people who supported the fallen government could face retribution.

The group confirmed the appointment of Mohammed al-Bashir as the country’s prime minister on Tuesday, according to SANA, Syria’s state-run news agency. Mr. al-Bashir has been tasked with running the transitional government until March 1, the agency reported. He previously served as the head of a rebel-run administration in northwestern Syria.

Hayat Tahrir al-Sham, an Islamist group that was once linked to Al Qaeda, has tried to gain international legitimacy by eschewing global jihadist ambitions and focusing on organized governance in Syria. But it has also come under criticism for using authoritarian tactics and cracking down on dissent.

Human rights groups say that more than half a million people including around 200,000 civilians, died in Syria’s civil war, which began in 2011, but attention has focused on atrocities committed by Mr. al-Assad’s government.

The whereabouts of around 136,000 people who were arrested arbitrarily by the Assad regime is unknown and in many cases the government did not acknowledge that people had been detained, leaving family members in the dark, according to the Syrian Network for Human Rights. In addition, at least 15,000 people were tortured to death by the government, the network said.

Human rights experts say a judicial process is necessary and that the options for the rebel alliance include building cases in national, foreign and international courts. But they caution that it would take time to set up a national system capable of rendering justice and it can only be done once security in the country has been established.

Human rights groups in Syria, working with international organizations, have spent more than a decade documenting evidence of crimes committed in the country, particularly in the regime’s notorious prison network.

In addition, there have been around a dozen prosecutions in courts outside Syria. In one high-profile example, a court in the city of Koblenz, Germany, sentenced a former intelligence officer to life in prison in 2022 after he was convicted of crimes against humanity.

The court found that the former officer, Anwar Raslan, 58, oversaw the torture of prisoners in Damascus and the killing of at least 27 people there, in addition to the sexual abuse and rape of detainees.

The ultimate target for human rights groups working on Syria would be to bring Mr. al-Assad to justice. Russia announced over the weekend that Mr. al-Assad had arrived in Moscow and would be granted asylum, apparently putting him out of reach.

Israeli officials said they were destroying weapons and military facilities to keep them out of the hands of Islamist extremists. The rebel group that led the toppling of the president, Bashar al-Assad, was formerly linked to Al Qaeda and is still designated as a terrorist group by the United States and the United Nations.

“We have no intention to meddle in Syria’s internal affairs, but we certainly intend to do whatever is needed to guarantee our security,” Prime Minister Benjamin Netanyahu of Israel said on Tuesday.

The Israeli campaign over the past two days has been exceptional in force and scope, trying to ensure that whoever ends up in power in Syria will be significantly disarmed.

It followed months of intensified Israeli airstrikes on Syria, including on weapons depots belonging to Iran and Hezbollah. But the large-scale bombings this week have been far more comprehensive and devastating to Syria’s military capabilities, analysts said.

The assault delivered a blow to the infrastructure in Syria that Iran used to transport weapons to Hezbollah in Lebanon. And Mr. Netanyahu warned the country’s future leaders to prevent Iran from using Syrian territory again for its own military purposes.

The Israeli defense minister, Israel Katz, congratulated the country’s missile ships on Tuesday for completing “the destruction of the Syrian Navy” the night before.

But the intense air assault on Syria at such a fragile moment raised alarm among some in the international community.

“This needs to stop,” the U.N. special envoy for Syria, Geir Pedersen, told reporters in Geneva on Tuesday. As Syrian factions attempt an orderly transition to a new government, it is “extremely important that we don’t see any action from any international actor that destroys the possibility for this transformation in Syria to take place,” he added.

The U.N. also expressed concern about Israel’s actions on the ground.

On Saturday, even before Mr. al-Assad fled the country, Israeli forces entered Syrian territory for the first time in 50 years. They have since taken control of a 155-square-mile demilitarized buffer zone in the Golan Heights that has been patrolled by U.N. troops since the 1973 Middle East war. The zone abuts Syrian-controlled territory.

