Rebels had seized most of Syria’s largest city, Aleppo, as of Saturday, according to a war monitoring group and to fighters who were combing the streets in search of any remaining pockets of government forces.

The antigovernment rebels said they had faced little resistance on the ground in Aleppo. But Syrian government warplanes responded with airstrikes on the city for the first time since 2016, according to the war monitoring group, the Syrian Observatory for Human Rights.

Aleppo came to a near standstill on Saturday, with many residents staying indoors for fear of what the sudden flip in control might mean, witnesses said. Others did venture out into the streets, welcoming the fighters and hugging them. Some rebels tried to reassure city residents and sent out at least one van to distribute bread.

The rapid advance on Aleppo came just days into a surprise rebel offensive launched on Wednesday against the autocratic regime of President Bashar al-Assad. The developments are both the most serious challenge to Mr. al-Assad’s rule and the most intense escalation in years in a civil war that had been mostly dormant.

The timing of the assault suggested that the rebels could be exploiting weaknesses across an alliance linking Iran to the militant group Hezbollah in Lebanon as well as the Assad regime in Syria and others.

In Aleppo on Saturday, well-armed rebel fighters dressed in camouflage patrolled streets still lined with the ubiquitous posters of Mr. al-Assad. The opposition forces said that although they were in control of nearly the entire city, they had not yet solidified their hold on it.

The rebels also announced the capture of towns and cities across three provinces, Aleppo, Idlib and Hama.

In the city of Aleppo, rebels announced a 24-hour curfew starting at 5 p.m. Saturday, saying it was for residents’ safety.

Within hours from Friday into Saturday, Syrian government soldiers, security forces and police officers fled the city, according to the war monitoring group. They were replaced by the Islamist and Turkish-backed rebels sweeping through on foot, motorbikes or on trucks mounted with machine guns.

Government military vehicles were parked along the western entrance of the city, apparently abandoned. Rebels took down the government flag, burning it in the streets, and raised the opposition flag, with its band of green, on a pole overlooking much of the city.

The offensive is the latest upheaval in the long-running Syrian civil war, which began in 2011. It displaced about half of the population and sent millions seeking safety in neighboring countries, like Turkey and Lebanon, and beyond, including in Europe.

The conflict was largely stagnant for years until Wednesday, when fighters from an array of armed opposition factions launched the surprise offensive.

Their advance came eight years after a bloody battle for control over Aleppo in 2016 that lasted for months. The rebels were ultimately routed in a big blow to their efforts to oust Mr. al-Assad.

Throughout the war, Mr. al-Assad has counted on military and political support from two of his closest allies, Iran and Russia. Russia once again came to Mr. al-Assad’s aid in the latest fighting.

The Russian Defense Ministry said on Friday that Moscow had carried out airstrikes against the rebel offensive in support of the Syrian military, although it did not say where. The war monitoring group said that Russian strikes had hit opposition-held areas of Aleppo and Idlib Provinces.

It was not immediately clear whether Russia had struck the city of Aleppo, the capital of the Aleppo Province, to back up the Syrian government airstrikes.

The Aleppo governor, police and security commanders, and other regime forces have fled central Aleppo, the monitoring group said on Saturday.

The Syrian state news media challenged the reports of a rebel takeover of most of Aleppo, saying the military had captured groups of “terrorists” who had been filming inside several neighborhoods to try to prove that they had taken control of them. Since the early days of the Syrian conflict, the government has characterized virtually all opposition figures as terrorists.

Residents of Aleppo described to The New York Times how control of their city seemingly switched from night to day.

Some former Aleppo residents returned with the advancing rebel offensive. They and the antigovernment fighters have been sharing photos and videos of themselves around Aleppo landmarks. One popular selfie spot on Saturday was in front of the ancient citadel that served as a military outpost for government forces at the height of battles for the city years ago.

One older man, apparently confused about the identity of the city’s new rulers, called out to a group of fighters, “May God protect you. May God protect al-Assad.”

The rebel alliance is led by the group Hayat Tahrir al-Sham, which was once linked with Al Qaeda, though it publicly broke with the terrorist group years ago. Turkish-backed rebel groups have also joined in.

Some restaurants and cafes rimming the edges of the citadel’s old walls opened as usual on Saturday, but suddenly they had new patrons: rebel fighters with their weapons and returning residents who had fled when the regime fully took over the city in 2016.

On Friday, the U.N. Office for Coordination of Humanitarian Affairs reported that the Aleppo airport was closed and all flights suspended.

On Saturday, Kurdish-led forces, who control large parts of Syria’s northeast, took over the airport, according to the observatory. But hours later, the rebels claimed to have taken control of the airport, confusion on the ground that may hint at chaos to come as territory quickly changed hands.

The Kurdish-led fighters — a separate force that is not aligned with the rebels — also took over abandoned checkpoints in some Aleppo neighborhoods, according to the war monitoring group and rebels.

The Kurdish-led forces are backed by the United States, which formed an alliance with them years ago to battle the Islamic State terrorist group when it took advantage of a power vacuum created by the civil war and seized large swaths of Syria. The United States still has hundreds of forces stationed on Syrian territory, mostly in Kurdish-controlled areas.

Former residents of Aleppo came back to survey their homes and neighborhoods and, in some cases, to reunite with family members they had not seen in years.

Some city residents said they were concerned about what a takeover by a hodgepodge of rebel groups could bring. But that is not their only fear.

The airstrikes on Saturday could portend more aerial attacks and a return to the years when rebels controlled parts of the city that were bombarded regularly by regime and Russian warplanes.

Before the war, Aleppo was Syria’s commercial center with a population of about three million. But large parts of the strategic city were destroyed during the years when rebels and the regime fought for control of it.

In December 2016, the Assad government, aided by Russia, recaptured all of the city after a prolonged siege of opposition-held neighborhoods. The takeover was exalted at the time by Mr. al-Assad as a turning point in the war. In the years that followed, his regime, supported by the Russian military and Iran-backed militias, clawed back large parts of the country.

