After months of losing ground to Russia in brutal, grinding battles in Ukraine, Kyiv shifted tactics with a surprise attack into Russian territory this week that caught Moscow off guard and opened a new front in the 30-month war.

Ukrainian forces have punched through Russian border defenses and seized several settlements in fighting that was still raging on Thursday, according to Russian officials, a Ukrainian soldier and analysts. The attack triggered a state of emergency in one region in the west of Russia. Ukrainian armored columns were filmed moving along roads as far as six miles inside Russia.

But the attack left some military analysts wondering why Ukraine would throw scarce resources into a risky assault in a new area at a time when it is fighting pitched battles to hold on to positions in its own territory.

It was unclear whether Ukraine would seek to hold the area. Whatever the next step by Ukrainian forces, the attack appeared to push the limits on attacking inside Russia with American-provided equipment and put the Russians in disarray. American-made armored vehicles were also filmed being blown up in a Russian counterattack.

The goal was to shift the fighting — and Russian soldiers and weaponry — onto Russian territory and ease the pressure of Moscow’s offensive in eastern Ukraine, a senior Ukrainian official said. He asked not to be cited by name, as Ukraine has not acknowledged its soldiers are fighting in Russia.

“We are at war,” he said of striking inside enemy territory. “Why Russia can and we cannot?”

So far the assault has played out “much more successfully” than previous cross-border raids, the senior Ukrainian official said.

Operating surreptitiously to evade Russian reconnaissance and spies, Ukraine gathered a force that Russia’s top general has estimated at 1,000 soldiers for a mechanized assault on Russia’s border, an audacious move after repeated setbacks over the past year and a half.

In Washington, a State Department spokesman, Matthew Miller, said Wednesday that the use of U.S.-supplied weapons and munitions in the attack by Ukraine did not violate U.S. policy.

“Nothing about our policy has changed, and with the actions that they are taking today, they’re not in violation of our policy,” Mr. Miller said.

Andriy Zagorodnyuk, a former Ukrainian defense minister who still advises the government, said the goal was not to hold the territory long-term, but rather to challenge the Russians, to “divert their forces, attention and resources.” And “show they have no reserves and no resilience capacity.”

Konrad Muzyka, a military analyst for Rochan Consulting in Poland, writing in Ukraine Conflict Monitor, said Ukraine could benefit if the attack reduced Russian attacks in the Donetsk region of Ukraine and allowed Kyiv to maintain a presence in the Kursk area, and thus improve its negotiating position. But Ukraine would be the loser if its troops were pushed back with high losses, he said.

“There is no middle ground here,” Mr. Muzyka wrote. “The operation is daring. Let’s see what the next few days bring.”

Military analysts say they are skeptical that Russia, which has a vastly larger army and arsenal of weapons than Ukraine, would be forced to divert forces from the fighting inside Ukraine to defend its border. Russia has reserves of conscript soldiers it is prohibited by its policies from deploying into Ukraine, but could on Russian soil.

Ukraine has remained mostly silent about the attack, which began on Tuesday. President Volodymyr Zelensky seemed to hint at an aim of raising the cost of the war for Russians without directly acknowledging the Ukrainian incursion in a speech Wednesday. “The more pressure is exerted on Russia — the aggressor that brought war to Ukraine — the closer peace will be,” he said.

Mr. Zelensky, in his nightly address on Thursday, again spoke only indirectly of the Ukrainian attack, saying, “Russia brought war to our land and should feel what it has done.”

Surprise played the pivotal role, Roman Kostenko, the secretary of the defense and intelligence committee in Ukraine’s Parliament, said in an interview. “It was obvious Russia was not prepared and it was a total surprise,” he said. “This is rare in modern war.”

Ukraine, he said, should fight wherever conditions are favorable, whether in Russia or along the front line inside the country. “Our readiness to strike in this way, here or somewhere else, will force Russia to deploy troops to respond.”

Ivan Kyrychevsky, a commentator on military affairs for Defense Express, said Ukraine’s official silence on the operation aided its success, in contrast to the highly telegraphed lead-up to Ukraine’s counteroffensive last year. “If we don’t understand what’s happening, it’s even less clear to the enemy,” he said.

The Russian and Ukrainian armies face off over about 600 miles of international border, in addition to the frontline inside Ukraine. In the Sumy region, Ukraine in June and July had been bracing for a Russian cross-border assault, officials had said. But when the fighting started, it was Ukrainian troops entering Russia.