Israel captured the Golan during the 1967 Middle East war and annexed most of it in 1981. Most of the world views the area as Israeli-occupied Syrian territory, but over the past decades, Israel has firmly defended the land.

Israeli’s foreign minister, Gideon Saar, said Israel’s incursion into the U.N.-controlled buffer zone was “limited and temporary.”

Stéphane Dujarric, spokesman for the U.N. secretary general, told reporters that the presence of Israeli forces in the area violated the 1974 agreement that created the buffer zone.

“There should be no military forces or activities in the area,” he said.

Mr. Netanyahu said on Monday that the 1974 agreement had collapsed because Syrian troops protecting some of the buffer zone had abandoned their posts.

Egyptian officials said Israel’s actions “violate international law, undermine the unity and integrity of Syrian territory and exploit the current instability to occupy more Syrian land.” Egypt called on the U.N. Security Council to “take a firm stance against Israeli attacks on Syria, ensuring its sovereignty over all its territories.”

The Israeli military said its airstrikes had destroyed much of Syria’s military capabilities. Its targets included airfields, hangars, military structures, launchers, firing positions, at least 15 naval vessels and dozens of weapons-production sites. The strikes against Syria’s Navy also destroyed dozens of sea-to-sea missiles with ranges of 50 to 120 miles, the Israeli military said.

Photographs from Syria on Tuesday showed sunken boats at a shipyard, crumbled buildings and the charred remains of a science research center that had been linked to the country’s chemical weapons program, according to the news agencies that distributed the images.

“I’ve just watched dots all over the place — it’s been literally hundreds of attacks in the last 48 hours,” said Miri Eisin, a retired Israeli colonel who has been tracking the strikes for the International Institute for Counter-Terrorism.

“Thank Israel that Israel just destroyed the chemical weapons that nobody else was willing to touch,” she said.

Under Mr. al-Assad, the Syrian government used chemical weapons — particularly sarin nerve agent — against citizens numerous times during the 13-year civil war.

Over the past years, Israel had periodically bombed targets in Syria, and the intensity of those attacks had escalated recently.

In April, Israeli forces bombed an Iranian Embassy building in Damascus, killing senior Iranian military and intelligence officials. And in September, Israeli struck a Hezbollah missile production site in Syria before commandos rappelled down from helicopters to collect evidence.

But nothing had come close to the scale of this week’s attacks.

Mr. Netanyahu said Israel would like to forge relations with the new Syrian government — on certain conditions.

“If this regime allows Iran to reestablish itself in Syria, or allow for the proliferation of Iranian weapons or any other weapons to Hezbollah or attack us, we will respond forcefully and exact a heavy price,” he warned.

Ms. Eisin, the retired colonel, said Israel took advantage of a window of opportunity in Syria.

“You are trying to cut what you could never do in the past because if you did, you would be in an all-out war against Syria, Hezbollah” and Iran, she said.

Mr. Katz, the defense minister, said Israel’s move into the buffer zone on the Golan Heights aimed to prevent militants from amassing near Israel, as had happened in Gaza and Lebanon.

He said that Israeli forces would “establish a defensive area clean of weapons and terror threats in southern Syria.”

The ground advances generated nervous speculation that Israel might go further in capturing territory.

Nadav Shoshani, an Israeli military spokesman, confirmed that Israel forces had moved outside the buffer zone in several areas. But he said that was because of the landscape and “not an offensive.”

Israeli forces “are not advancing toward Damascus,” Mr. Shoshani told reporters on Tuesday.

“We are not a side in this conflict, and we do not have any interests other than protecting our borders and the security of our civilians.”

Reporting was contributed by Johnatan Reiss, Nick Cumming-Bruce, Aaron Boxerman, Gabby Sobelman and Vivian Yee.

“It’s a wreck now but we want to fix it,” said a fighter with his face covered who gave only his nom de guerre, Abu Oweis. Of the palace, he said: “It is beautiful, but it was all for Bashar.”