That control may now be under threat, especially with Mr. Assad’s allies, including Russia, Iran and Hezbollah, distracted and weakened by other regional conflicts — chief among them Israel’s war against Hezbollah in Lebanon and Russia’s war in Ukraine.

For some returning residents of Aleppo, it was hard to fathom the rapid turn of events that allowed them to once again go home.

When Abdulkafi al-Hamdo, a professor of English and antigovernment campaigner from Aleppo, heard the news that rebels had breached the city on Friday, he said that he did not believe it. He said he had fled the city in 2016 and has been living with his family in the countryside since then.

“Even dreaming, it was impossible for Aleppo to be liberated once again,” he said. “But I had faith.”

Once he heard the news of the partial rebel takeover early on Saturday, he said, he had rushed back.

“Even as I was entering Aleppo, I was saying to myself, ‘How did this happen? This was impossible,’” he said.

He ventured back to familiar neighborhoods and landmarks, including the university he graduated from, he said, adding that he had taken a selfie in front of the citadel.

As an opposition supporter, he said he had tried to reassure city residents.

“The people are afraid,” he said. “The regime planted in them fear of the rebels.”

Hwaida Saad contributed reporting.

Prime Minister Justin Trudeau of Canada went to Florida on Friday night to see President-elect Donald J. Trump at Mar-a-Lago, two officials with direct knowledge of the visit said, after a threat by Mr. Trump to impose across-the-board tariffs on goods from Canada and Mexico on Day 1.

The visit makes Mr. Trudeau the first head of government from the Group of 7, a key forum of global coordination consisting of the world’s wealthiest democracies, to visit the president-elect.

Mr. Trudeau and Mr. Trump dined together on Friday evening, one official said, along with a delegation of senior Trump allies poised for top trade and security positions in his new administration.

Mr. Trudeau was accompanied on his visit by Dominic LeBlanc, Canada’s minister of public safety. The Canadian prime minister stayed in the area overnight, but not at Mar-a-Lago, Mr. Trump’s private club and home in Palm Beach, Fla.

The Trump transition team did not respond to requests for comment, and there was no information released about Mr. Trump’s schedule on Friday. Mr. Trudeau, on returning to his hotel after spending about three hours at Mar-a-Lago, did not respond to questions about what was discussed over dinner.

In a statement posted to Truth Social on Saturday, Mr. Trump said that the meeting with Mr. Trudeau had been productive, and that the two men had discussed important topics such as the fentanyl drug crisis, fair trade deals and the United States’ trade deficit with Canada.

Mr. Trump said that Mr. Trudeau had made a commitment to work with the United States to end the cross-border flow of fentanyl that has caused “terrible devastation” for U.S. families.

Mr. Trump also said that the pair had discussed “many other important topics like energy, trade and the Arctic. All are vital issues that I will be addressing on my first days back in Office, and before.”

Mr. Trudeau has been scrambling to formulate a plan to respond to the threat made this week by Mr. Trump to impose a 25 percent tariff unless Mexico and Canada take action to curb the arrival of undocumented migrants and drugs across their borders into the United States.

Mr. Trump said he would sign such a measure on his first day in office, a move that could potentially cripple trade across the continent.

The threat was also seen as an opening salvo in what would most likely be a long renegotiation of the trade agreement among the three North American nations known as the USMCA —United States, Mexico, Canada trade agreement, the successor to the North American Free Trade Agreement.

Mr. Trudeau has tried to project calm and confidence, saying that he believed that Mr. Trump would see that tariffs would harm both countries, which are each other’s biggest trading partners. About 80 percent of Canada’s oil and 40 percent of its gas are exported to the United States, and the two countries are deeply intertwined through the joint manufacturing of cars, as well as in multiple other industries.

The Mar-a-Lago visit on Friday, which was initially reported by The Globe and Mail newspaper on the basis of tracking Mr. Trudeau’s plane, is intended as a direct effort by Mr. Trudeau to show that he has a plan to address Mr. Trump’s border concerns, and that tariffs should be avoided for the sake of both nations’ economies.

Mr. LeBlanc, the Canadian public safety minister, was given the task of drafting tougher border measures. He said this week that he was preparing to deploy additional staff, as well as drones and helicopters if needed, to better manage the border between the two countries.

The security and trade issues at the heart of the Friday meeting were also reflected on the Trump side of the table: The dinner, an official said, was attended by Gov. Doug Burgum of North Dakota, whom Mr. Trump picked to be interior secretary; Howard Lutnick, who is Mr. Trump’s selection for commerce secretary; and Mike Waltz, Mr. Trump’s choice for national security adviser.

Mr. Trudeau’s response to Mr. Trump’s tariff threats contrasts sharply with that of Mexico’s president, Claudia Sheinbaum. Mr. Trudeau and other senior Canadian officials have sought to distinguish Canada’s position from Mexico’s, highlighting that Canada is better aligned with the United States and Mr. Trump’s agenda on three key issues: the borders, restrictions on China and bringing well-paying jobs back home.

Ms. Sheinbaum has been firmer, threatening retaliatory tariffs if Mr. Trump made good on his threat.

The split has driven a wedge between two allies, Canada and Mexico, who were able to leverage their relationship to negotiate a favorable agreement during Mr. Trump’s first presidency. The trade pact is officially up for renegotiation in 2026, but analysts say they believe that Mr. Trump’s decision to put tariffs against the two U.S. neighbors on the table even before he takes office indicates that a renegotiation of that agreement could come sooner than planned.

Mr. Trudeau is facing considerable pressure over how to handle this difficult moment. The imposition of tariffs on Canada would throw the country into recession, economists believe, as doing business with the United States is the lifeblood of the economies of several Canadian provinces.

But Mr. Trudeau is in a precarious political moment, too, after nine years in power. He faces elections in the coming months, and polls show his party, the Liberal Party, is set to lose badly to the Conservatives led by Pierre Poilievre.

And as Mr. Trudeau has been trying to define a negotiating position to counter Mr. Trump’s unilateral tariff threat, the leaders of some provinces have charged ahead with their own proposals. Most prominently, Doug Ford, the premier of Ontario, has said that Canada should ditch Mexico and directly negotiate a new deal with Mr. Trump.