“They were not expecting us, and they fled wherever they could,” said a Ukrainian soldier who fought in the assault and who asked to be identified by only his first name, Oleksandr, in keeping with Ukrainian military protocol. His unit took prisoners and captured a tank, he said in a telephone interview.

The Ukrainians, he said, were motivated to take the fight into Russia. “To be honest, we all have joy in our hearts,” Oleksandr said, though he acknowledged there were still risks.

Ukraine has not advanced so quickly since reclaiming the Kherson region in the country’s south in November 2022. In May, Russia had surprised Ukraine when it sent troops across the border in the area north of Kharkiv, where it still has a narrow foothold. In the Donbas region of eastern Ukraine, understrength Ukrainian units are falling back, and Russia is fighting in or close to the strategic hilltop town of Chasiv Yar and rail and road hubs in Kostyantynivka and Pokrovsk.

The incursion into the Kursk region is the third significant Ukrainian ground assault on Russian territory since the start of the full-scale invasion of Ukraine more than two years ago. But it appears to be the largest, according to open-source intelligence analysts studying photographs and videos from the area of Russia under attack, a rolling expanse of farm fields, forests and small towns.

Some analysts estimate that Ukraine has sent hundreds of troops into Russia, a major commitment as its forces are under pressure in the east. Russia’s chief of the general staff, Valery Gerasimov, said in televised remarks that Ukraine deployed 1,000 soldiers.

In a statement on Thursday, Russia’s Ministry of Defense said it was destroying Ukrainian formations and repelling the attack.

But a prominent Russian military blogger who writes under the name of Rybar said Thursday that Ukrainian forces had Sudzha, the main town in the area, “practically under full control.” Videos from Sudzha, verified by The New York Times, show cars trying to leave amid sounds of gunshots and roads littered with burned vehicles and what appeared to be mines.

The Kremlin sought to maintain calm as the state news media portrayed the incursion as akin to a natural disaster rather than a military crisis, Mikhail Vinogradov, a Moscow political analyst, said. State media highlighted efforts to help civilians while doing little to place the attack in the context of Russia’s invasion of Ukraine.

A noon news broadcast on Russia’s state-controlled Channel 1 on Thursday showed bottled water being dispatched for evacuees and supportive vigils in cities like Sochi on the Black Sea.

“Kursk, we are with you!” a person there was shown saying.

Mr. Vinogradov, in a phone interview, said the restrained coverage reflected the Kremlin’s avoidance of “politicization” of the Russian public, with Russian officials instead preferring to limit people’s exposure to the ups and downs of the war.

Dmitri Kuznets, an analyst of the war for Meduza, an independent Russian news outlet based in Latvia, said the Ukrainian Army had clearly found a loophole in Russian defenses and caught thin and ill-equipped troops off guard.

Mr. Zelensky’s administration offered its first commentary on the incursion on Thursday in a statement by a senior adviser that did not acknowledge any Ukrainian role but portrayed Russians as collectively backing Mr. Putin’s invasion, signaling the intention was not to affect morale inside Russia.

The adviser, Mykhailo Podolyak, said Russians were not going to “come out with flowers to greet the anti-Putin tanks,” and noted that a million Russians had volunteered to serve in the country’s military.

The Ukrainian push involves regular army units, analysts have noted, in a change from the previous incursions, which were carried out by armed groups of Russian exiles backed by Kyiv’s army. That tactic had offered Kyiv a veneer of noninvolvement.

The European Union spokesman, Peter Stano, told the Ukrainian news outlet Suspilne that Ukraine “has the legal right to defend itself, including by striking at the aggressor on its territory.”

Photographs and videos posted online and verified by independent military analysts suggested that Ukrainian forces had overran a Russian metering station for a pipeline exporting natural gas to Europe across Ukraine, which had remained active despite the war. Gazprom, the Russian gas company, told Russian news media the flows declined slightly. In Europe, natural gas prices rose.

A video filmed by a Russian drone and posted by a Russian military blogger who writes under the handle Dva Mayora, or Two Majors, showed a damaged, U.S.-made armored vehicle at a road intersection six miles inside Russia.

Reporting was contributed by Eric Schmitt, Anton Troianovski, Ivan Nechepurenko, Constant Méheut, Sanjana Varghese, Dzvinka Pinchuk and Stas Kozljuk.

After more than a week of sporadic far-right violence, a fever seemed to ease in Britain on Wednesday night. An unconfirmed list of more than 30 target sites associated with the migration system, widely circulated online, summoned few would-be rioters but drew a heavy police presence and large crowds of protective counterprotesters.