He is a fighter with Hayat Tahrir al-Sham, the group that led the rebel charge from Idlib Province in the northwest all the way to Damascus, forcing Mr. al-Assad to flee. Abu Oweis said he was 20, had not finished high school and had never left Syria or visited its largest cities, Aleppo or Damascus.

“It’s a big city,” he said of the Syrian capital. “Really big.”

Outside the presidential palace, a family and their friends were wandering around, marveling at the grandeur of the structure and the landscaped gardens around it.

“I wanted to see all the blood that they drew from us to build this,” said Mohammed Abu al-Kheir, 42, an electronics trader.

“This is really rich, unnaturally rich,” he said, shaking his head. “Just the irrigation needed its own budget.”

In the fallout created by Mr. al-Assad’s downfall, there are big questions about who will step in.

Hayat Tahrir al-Sham

Hayat Tahrir al-Sham, whose name means Organization for the Liberation of the Levant, is a former affiliate of Al Qaeda that broke with the older group years ago and came to dominate the last stronghold of Syria’s opposition.

It was the main rebel group leading the latest offensive, launching a surprise assault in late November out of its base in northwestern Syria that quickly led to the fall of the Assad government.

Members of the group had early links to the Islamic State, and then to Al Qaeda. In 2016, they tried to shed their extremist roots, banding together with several other factions to establish Hayat Tahrir al-Sham. The United States and other Western countries still consider it a terrorist group.

The group’s leader, Abu Mohammad al-Jolani — who as of Monday shed that nom de guerre and is now going by his real name, Ahmed al-Shara — told The New York Times that his primary goal was to “liberate Syria from this oppressive regime.” He has tried to gain legitimacy by providing services to residents in his stronghold of Idlib Province.

Because of its roots and its designation as a terrorist group, Hayat Tahrir al-Sham has struggled to raise funds, according to the Center for Strategic and International Studies, a Washington research institute. The group raises money from border tariffs, collecting taxes on residents and holding a monopoly over utilities. Analysts say it has also been involved in trafficking the synthetic stimulant captagon.

Syrian Democratic Forces

Forces from Syria’s Kurdish ethnic minority, which makes up about 10 percent of the population, became the United States’ main local partner in the fight against the Islamic State in Syria, under the banner of the Syrian Democratic Forces.

After the Islamic State was largely defeated in 2019, the Kurdish-led forces consolidated control over towns in the northeast, expanding an autonomous region they had built there. But Kurdish fighters still had to contend with a longtime enemy, Turkey, which regards them as linked to Kurdish separatist insurgents inside Turkey.

Even as rebels took control of Damascus, fighting flared between Turkey and the Kurds in the northeast of Syria, centered on Manbij, a Kurdish-controlled city near the Turkish border. At least 22 members of the Syrian Democratic Forces were killed in and around Manbij, and 40 others were wounded, according to the Kurdish group.

The Syrian National Army

This umbrella group includes dozens of groups with different beliefs. It receives funding and arms from Turkey, which has long been focused on expanding a buffer zone along its border with Syria to guard against the activities of Kurdish militants based in the region that it sees as a threat.

Turkey wants to create an area where it can resettle some of the three million refugees who have fled Syria and are living within its borders. But it has struggled to harmonize the ragtag groups that make up the Syrian National Army.

The group is largely composed of the dregs of the Syrian civil war, including many fighters whom the United States had rejected as criminals and thugs. Some received training from the United States early in the war, but most were dismissed as too extreme or too criminal. Most have no clear ideology and had turned to Turkey for a paycheck of about $100 a month when the group was formed.

On Monday, there were fierce battles in the northern city of Manbij between the Syrian National Army, supported by Turkish airstrikes and artillery, and the Kurdish Syrian Democratic Forces.

According to the Syrian Observatory for Human Rights, a war monitoring group based in Britain, the city was captured by the Syrian National Army. A spokesperson for the Syrian Democratic Forces said fighters with the Syrian National Army had taken only 60 percent of the city. The claims could not be independently verified.