“To compare us to Mexico is the most insulting thing I’ve ever heard from our friends and closest allies, the United States of America,” Mr. Ford said earlier this week, adding that Mr. Trump’s descriptions of Canada were “like a family member stabbing you right in the heart.”

Taiwan’s president set off on a mission on Saturday to shore up relations with some of his island democracy’s shrinking band of diplomatic allies: three tiny Pacific Island nations that have taken an outsize importance in Taiwan’s struggle against Chinese efforts to push it off the international stage.

Lai Ching-te, the Taiwanese president, is scheduled to visit the Marshall Islands, Tuvalu and Palau, which amount to one-quarter of the dozen states that maintain diplomatic ties with Taiwan. Since the 1970s, dozens of countries have shifted ties to China. Beijing claims the self-governed island of Taiwan as its territory, and insists that governments end diplomatic relations with Taipei if they want full relations with China.

Mr. Lai’s weeklong trip comes as his government tries to fathom what changes President-elect Donald J. Trump will bring to U.S. dealings with Taiwan, and with China. Mr. Trump has called for Taiwan to sharply increase its military spending and has complained about Taiwan’s global dominance in making semiconductors. But Mr. Trump’s proposed cabinet includes Republicans who have been deeply distrustful of China and sympathetic to Taiwan.

In uncertain times, experts say, Taiwan needs every edge of international advantage that it can get, including from its small allies in the Pacific. Their total population is about 67,000, according to United Nations estimates, compared with Taiwan’s more than 23 million people. But they are members of the United Nations and its bodies, while Taiwan is generally excluded.

“The advantage that Taiwan gets from showing good will to these diplomatic partners is that naturally they help us speak out internationally, in all kinds of international settings where Taiwan can’t do it,” said Ian Tsung-yen Chen, a professor who specializes in Asia-Pacific relations at National Sun Yat-sen University in Kaohsiung, southern Taiwan.

In addition to visiting the Pacific Island countries, Mr. Lai will spend two nights in Hawaii and stop for a day on Guam, an American island territory in the Pacific, Taiwanese officials have said. His trip has already drawn condemnation from Beijing. In the coming days, China may, if precedent is a guide, display its anger over Mr. Lai’s trip by piling invective on him, and Taiwanese security officials have said they expect Beijing to also stage increased military activities around the island, including coast guard maneuvers.

China says that Taiwanese leaders’ stopovers on American soil violate Washington diplomatic understandings with Beijing; U.S. officials say the transit visits are a courtesy for Taiwan’s leaders. Taiwanese leaders have used their brief stops to promote stronger ties with the United States, which, even without full diplomatic relations, remains their most important partner — a vital market for Taiwan’s goods and a supplier of most of its military weapons.

This time, Mr. Lai will not set foot in the continental United States, reducing opportunities for high-profile meetings. Beijing’s reaction may also be relatively muted as Chinese leaders focus on preparing for the second Trump administration. More attention may fall on Mr. Lai’s efforts to preserve Taiwan’s diplomatic footprint, especially in the Pacific.

Faced with global challenges, “Taiwan must actively engage with the world,” Mr. Lai told an audience in Taipei shortly before boarding his plane for the trip — his first abroad since becoming president in May. The three Pacific allies, he added, were “supporting Taiwan’s participation in many international forums.”

Mr. Lai is likely to be especially eager to make sure that the Marshall Islands, Tuvalu and Palau speak up for Taiwan in the Pacific Islands Forum, the region’s main diplomatic gathering, said Mihai Sora, the director of the Pacific Islands Program at the Lowy Institute, a foreign policy think tank in Sydney. The Forum is one of the few multilateral venues where Taiwan has a steady presence — as a “development partner” — to the consternation of diplomats from China, which also has a role in the dialogue.

When leaders met for the Pacific Islands Forum in Tonga this year, a Chinese official denounced a draft communiqué that confirmed Taiwan’s status, the Guardian reported. But the Solomon Islands, which has had strong ties with China, may try to diminish Taiwan’s role when it hosts the leaders meeting next year. Taiwan’s diplomatic partners could help counter that risk.

“Our country is closely tracking developments regarding the Solomon Islands and other countries that follow China’s lead and who may try to exploit opportunities to thwart our participation in the Pacific Islands Forum in 2025,” Jeff Liu, a spokesman for Taiwan’s Ministry of Foreign Affairs, said in a written response to questions. Taiwan, Mr. Liu wrote, would “work closely” with its diplomatic allies and partners “to firmly defend our rights and interests in participating” in the forum.

Over the past decade, China has used generous offers of aid and investment to draw away Taiwan’s remaining diplomatic partners, partly in an effort to stifle Taiwan’s global standing and also as retaliation against successive governments led by the Democratic Progressive Party. The party rejects the idea that Taiwan is a part of China and says the island is, for practical purposes, independent and should stay that way.

Days after Mr. Lai won Taiwan’s presidential election in January, China announced that it had persuaded Nauru, a speck in the Pacific Ocean, to shift diplomatic ties from Taipei to Beijing. In 2019, two Pacific Island states — Kiribati and the Solomon Islands — switched to Beijing. This week, China’s leader, Xi Jinping, hosted Fiame Naomi Mata’afa, the prime minister of Samoa, in Beijing, presenting China as a firm friend to the Pacific Island nations.

People on the Pacific Islands, threatened by rising seas caused by global warming, have learned to cope with geopolitical rivalries spilling into their region, said Sione Tekiteki, a former official at the Pacific Islands Forum Secretariat who now lectures at the Auckland University of Technology in New Zealand.

“The problem, I would say, is the fact that, rather than trying to deal with the issues themselves, they work through their affiliated Pacific nations, which in turns creates tensions among those Pacific nations,” Mr. Tekiteki said of China and Taiwan. “It’s only a big issue for us because everyone else talks about it and pulls in Pacific nations.”