During the previous days, racist and anti-immigrant rioting had flared in more than a dozen towns and cities across England and in Northern Ireland. Over 400 people were arrested, according to a police chiefs’ group. Many have gone to court. Some are already beginning prison sentences.

The spark for the rioting was anger over a knife attack that killed three young girls and falsehoods that spread online about the perpetrator.

Those participating in the riots have remained a small and strongly unpopular fringe, disowned even by politicians who seek to channel broader public misgivings about immigration. But that has not made the violence easy to stop.

Here is a timeline of how the unrest developed.

Monday, July 29

A stabbing attack that shocked the nation

A knife-wielding attacker burst into a dance and bracelet-making class for young Taylor Swift fans in Southport, a coastal town in northwestern England. His assault killed three girls: Alice Dasilva Aguiar, 9; Elsie Dot Stancombe, 7; and Bebe King, 6. Eight other children and two adults were seriously wounded.

The police arrested Axel Rudakubana, a teenager from the nearby village of Banks. Under strict privacy rules for suspects under 18, the authorities initially identified him only by his age and where he lived, adding shortly afterward that he had been born in Cardiff, Wales. British news reports later indicated that his parents had come to the country from Rwanda, and that the family regularly attended church.

Within hours of the attack, far-right accounts on social media began spreading a fictitious Arabic-sounding name for the killer and claiming that he was a Muslim asylum seeker who had illegally arrived in the country by boat.

Tuesday, July 30

A riot in a town in mourning

The next night, far-right activists called for a rally in Southport after a vigil for the three girls. The rally quickly turned to violence, with hundreds of rioters attacking a mosque near the scene of the stabbing. They hurled bricks, set cars on fire and injured more than 50 police officers.

Wednesday, July 31

A far-right rally in the heart of London

Another violent far-right gathering followed in Whitehall, in the heart of London’s government district, resulting in more than 100 arrests. As they had in Southport, the demonstrators adopted the slogan “Enough is enough.” They borrowed another rallying cry — “Stop the boats,” meaning the small boats used by people smugglers to ferry migrants across the English Channel — from Britain’s former Conservative government.

Angry crowds gathered that evening in Manchester, in northwestern England, and in the southern town of Aldershot, outside hotels that they believed were being used to house asylum seekers.

The next day, at an initial court hearing, a judge took the unusual step of easing reporting restrictions about the stabbing suspect, allowing his name to be published. If the hope was to halt the violence, it did not appear to work.

Friday, Aug. 2

Flames and counterprotests

As the unrest grew, antiracist organizations and religious and community groups mounted a response. In Liverpool, the nearest large city to Southport, calls for a Friday-night rally outside a mosque brought out a far larger crowd of protesters to protect it.

But violence continued to erupt. That night in Sunderland, an industrial port city in northeastern England, a far-right mob attacked police officers, looted stores, burned buildings and set a car on fire.

Saturday, Aug. 3

The flames spread

The weekend brought violence in a dozen towns and cities across England and in Belfast, Northern Ireland, as far-right rallies faced off against both riot police officers and counterprotesters.

A library and a food bank were burned in Liverpool as far-right groups damaged and looted businesses, and in Hull, fires were set and storefronts smashed in the city center.

A cafe owner in Belfast, Mohammed Idris, described to the BBC how rioters attacked his business — shouting, “Where is Mohammed?” — and then set it alight. Some counterprotesters in the city hurled insults at the rioters, while others chanted, “Refugees are welcome here.”

In Bristol, in southwestern England, counterprotesters formed a ring around a hotel that far-right activists had targeted in the belief that it contained asylum seekers.

Sunday, Aug. 4

Rioters storm a hotel

On Sunday, Prime Minister Keir Starmer denounced the riots as “organized, violent thuggery” and warned that anyone participating in the violence would “face the full force of the law.”

Hours later, far-right groups in the northern town of Rotherham attacked a Holiday Inn that had been used to shelter asylum seekers, clashing with the police and appearing to set fires before some stormed into the building. Video showed a handful of people looking out of windows as a crowd of demonstrators surrounded the hotel, chanting, “Get them out.”

Monday, Aug. 5

A peaceful memorial

A week after the knife attack, people in Southport held a vigil that could hardly have been more different from some of the scenes the previous Tuesday night. Even as unrest continued elsewhere in the country, families and children gathered around the flowers and toys that had piled up in tribute to the three girls killed. As evening came, they blew bubbles together.

“It’s blowing kisses to those girls up there,” one woman attending told BBC local radio.