The Druse militia

Syria’s Druse minority is concentrated in Sweida, an area in the southwest of Syria that has seen rare antigovernment demonstrations over rising costs of living, and many Druse men have refused military service. This week, Druse fighters joined the push to topple the Assad regime, launching an offensive in the southwest and clashing with government forces, according to media reports.

The Druse fighters are part of a newly formed group of Syrian rebels, which includes fighters from other backgrounds, working under the name the “Southern Operations Room.”

The Druse are a religious group that practices an offshoot of Islam, developed in the 11th century, that contains elements of Christianity, Hinduism, Gnosticism and other philosophies. There are more than one million Druse across the Middle East, mostly in Syria and Lebanon, with some also in Jordan and Israel.

The Islamic State in Iraq and Syria

The Islamic State in Iraq and Syria, better known as ISIS, seized vast stretches of territory in Syria and Iraq in 2014, establishing a brutal regime before it was beaten back by a U.S.-led coalition. Now its members are largely in hiding.

Lately, there have been signs of the group’s resurgence in Syria amid wider instability in the region. The Pentagon warned in July that Islamic State attacks in Syria and Iraq were on track to double compared to the previous year. The group has repeatedly tried to free its members from prisons and has maintained a shadow governance in parts of northeastern Syria, the U.S. said.

President Biden announced on Sunday that the U.S. military has been conducting airstrikes in Syria to keep the Islamic State from reasserting itself in the power vacuum created by Mr. al-Assad’s ouster.

The United States has about 900 troops in Syria to help contain and defeat what remains of ISIS there. The U.S. has not given a date for ending its presence in the country, saying it was contingent on conditions within the war-torn country. Those conditions have now changed dramatically.

“We’re cleareyed about the fact that ISIS will try to take advantage of any vacuum to reestablish its capability, to create a safe haven,” Mr. Biden said. “We will not let that happen.”

Now, after a lightning advance by rebels and the rapid-fire collapse of Mr. al-Assad’s government, great powers — albeit with different ones in ascendence — are jockeying for advantage within Syria’s borders. For the first time in years, the skies are empty of Syrian and Russian bombers — but airstrikes by Israel, Turkey and the United States continue.

Here is a look at the foreign countries that have been present in Syria — often operating through local allies — and what they stand to gain and lose from the end of the Assad regime.

Turkey

Turkey has two primary concerns in Syria: Kurds and refugees.

Eastern Syria is home to a sizable ethnic Kurdish population, which the Turkish government sees as allied with Kurdish separatist groups in Turkey. The country is also hosting nearly four million Syrian refugees who fled persecution by the Assad government, and President Recep Tayyip Erdogan would like them to return home.

Turkey, which was once the seat of the vast Ottoman Empire that included much of Syria, has backed an array of different rebel groups that hold territory along the Syrian-Turkish border. One of those groups, Hayat Tahrir al-Sham, which led the offensive that toppled Mr. al-Assad, had benefited from Turkey’s military presence in the area.

It is unclear whether Turkey approved the group’s decisive offensive.

However, Ankara’s closest ties are with the Syrian National Army, which has functioned almost as a proxy force for Turkey — in the past, the group’s leadership has said it received funding and weapons — and in turn it has pushed the Syrian Kurds, whom the Turks view as a security threat, back from the border. Just in the last few days, as the rebels led by Hayat Tahrir al-Sham took control of the capital, Damascus, fighting flared between the Syrian National Army and the Kurds in northeastern Syria, centered on Manbij, a Kurdish-controlled city near the border with Turkey. Turkey supported the Syrian National Army with airstrikes.

Turkey now seems to be the foreign power with the most access and influence with the armed groups now in charge and in a prime position to pursue its own goals in Syria. That could mean further attacks against the Syrian Kurds, and the return of refugees who are currently in Turkey.

Israel

Israel has fought three wars with Syria, and has had many more armed confrontations. It holds much of the mountainous territory in the southwest of Syria known as the Golan Heights, an annexation that is not recognized by the United Nations and many other countries.