The Lowy Institute’s latest annual survey of aid received by the Pacific Island countries combined estimated that in 2022 — the most recent year when the data is available — Taiwan dropped out of the top 10 bilateral aid donors to the region. The drop appeared to reflect how, when Taiwan’s diplomatic partners in the Pacific switch to Beijing, they no longer take Taiwanese aid. China was the second-biggest donor, some distance behind Australia.

Taiwan’s remaining diplomatic allies in the Pacific all have strong ties with the United States or Australia, and that may discourage them from shifting ties to Beijing, Mr. Sora of the Lowy Institute said. The Marshall Islands and Palau receive U.S. economic support and other benefits through what are called “Compacts of Free Association,” and Tuvalu last year signed a wide-ranging agreement with Australia.

“It’s an important element in terms of adding ballast to these three countries’ current diplomatic ties with Taiwan,” Mr. Sora said. “Those countries might not feel pressed or compelled to look for new diplomatic partnerships.”

The fragile peace between Israel and Hezbollah largely hangs on 10,000 soldiers in the Lebanese Army.

The last time it was tasked with enforcing a cease-fire, it plainly failed.

The current cease-fire, which came into effect on Wednesday, calls for a 60-day truce between Israel and the Lebanese militant group Hezbollah, during which time Israeli forces gradually withdraw from Lebanon, and Hezbollah moves away from Lebanon’s border with Israel.

To ensure Hezbollah’s retreat, the agreement relies heavily upon the Lebanese Army, a national military strained under competing priorities and sectarian complexities that has long proved unable — or unwilling — to rein in Hezbollah.

In a new buffer zone along the border — a strip of land ranging from a few miles to 18 miles wide — the Lebanese Army is responsible for destroying all Hezbollah military infrastructure, confiscating any unauthorized weapons and blocking the transfer or production of arms. United Nations peacekeeping forces will sometimes accompany the Lebanese soldiers in a supporting role. On Wednesday, the army began deploying more soldiers to the region.

But that approach has been tried before — and it did not work.

The Israel-Hezbollah cease-fire that ended the 2006 Lebanon War, known as Resolution 1701, also called on the Lebanese Army to keep Hezbollah away from the border, with U.N. peacekeepers assisting. Years later, Hezbollah emerged even stronger than before, with extensive weaponry, infrastructure and tunnels across the border region.

Yet despite those past failures, the international community is once again banking on the Lebanese Army. In recent months, the United States and other nations revived an effort to train, equip and fund Lebanese forces.

But, analysts say, it is a risky bet.

“We’ve tried this before,” said Hanin Ghaddar, a senior fellow at the Washington Institute studying Hezbollah. The Lebanese Army, she added, “has never, ever confronted Hezbollah. So this would be highly unusual.”

Knowing that history well, Israel does not appear convinced that Lebanon’s army will be able to fulfill its mission.

One of the final sticking points of the cease-fire was Israel’s demand for a side agreement that, if the Lebanese Army fails to ensure that Hezbollah is disarmed, Israel will have U.S. approval to respond with force. Israel plans to monitor Hezbollah’s compliance via aerial surveillance and intelligence operations, an Israeli security official told reporters on Wednesday during a call in which government spokespeople required the official not to be named.

“If Hezbollah breaks the agreement and seeks to arm itself, we will attack,” Prime Minister Benjamin Netanyahu of Israel said in a televised address. President Biden reiterated that point in his own address on the cease-fire.

The Lebanese Army has long been central to the international strategy for peace in the region. The United States has given the force more than $3 billion since 2006. Saudi Arabia pledged another $3 billion in 2013, though it halted payments three years later. And last month, France helped raise $200 million.

Yet Hezbollah has long surpassed the power of Lebanon’s military, with bigger weapons and more experienced fighters.

Ms. Ghaddar said that the Lebanese government deserved some of the blame. It has been unwilling to give the Lebanese Army a clear mandate to suppress the military wing of Hezbollah, which is also an influential political party, because some governing factions are sympathetic or aligned with the group, she said.

“They are capable of fighting and winning,” Ms. Ghaddar said of the Lebanese Army. “The problem is we need a political decision.”

The Lebanese Army has rarely operated as a traditional military, largely ceding responsibility of securing borders and rarely getting involved in Israel’s wars against Hezbollah. Army officers have said that the diverse force, made up of various sects, would undermine national stability if perceived to be targeting one sectarian group. So Lebanese soldiers have rarely, if ever, searched private homes and stopped cars to search for Hezbollah members and arms in southern Lebanon.

Yet the new cease-fire doubles down on the force.

The agreement gives Lebanese soldiers wide authority to destroy weapon-making facilities and prevent any entry of unauthorized weapons into Lebanon. It calls on them to deploy along all of Lebanon’s borders and to set up roadblocks and checkpoints in the buffer zone between Israel and Hezbollah. And it charges the United States and France with helping the Lebanese Army deploy 10,000 soldiers to that buffer zone.

Today, there are at least 4,000 Lebanese soldiers in the area, according to two senior Lebanese security officials, who requested anonymity to discuss sensitive military matters.

In the White House Rose Garden on Tuesday, Mr. Biden made clear that the United States thought the Lebanese Army was up to the task. “Over the next 60 days, the Lebanese Army and the state security forces will deploy and take control of their own territory once again,” he said. “Hezbollah terrorist infrastructure in southern Lebanon will not be allowed to be rebuilt.”

In a briefing to reporters, White House officials stressed that the new cease-fire has a better chance at lasting peace than the 2006 agreement because the United States would lead a process to review reported violations of the truce — such as a stockpile of weapons in the buffer zone — and make sure the Lebanese forces are responding.

U.N. peacekeeping forces in Lebanon, known as UNIFIL, will also help monitor and report any violations, as they have since 1978. The 10,000 U.N. forces come from 48 countries, including India, Indonesia and Italy, and were also a key part of the 2006 deal, often accompanying the Lebanese Army in the border area. They will play a similar role this time.

“We are supporting the Lebanese Army, because the Lebanese Army will eventually be fully in charge of the situation,” said Andrea Tenenti, UNIFIL’s spokesman. The U.N. forces are armed, but are only authorized to use force in self-defense.