British officials on Thursday expressed relief and cautious optimism after far-right protests failed to materialize the previous night and thousands of antiracism demonstrators took to the streets instead.

The authorities had been braced for further violence after anti-immigrant protests in previous days had descended into riots, fueled by disinformation about the fatal stabbing of three young children at a dance class in Southport, northwestern England, on July 29.

But with around 6,000 officers on standby, Wednesday evening passed with little violence. Pockets of far-right protesters were easily outnumbered by antiracism counterdemonstrations in several cities, including Birmingham, Bristol, Liverpool and London.

That raised hopes that the law enforcement authorities had begun to regain control after riots over the previous weekend. Hundreds of people were arrested and dozens of police officers were injured in those outbreaks of violence, which saw rioters set cars on fire and target mosques and hotels housing asylum seekers.

Britain’s prime minister, Keir Starmer, said on Thursday that a repetition of that violence was largely avoided overnight because “we had police deployed, in numbers, in the right places giving reassurance to communities.”

Mr. Starmer, a former chief prosecutor, has pushed hard to ensure that people arrested during the riots were tried quickly in order to deter others from participating in future disorder.

On a visit to the West Midlands on Thursday, he said that the sentencing a day earlier of a rioter to three years in prison had sent a “powerful message.”

Two more people who took part in rioting were jailed on Thursday: John O’Malley, 43, and William Nelson Morgan, 69, were each sentenced to two years and eight months in prison for violent disorder.

And a 55-year-old woman was arrested Thursday by Cheshire police over social media posts “containing inaccurate information about the identity of the attacker” in the Southport stabbings.

But Mr. Starmer said it was important that there was no “let up” from the authorities, and that he planned to convene a meeting to reflect on the lessons learned from Wednesday night and to plan for days ahead.

Not everywhere was free of violence on Wednesday night, however, and a small number of arrests were made. In Belfast, Northern Ireland, objects were thrown at the police, fires were lit and officers in riot gear were deployed.

But in London, Mark Rowley, commissioner of the city’s Metropolitan Police, described Wednesday as a “very successful night.”

“The show of force from police — and frankly the show of unity from communities — together defeated the challenges we have seen,” he said.

Efforts to combat calls on social media for violence could also have played a role. Both the government and the police had expressed concern about the use of messaging apps like Telegram by far-right groups intent on stoking disorder.

On Wednesday, Telegram said that its moderators were removing channels and posts containing calls to violence. The company also said that it was using A.I. tools and user reports “to ensure content that breaches Telegram’s terms is removed.”

Experts said that far-right sympathizers were raising questions and circulating conspiracy theories online on Thursday about the low turnout the night before. Some people posting online claimed that warnings about potential trouble had been a false-flag operation by the government designed to embarrass the far right when it failed to materialize, according to Al Baker, the managing director of Prose Intelligence, a technology company that monitors Telegram channels.

Mr. Baker’s own interpretation of what happened on Wednesday was that online support for far-right actions that night had not translated to real-world support on the streets.

The far right has evolved into “a movement built around online spaces and influencers, rather than organizers,” Mr. Baker said, and lacks “the central organizing force which might have orchestrated these things from beginning to end.”

He predicted, however, that far-right activists might try to “revive some of the momentum” and try again to organize.

Those fears were also echoed by Tiffany Lynch, acting national chair of the Police Federation, which represents rank and file officers. She told the BBC that it was “absolutely not” possible to say that the run of violence was now at an end.

The youths accused of planning to attack a Taylor Swift concert in Vienna had hoped to kill as many people as possible, the Austrian authorities said on Thursday, outlining a plot designed to copy some of the worst terrorist assaults of the last decade.

“The suspects actually had very specific and detailed plans to cause a tragedy on the scale of Paris, Manchester or Moscow,” Karl Nehammer, the chancellor of Austria, said Thursday afternoon at a news conference, referring to attacks that killed hundreds of concertgoers in all. Mr. Nehammer said the two, arrested less than 24 hours earlier, wanted to leave a “trail of blood.”

Ms. Swift had scheduled three concerts in Vienna, the first on Thursday, and she had been expected to draw more than 200,000 fans from across the world. Barracuda Music, the promoter for the Vienna run, canceled the shows on Wednesday night in what it characterized as a decision coordinated with Ms. Swift’s management.

The Austrian authorities did not publicly identify either of the people arrested. They described the main suspect as a 19-year-old man who was radicalized online and swore an oath of allegiance to the Islamic State.