During the civil war, Israel regularly conducted airstrikes against Iran’s and Hezbollah’s weapons stores and personnel in Syria. In April, it bombed an Iranian Embassy building in Damascus, killing senior Iranian military and intelligence officials

Within hours of the fall of the Assad regime, Israel moved troops into the Golan Heights, advancing beyond the demilitarized zone in its first overt entry into Syrian territory since the 1973 October War.

As soon as it became clear on Sunday that there would be regime change in neighboring Syria, Israel began a sweeping aerial campaign.

By Tuesday, at least 350 airstrikes had leveled military assets across Syria, taking out the Navy, fighter jets, drones, tanks, air-defense systems, weapons plants and a wide array of missiles and rockets, according to the Israeli military.

The Syrian Observatory for Human Rights, an independent group based in Britain that has tracked the civil war in Syria for years, said that it had documented 322 Israeli strikes in Syria since Sunday, when President Bashar al-Assad fled the country.

Gideon Saar, Israel’s foreign minister, told reporters on Monday that the leaders of Syria’s rebels “are people with an extreme ideology of radical Islam.” Israeli forces are destroying the weapons “in order that they don’t fall in the hands of extremists,” he added.

Israel has touted its actions against Hamas and Hezbollah as instrumental in the overthrow of the Assad government. But it is not clear that a new government in Damascus, dominated by militant Islamists, will make Israel any safer.

Iran

Iran’s relationship with Syria dates back almost 50 years, when Syria’s president at the time, Hafez Assad, supported Iran in its eight-year war with Iraq. As Iran built a network of like-minded groups across the Middle East as a counterweight against the United States and Israel, Syria was the only state to become part of what Iran called its “Axis of Resistance.”

Syria became Iran’s main overland supply route to transport weapons to Hezbollah in Lebanon. In return, Iran sent military advisers to support the Assad regime during the civil war, along with fighters from its ally Hezbollah and two brigades under the command of Iran’s Quds Forces that was made up of refugees from Pakistan and Afghanistan who had fled to Iran.

With Mr. al-Assad forced to flee Syria, Iran will lose much of its military leverage in Lebanon and Syria. Its hopes for an “Axis of Resistance” that reached the Mediterranean have unraveled — at least for now.

Russia

The Russian relationship with Syria dates to the days of the Soviet Union. At the end of the Cold War, with the United States asserting its presence in Arab countries, Russia saw the Assad government as a crucial ally in the Middle East, one that could provide a counterweight to the American presence.

During the Syrian civil war, Russia made it a priority to keep its ally in power. It also saw the Syrian leader as a bulwark against Islamist extremism from Al Qaeda and the Islamic State. Russia sold weapons to the Assad government, deployed fighters from Russia’s Wagner group, expanded its naval base in Tartus, Syria, and opened an air base near Damascus.

With the demise of the Assad regime, Russia could lose much of its influence in Syria, but analysts say it probably will try to keep its Tartus base, which is its only Mediterranean port for its Black Sea fleet It is making conciliatory gestures toward the rebel forces who now control the country, and has said it is too soon to make any decisions about the fate of its military bases in Syria.

United States

The U.S.-Syria relationship has never been particularly friendly. The United States severed diplomatic relations in 1967 during the Arab-Israeli war, and placed Syria on its list of state sponsors of terrorism in 1979.

The main U.S. interest in Syria now is the defeat of the Islamic State, which maintains a presence in the northeastern and central parts of the country. In 2019, during Donald Trump’s first term as president, he pulled most U.S. forces out of Syria, but about 1,000 U.S. Special Operations troops remain, and they work closely with Syrian Kurdish troops trained by the United States.

President Biden said on Sunday that the U.S. military has been conducting airstrikes in Syria to keep the Islamic State from reasserting itself in the power vacuum created by Mr. al-Assad’s ouster. He said the United States would support the region “should any threat arrive from Syria during this period of transition.”

“We’re cleareyed about the fact that ISIS will try to take advantage of any vacuum to re-establish its capability, to create a safe haven,” Mr. Biden said. “We will not let that happen.”

Additional reporting by Jack Nicas.