Mohanad Hage Ali, a researcher who studies the conflict at the Malcolm H. Kerr Carnegie Middle East Center in Beirut, said the U.N. forces helped, but the Lebanese Army must lead because they know the territory and are more trusted by locals. “At the end of the day, this is a state job,” he said.

The Lebanese Army has the ability to succeed in the short term, he added, “but the question is the long-term stability of these operations.”

Lebanon is mired in a yearslong financial crisis, which has forced the cash-strapped government to increasingly rely upon the military for domestic security. At the same time, Lebanon’s currency has collapsed, prompting many rank-and-file soldiers to look for second jobs.

“Basically it became more of a part-time job, because it doesn’t really sustain you,” Mr. Hage Ali said. “So any plan going ahead should address that.”

Last year, the United States paid more than 70,000 Lebanese Army personnel $100 a month for six months. This year, Qatar began funding similar $100 monthly payments. Italy has been leading another initiative to better train and equip the troops, which has included providing watch towers from Britain and mine-clearing tools from France.

In October, France organized a conference to raise more support for Lebanon, including its military. In a speech, President Emmanuel Macron of France called on more nations to help.

The Lebanese Army has “a decisive role to play today,” he said, “even more than yesterday.”

Euan Ward contributed reporting from Beirut, and Aurelien Breeden from Paris.

Robert F. Kennedy Jr., who is in line to lead the Department of Health and Human Services in the next Trump administration, is well-known for promoting conspiracy theories and vaccine skepticism in the United States.

But Mr. Kennedy, an environmental lawyer, has also spent years working abroad to undermine policies that have been pillars of global health policy for a half-century, records show.

He has done this by lending his celebrity, and the name of his nonprofit group, Children’s Health Defense, to a network of overseas chapters that sow distrust in vaccine safety and spread misinformation far and wide.

He, his organizations and their officials have interfered with vaccination efforts, undermined sex education campaigns meant to stem the spread of AIDS in Africa, and railed against global organizations like the World Health Organization that are in charge of health initiatives.

Along the way, Mr. Kennedy has partnered with, financed or promoted fringe figures — people who claim that 5G cellphone towers cause cancer, that homosexuality and contraceptive education are part of a global conspiracy to reduce African fertility and that the World Health Organization is trying to steal countries’ sovereignty.

One of his group’s advisers, in Uganda, suggested using “supernatural insight” and a man she calls Prophet Elvis to guide policymaking. “We do well to embrace ethereal means to get ahead as a nation,” she wrote on a Ugandan news site this year.

These people, more than leading scientists and experienced public health professionals, have existed in Mr. Kennedy’s orbit for years. The ideas spread by him and his associates abroad highlight the unorthodox, sometimes conspiratorial nature of the world occupied by a man who stands to lead America’s health department, its 80,000 employees and its $1.8 trillion budget.

Mr. Kennedy did not respond to a list of questions about his organization’s work abroad. His personal email automatically replied with a link to a Google form for people to apply to work with him in government — and name their own job titles. Mary Holland, the chief executive of Children’s Health Defense, said that Mr. Kennedy was the group’s “chairman on leave” and had not been involved in the day-to-day operations in over a year.

As health secretary, Mr. Kennedy would have the opportunity to reshape health policy. The department has a hand in negotiations for an international pandemic-response treaty, is the parent agency of the Centers for Disease Control and Prevention, and finances global projects like vaccine campaigns.

He undermined confidence in the measles vaccine ahead of a deadly outbreak in Samoa.

Mr. Kennedy visited the Pacific island of Samoa in June 2019 in the aftermath of a public health tragedy.

During routine measles immunizations a year earlier, nurses had mistakenly mixed the vaccine with a muscle relaxant, leading to the death of two infants.

Measles, a highly contagious disease, is preventable, thanks to vaccines that have been proven safe since the 1960s.

But vaccine skeptics seized on the death of the two children as evidence that the vaccines should not be trusted. The Samoan government temporarily suspended its immunization program.

Mr. Kennedy arrived in Samoa, on the invitation of a local anti-vaccine activist, and amplified doubts about the vaccine’s safety. It was a crucial moment. Vaccination rates had plummeted, and the World Health Organization called for Samoa to ramp up immunization as soon as possible.

Mr. Kennedy met with the prime minister and other officials. He told activists that vaccines shipped to Samoa might be of a lower quality than those sent to developed countries.

“With his last name, and the status attached to it, people will believe him,” said Dr. Take Naseri, who met with Mr. Kennedy at the time as Samoa’s director general of health.

A measles outbreak began a few months after his visit. Eighty-three people died, most of them children, a staggering loss in a nation of about 200,000 people.

During the outbreak, Mr. Kennedy falsely suggested that defective vaccines could have caused the deaths. He later dismissed the outbreak as “mild” and denied any connection to it. “I never told anybody not to vaccinate,” he said last year.

When Edwin Tamasese, the anti-vaccine campaigner who arranged Mr. Kennedy’s visit, was arrested and charged with incitement for interfering with vaccinations, Children’s Health Defense helped him obtain legal advice and paid for his lawyers, according to Mr. Tamasese.

The measles outbreak in Samoa ended after 95 percent of the eligible population received vaccinations, according to the W.H.O.

He and his organization promote AIDS falsehoods.

Sex education has been central to the global fight against the spread of AIDS in Africa for decades.

But officials with Children’s Health Defense Africa, one of Mr. Kennedy’s nonprofit groups, see a conspiracy at play.

Wahome Ngare, a Kenyan physician who sits on the group’s advisory board, argued at a conference in Uganda this year that contraception and health education were part of a global plot to reduce Africans’ fertility. He attended the conference alongside the head of the Children’s Health Defense Africa, who presented slides bearing the organization’s logo and web address.

Mr. Kennedy himself has questioned the accepted science behind AIDS. He falsely said that AIDS may have been caused by the recreational use among gay people of the drug amyl nitrite. It is caused by the virus H.I.V.

Last year, Children’s Health Defense posted a video promoting a book that questions the link between H.I.V. and AIDS. Another of the group’s interview subjects this year said that the former U.S. government scientist Anthony Fauci should be imprisoned or “taken off this Earth.”