Franz Ruf, the head of public safety and Austria’s highest-ranking police officer, said at a news conference earlier on Thursday that the suspect had confessed to the terror plans after being arrested, providing detailed insight into his intentions, which included using explosives and weapons to kill attendees.

Searching the young man’s home in the town of Ternitz, about 40 miles south of Vienna, where he lived with his parents, the police found machetes, knives, explosives, timers and chemicals to make explosives, as well as steroids, Islamic State propaganda and 21,000 euros in counterfeit bills, Mr. Ruf said. The man had successfully fabricated bombs using instructions found online, the police said.

He did not have a ticket to any of the concerts. But the authorities found a police siren system, which they said could have allowed an attacker to enter or flee the site in a car.

When the suspect, who was born in Austria but has Macedonian roots, quit his job last month, he told colleagues he had big plans, according to investigators. He then changed his appearance, they said.

The tip that led to the arrests came from U.S. intelligence, according to U.S. officials. Omar Haijawi-Pirchner, the head of Austria’s domestic intelligence agency, acknowledged on Thursday that the tip had come from abroad but did not attribute American officials.

Earlier this year, U.S. officials warned their Russian counterparts of an imminent threat in Moscow, but Russian security forces failed to stop an attack on a concert.

The second person arrested in Austria was a 17-year-old suspected of being an accomplice. Mr. Ruf said he was known to the police and had recently started a job for an events services provider working at the Ernst Happel Stadium, where Ms. Swift was scheduled to perform. He was arrested on Wednesday at the stadium, and had broken up with his girlfriend last week, in what investigators think was preparation for the attack.

A 15-year-old was also questioned and confirmed many details of the main suspect’s confession, Mr. Ruf said. The police believe the boy was not an active participant in the plot but knew of its details, he added.

With the suspects in custody, Mr. Ruf said there was no longer an imminent threat.

Mr. Haijawi-Pirchner, the domestic intelligence director, said his agency had not pushed to cancel the concerts, but understood the reasoning for doing so. Later, Chancellor Nehammer called it a “very responsible and understandable” decision.

Gerhard Karner, Austria’s interior minister, said “the situation was serious,” noting that Austria had been on a higher alert since the Hamas-led attacks on southern Israel last October. “The danger from Islamist extremism in Europe has increased significantly following the terrible terrorist attack by Hamas on Israel,” he said.

Concerts have been targets elsewhere in Europe in recent years. In 2015, gunmen attacked a concert venue in Paris, killing more than 90 people and wounding hundreds. In 2017, a suicide bombing at an Ariana Grande concert in Manchester, England, killed 22 people. And in March, gunmen attacked the Moscow concert, killing more than 140. Those assaults were carried out by men who were inspired by or linked to the Islamic State or its ISIS-K, an ambitious affiliate that has set its sights on attacking Europe.

Ms. Swift’s fans were also victims in England last week, when three girls were fatally stabbed and others wounded during a Swift-themed dance class. A suspect, who just turned 18, was arrested in connection with the assault, which the police have said is “not being treated as terrorist-related.”

Next week, the singer’s global tour is scheduled to begin a run of five sold-out shows at Wembley Stadium, a 90,000-capacity arena in London.

Neither Wembley Stadium nor A.E.G. Presents, the promoter for the tour’s British dates, immediately responded to emails asking about how the events in Austria might affect those shows. But a spokesman for London’s Metropolitan Police said in a statement that there was “nothing to indicate that the matters being investigated by the Austrian authorities will have an impact on upcoming events here.”

Eric Schmitt contributed reporting from Washington, Sarah Maslin Nir from Vienna, and Alex Marshall from London.

Faced with a shrinking population, China’s top leadership has tried everything to get women to have more babies. Everything, it turns out, except allowing unmarried women to freeze their eggs.

A Beijing court this week chose to uphold a longstanding rule that only married women may use the procedure. Rights activists say the rule is unfair because it excludes single women from a reproductive measure that gives them the option to put off childbirth.

The ruling centers on a lawsuit filed by Teresa Xu, against an obstetrics hospital after a doctor denied her access to egg freezing services and instead told her that she should get married and have children quickly.

On Wednesday, Ms. Xu said the Chaoyang Intermediate People’s Court in Beijing had rejected her lawsuit, exhausting her legal options in a six-year battle for reproductive rights. The court had argued that her rights were not violated.

In a livestream video, Ms. Xu, 36, a freelance writer in Guangzhou, said she wasn’t surprised by the court’s decision. “I was mentally prepared for it,” she said in the video that was later posted to her social media account. “This result wasn’t all that unexpected.”