Dr. Ngare is among the many people in Mr. Kennedy’s orbit whose views conflict sharply with those of the health agency that Mr. Kennedy stands to lead.

In an interview with NPR in 2015 before joining Children’s Health Defense Africa, Dr. Ngare mused about stories that “vaccines have been used for spread of H.I.V.” and called for a boycott of polio vaccines. The U.S. government is a major sponsor of polio vaccine campaigns worldwide. Dr. Ngare did not respond to requests for comment.

Ms. Holland, the chief executive of Children’s Health Defense, said those were Dr. Ngare’s personal views, not those of Mr. Kennedy’s organization.

At the conference in Uganda, Dr. Ngare spoke to far-right lawmakers and activists who support draconian punishments, including life in prison, for people convicted of having gay sex.

The United States has imposed sanctions on Ugandan officials over that law.

He aligned himself with fringe figures, including people who ended up on German security watch lists.

When Mr. Kennedy started his nonprofit group’s European chapter in August 2020, he floated questions about whether the Covid-19 pandemic was part of “a sinister game” played by governments to control people.

“A lot of it feels very planned to me,” he said in Berlin.

The next day, he rallied about 38,000 people at a protest over Covid-19 measures. The protest was organized by a German group called Querdenken. Its leaders have since ended up on a government watch list for fomenting antigovernment sentiment.

Promoters used Mr. Kennedy’s name to drum up attendance, saying that he personally wanted people to take to the streets and fight back. After the event, hundreds of protesters tried to storm the Reichstag, Germany’s Parliament.

Mr. Kennedy was not in attendance at the Parliament. “That whole Reichstag thing was completely unrelated to the demonstration,” Ms. Holland said.

Mr. Kennedy’s influence in Germany lives on, at least in online forums. Recent data from CeMAS, a research group that monitors conspiracy movements, shows that his name is often invoked on conspiracy-focused German Telegram channels, coming up more than 6,000 times this year alone.

His European chapter paid a British lawmaker to speak at a conference promoting vaccine skepticism.

Children’s Health Defense’s chapter in Europe has cultivated relationships with members of the European Parliament.

In January 2022, the organization held a news conference in Brussels demanding a “moratorium on health restrictions.” An anti-vaccine rally that followed the event turned violent, with protesters smashing windows at the European Union’s diplomatic headquarters.

In April 2023, Children’s Health Defense Europe helped organize a conference on the grounds of the European Parliament in Strasbourg, France. At the conference, lawmakers criticized a proposed pandemic treaty being considered at the World Health Organization.

The chapter has hosted press events with European lawmakers and encouraged Parliament to reject vaccination certificate rules.

In 2023, the European chapter paid a member of Britain’s Parliament, Andrew Bridgen, to speak at a conference it had helped organize. The conference discussed opposition to government pandemic measures and promoted vaccine skepticism. The sum was small, at just under $800, according to Mr. Bridgen’s financial disclosures. Such payments are legal in Britain.

Mr. Bridgen has repeatedly compared the Covid-19 vaccine rollout to the Holocaust, including in an interview with the Children’s Health Defense online television station.

Children’s Health Defense spent $315,000 in Europe last year, including in Iceland and Greenland, its U.S. tax filings show. Ms. Holland said that as of this year, the European chapter was run by volunteers and no longer funded by the U.S. operation.

His Africa chapter pushes measles misinformation and risky remedies.

In 2021, a South African herbalist named Toren Wing reached out to Mr. Kennedy about his effort to ban 5G cellphone towers over health concerns.

In an email, Mr. Wing recalled in an interview, he invoked a rousing speech about liberty that Mr. Kennedy’s father had delivered as a senator visiting apartheid South Africa in 1966.

“This is so cool,” Mr. Kennedy responded, according to a copy of the email. He looped in a Children’s Health Defense lawyer. The anti-5G effort fizzled, Mr. Wing said, but it laid the groundwork for a Children’s Health Defense chapter in Africa.

At the chapter’s launch, Mr. Kennedy said the continent was “a testing and clinical trial laboratory for multinational pharmaceutical companies that see African people as commodities.” His group sent just over $15,000 for “setup expenses” in 2022, U.S. tax filings show.

Shabnam Palesa Mohamed, who leads the chapter, is a frequent host of the nonprofit’s online show. She interviews doctors promoting unproven Covid-19 remedies and rails against vaccines.

After a measles outbreak started in Cape Town, Ms. Mohamed appeared in a video discussing supposed negative effects of “alleged measles injections” in South Africa.

⁠In 2023, Unicef reported a 30 percent decline in confidence in childhood vaccines in South Africa after the Covid-19 pandemic, coming amid the world’s “largest sustained backslide in childhood immunization in 30 years.” The group cited factors including “growing access to misleading information.”

Ms. Mohamed and others affiliated with Children’s Health Defense Africa pushed the discredited theory that the drug ivermectin will treat Covid-19. They also sued the South African government, unsuccessfully, to stop Covid-19 vaccinations. Ms. Mohamed thanked Children’s Health Defense for supporting the case.

Ms. Mohamed has promoted conspiracy theories against the World Health Organization, Bill Gates and two of the world’s biggest money managers, BlackRock and Vanguard. Ms. Mohamed declined to answer questions about her work.

“I don’t think she was speaking on behalf of C.H.D.,” said Ms. Holland, who said the Africa chapter was a volunteer organization. “She’s an individual. She has her own views.”

Kimon de Greef contributed reporting.

Israel on Saturday said it had killed a World Central Kitchen worker it accused of taking part in the Hamas-led attack that started the war in Gaza last year, in the second Israeli strike to kill workers affiliated with the aid group.

A spokeswoman for World Central Kitchen, a U.S.-based relief group, said on Saturday that three of its contractors had been killed in an Israeli airstrike on a vehicle. In a statement, the aid group said that it “had no knowledge that any individual in the vehicle had alleged ties” to the Hamas-led attack.

“To the best of our knowledge, no WCK team members are affiliated with Hamas,” the spokeswoman, Linda Roth, said in an email.

The Israeli military declined to comment on the two other workers World Central Kitchen reported were killed.

The aid group said it was pausing operations in Gaza, where a dire humanitarian crisis is unfolding for some two million people. The organization took a similar action in April, after seven of its workers were killed in an Israeli airstrike.

The Israeli military said on Saturday that the person it targeted in the latest strike had taken part in the Oct. 7 attack on Kibbutz Nir Oz, an Israeli village near the Gaza border where dozens of people were abducted. He had been monitored by Israeli “intelligence for a while and was struck following credible information regarding his real-time location,” the military said.

It added that it had targeted “a civilian unmarked vehicle, and its movement on the route was not coordinated for transporting of aid.”

Ms. Roth said that the vehicle was not branded, and that the aid group was investigating the situation.

COGAT, the Israeli agency responsible for coordinating aid deliveries into Gaza, said it had demanded that World Central Kitchen investigate its hiring practices after the strike on the vehicle.

The aid group said it thoroughly vetted its workers. “All World Central Kitchen contractors are queried against the U.S. government’s OFAC list,” Ms. Roth said, referring to a U.S. Treasury list of people, institutions and countries considered national security threats. “Staff employees undergo additional background checks.”

The strike and the accusation that followed threatened to reignite tensions between Israeli officials and humanitarian leaders, who have criticized each other over a year of war and worsening conditions in Gaza. Relief groups have accused Israel of allowing too few supplies into the territory and hindering the flow of aid; Israeli officials say they do not restrict the amount of aid that can enter Gaza and argue that aid agencies should be doing more.

Aid workers operating in Gaza also face the dangers of the ongoing war and widespread lawlessness in the territory. A large convoy of trucks was “violently looted” in Gaza this month, according to the main U.N. agency that helps Palestinians, known as UNRWA.

The Israeli attack on the contractors associated with World Central Kitchen, a disaster relief organization founded by the celebrity chef José Andrés, left the group once again to grapple with the loss of its workers in the Gaza war zone.

In early April, the group suspended work in Gaza and across the region after the airstrike that killed seven of its workers.

After an investigation, the Israeli military called that attack a “grave mistake” and cited a series of failures, including a breakdown in communication and violations of the military’s operating procedures.

The aid group resumed operations a few weeks later with local Palestinian employees, and it remains one of several aid providers that have operated in the territory, which faces a high risk of famine and crises of mass displacement, disease under conditions of wintry weather and continued fighting.

In March, World Central Kitchen became the first group in nearly 20 years to deliver aid to Gaza by sea, briefly using a makeshift jetty fashioned from rubble. The U.S. military also attempted to use a temporary pier to get aid into Gaza, but shut down the project this summer because of rough seas.

Israel has also accused a handful of UNRWA employees of participating in the Hamas-led Oct. 7 attacks. U.N. investigators in August cleared 10 employees of the agency, which says it submits its employees’ information to Israel, and fired nine other workers because of possible involvement.

Israel’s ambassador to the United Nations criticized the investigation, and in October the Israeli lawmakers passed two laws that could threaten its work by barring operations in the country.

Aaron Boxerman contributed reporting.

Russian troops in eastern Ukraine have seized at least 10 villages and settlements in roughly as many days, according to a group with ties to the Ukrainian Army that maps the battlefield, as Moscow presses on with slow but steady advances that have heightened pressure on Ukraine’s authorities to start cease-fire talks.

The situation looks particularly precarious for Ukrainian forces in Donetsk, in Ukraine’s east, where Russian forces are closing in on their last two strongholds in the southern part of the region, according to the analysis by the group, DeepState. The fall of the strongholds, Kurakhove and Velyka Novosilka, could pave the way for a Russian takeover of the area, experts say.

Russia, which annexed Donetsk in 2022 and controls about two-thirds of the region, is seeking to consolidate power over the whole territory. It has concentrated its attacks in the south of the region, searching for weak points in the Ukrainian lines and attacking from many directions, giving outnumbered Ukrainian troops little choice but to withdraw.

“This is indeed the most difficult situation in almost three years of war,” Andrii Biletskyi, the commander of the Third Assault Brigade, one of Ukraine’s top fighting units, told the newspaper Ukrainska Pravda last week, referring to the overall state of the battlefield.

Since this summer, Russian troops have advanced in the Donetsk region at a pace unseen since 2022, capturing hundreds of miles of territory. Experts say the situation, although difficult, is not catastrophic for Ukraine because Russia has yet to reach any major city in the area.

Nevertheless, concerned about their losses, the Ukrainian authorities are warming up to the possibility of opening peace talks with Moscow, which is demanding that it be able to hold onto its gains in Ukraine.

In an interview with Sky News released on Friday, President Volodymyr Zelensky of Ukraine suggested that a way to stop the “hot phase” of the war would be to let Ukraine join NATO and bring the land it controls under the protection of the military alliance. Kyiv would then try to reclaim the territory occupied by Russia “in a diplomatic way,” he said. It was one of Mr. Zelensky’s clearest concessions that the war could end without a quick return of occupied land, a sensitive issue in Ukraine.

NATO members have so far resisted extending an invitation for Ukraine to join the alliance during the war, concerned that it would make the alliance an active participant in the fighting. The prospects seem even dimmer with President-elect Donald J. Trump set to take office, as his team has signaled reluctance to support Ukraine’s membership in the near future.

The Russian advance in the east is also likely to complicate any discussions about Mr. Zelensky’s proposal, particularly regarding the territory that would fall under NATO’s protection, as Moscow is chipping away at it day after day.

Kurakhove is now at risk of being caught in a pincer movement, as Russian forces advance from the north and south and have entered its eastern outskirts. Russian troops have secured positions within five miles of the main supply road into Kurakhove, on both sides, according to DeepState and battlefield maps, enabling them to target Ukrainian forces and vehicles moving in with drones and artillery.

“The clock is already ticking. It won’t be weeks, maybe just days before the city will have to be abandoned,” Ivan Stupak a Ukrainian military expert, told Ukrainian television this past week, noting that almost 70 percent of Kurakhove was surrounded.

Russian forces are also advancing on the town of Velyka Novosilka, 20 miles southwest of Kurakhove. The Center for Defense Strategies, an independent Ukrainian research organization, said the town was a critical logistical hub for Ukrainian forces, one of the launch points for Ukraine’s southern counteroffensive last summer.

The Russian advance in southern Donetsk has also strengthened Moscow’s logistical routes into the vast land corridor it has captured in southern Ukraine. By pushing Ukrainian forces out of artillery range of key railways, Russian troops can now freely operate railway hubs like Volnovakha, about 28 miles south of Kurakhove, to move troops and matériel from the east to the south, experts say.

This advantage could support offensives in nearby areas, such as in the Zaporizhzhia region, where Ukrainian officials have warned that Russia was building up forces in preparation for a potential new assault.

Gen. Oleksandr Syrsky, the top commander of the Ukrainian Army, said on Friday during a visit to the southern Donetsk front that he was sending additional troops, ammunition and weapons to the units deployed there to fend off the Russian advance.

A recent positive note for Ukraine has been its continued defense of the territory it captured this summer in Russia’s western Kursk region, an area Mr. Zelensky views as a potential bargaining chip in negotiations with Moscow.

Though Russia has managed to retake at least a third of the land in Kursk over several months, recent counter-assaults aimed at driving out Ukrainian forces have made little progress, according to battlefield maps based on satellite images and videos of the fighting. Russian troops have also suffered heavier equipment losses than Ukraine has, including of armored fighting vehicles, according to Naalsio, an open-source intelligence researcher analyzing combat footage.

Oleksandr Musiienko, the head of the Center for Military Law Research, said Ukrainian forces had managed to stabilize their flanks in the area and secure logistical lines running back into Ukraine. But he cautioned that Russia may escalate efforts to reclaim the land, potentially deploying more troops, including from North Korea.

The Israeli military said on Saturday that it had struck several Hezbollah sites in Lebanon, keeping the country on edge over a fragile cease-fire, as its forces pressed on with their campaign in Gaza with little sign of letting up.

The cease-fire in Lebanon forged this week ended months of destructive airstrikes and fighting, but it is still not clear when all of the roughly one million people displaced from their homes will be permitted to safely return. Under the deal, Israeli forces are to gradually withdraw from southern Lebanon over 60 days, as Hezbollah fighters also retreat north of the Litani River.

The Israeli military said it hit a site at the Lebanon-Syria border that was “actively being used” by Hezbollah to transport weapons to Lebanon from Syria, in what it called a violation of the cease-fire agreement.

It also said it struck three other sites in southern Lebanon where it had observed militants either “approaching Hezbollah structures,” loading vehicles with weapons or engaging in “terrorist activity” at a Hezbollah facility. It declined to provide further details about any of those strikes.

Hezbollah did not immediately comment on the strikes. Israel struck at least two other sites that it described as Hezbollah infrastructure this week after the cease-fire went into effect on Wednesday.

Despite the sporadic attacks, the cease-fire appeared to largely hold in Lebanon. The Israeli military announced on Saturday night that it was loosening restrictions on gatherings and movement in communities near the border with Lebanon.

The move appeared to be another step toward facilitating the return of tens of thousands of Israelis who fled their homes over a year of conflict with Hezbollah.

In Gaza, however, any pause in the war feels distant.

Israel is pressing a weekslong offensive in northern Gaza that has continued to isolate the area. Dozens of people were killed in Israeli airstrikes in northern Gaza on Friday and more were trapped under debris, according to emergency rescue workers in the territory.

Palestinian Civil Defense in Gaza, an emergency response group, said it believed that more than 75 people had been killed in strikes in Beit Lahia, a farming town north of Gaza City, although it said it had been unable to reach the site because of an Israeli blockade. Civil Defense does not differentiate between civilians and combatants in its death tolls, but it said that families were among the dead.

“Entire families were wiped out in northern Gaza and we don’t know anything about them,” the group said in a statement on Friday night. “There are survivors who remain under the rubble for a long time, and there is no civil defense to rescue them.”

Rescue workers have been unable to operate in northern Gaza since an Israeli offensive began almost two months ago. Internet and phone service to the area has also been unreliable in recent days, leaving rescue workers and the families of those killed and missing with few ways to obtain reliable information.

The Israeli military dismissed reports of airstrikes in Beit Lahia as “false Hamas propaganda” on Saturday, but said it was continuing its “counterterrorism activity against Hamas in the Gaza Strip.”

The military “operates following precise and credible intelligence against Hamas terrorists and terror targets, not against the civilians in Gaza,” the military said in a statement. “We emphasize that the area in question is an active war zone.”

The Israeli military also said on Saturday that it had struck a Hamas fighter who took part in the Oct. 7, 2023, attack that set off the war in Gaza.

Separately, Israel on Saturday said it had killed a World Central Kitchen worker it accused of taking part in that initial Hamas-led attack on Israel. It was the second Israeli strike to kill workers affiliated with the aid group.

The aid agency Save the Children said one of its workers had also been killed in an Israeli airstrike in Khan Younis, in southern Gaza, on Saturday, Reuters reported.

Israel began a major military offensive in northern Gaza in early October to combat what its military describes as Hamas’s resurgence in the area. Local residents and aid groups say the humanitarian effect of the operation has been severe.

At the start of the offensive, the United Nations estimated that 400,000 people were trapped in the combat zone, and tens of thousands have fled the area since then. The fighting has also brought the flow of humanitarian aid and commerce in the north to a near standstill, leaving the civilian population with little access to food, water, medicine and other essentials.

Momen Ahmed, 27, said he spent hours fruitlessly calling his relatives and neighbors in Beit Lahia when he heard that there had been strikes nearby. “No internet or communications are there,” he said.

Finally Mr. Ahmed reached a neighbor, who he said told him that the family house had “been flattened and everyone who was inside the building when the strike took place must have been killed.”

Abu Bakr Bashir and Aaron Boxerman contributed reporting.