In China, the ruling Communist Party continues to have a large say over who may have children, and how many. For years, it allowed families to have only one child. As births slowed significantly, threatening growth, officials loosened the one-child policy to allow for two children and then three.

Most hospitals in China require women to be married before freezing their eggs. Single women who are pregnant are regularly denied access to public health care as well as benefits like maternity leave. And children born to single parents struggle to get social benefits such as education and medical insurance.

Yet the reasons for Ms. Xu’s decision to freeze her eggs are ones shared by many young Chinese women: She wanted to have a baby at some point, but she wanted to work and save money first, for her future.

China’s ban on access to reproductive treatments for single women has forced many women who have the financial means and the determination to travel overseas and spend tens of thousands of dollars to get their eggs frozen in countries like Thailand and Malaysia.

On the surface, the desires of Ms. Xu and other young women would seem to fit with Beijing’s own goals. Urgently trying to address a declining population, Chinese officials have tried doling out cash and subsidies as incentives for families to have more children, making in vitro fertilization and other reproductive services more widely affordable.

Nevertheless, its birthrate remains historically low — and continues to fall. Many young Chinese women say they prefer to be alone. Even people who are in a relationship are forgoing marriage and children, some because of uncertainty about the country’s prospects as its growth slows. The number of marriages in the first half of this year dropped to the lowest level in a decade, according to official data released this week.

Ms. Xu, an advocate of women’s rights, has argued that the rules around egg freezing are sexist. Men can choose to freeze their sperm with no conditions. In addition to being married, women seeking to freeze their eggs must show that they have a license to give birth to a child. They must promise not to exceed the number of children they are allowed to have, and show proof they are either infertile or are undergoing treatment that could make it more difficult for them to conceive.

In its ruling, the Beijing court said that the decision by the hospital to deny Ms. Xu egg freezing services had complied with current rules and was “consistent with laws and regulations as well as common sense.” But the court also left room for future changes that could be made to China’s fertility policy and said that “when conditions are met, Ms. Xu may resolve the relevant disputes separately.”

In her livestream on Wednesday night, Ms. Xu vowed to continue to fight for the reproductive rights of single women, saying the ruling this week was “not the end,” adding that she would “actively formulate the next strategy.”

As she signed off from her livestream, she added, “We have a long road ahead of us.”

No country competing in the track and field competition at the Paris Olympics has had its athletes subjected to more drug testing than Kenya in recent years. There is a reason for that.

Not only is the East African nation a running powerhouse, it has also been trying to emerge from a major cheating crisis first detected nearly a decade ago.

About 300 athletes from Kenya have been punished for using banned substances since 2015. The situation was so bad at one point that track and field officials had discussed the possibility of the unthinkable: a ban similar to those imposed on Russia, another sporting powerhouse whose doping past — among other issues — has rendered it conspicuous by its absence in Paris.

Kenya’s vast footprint, and dominance, of track and field since Naftali Temu brought home the country’s first gold medal at the 1968 Games meant losing the nation would have diminished the entire sport, said Barnabas Korir, an executive committee member of Kenya’s athletics federation.

Without Kenya, he said, “other countries will feel they will not get a proper competition because this country has some of the best athletes, deeper talent than any country”

“That is why,” he added, “the chance was given to Kenya to redeem itself from this problem.”

For Brett Clothier, the head of the independent unit responsible for drug testing in global athletics, the threat of a ban represented a “come-to-Jesus moment” for Kenya, a nation whose global standing, and sense of itself, is in many ways linked to its champion runners.

That much is clear on the roads out of Jomo Kenyatta airport in Nairobi, where the faces of top stars like Faith Kipyegon and Eliud Kipchoge appear on billboards, and where Kenyan runners come and go on their way to races around the world. Their success has been so consistent, the country’s gold-medal haul now so consistent, that winning well — breaking a record — is now the true measure of success.

“We say they are ambassadors for our country,” Mr. Korir said.

Risking all of that was not an option, Kenyan officials decided. So the Kenyan government bowed to demands (and threats) to invest a vast sum to create a new antidoping agency capable of tackling a problem that had spread through the entire Kenyan running pyramid.

In 2023, the government committed to spend $5 million annually to rebuild its broken antidoping program. The total is half the contribution of the British government to its local body and only a third of the United States’ contribution to testing there. But, according to Mr. Clothier, the head of track and field’s antidoping body, the Athletics Integrity Unit, it is an “incredible” sum considering the relative financial might of Kenya.

Testing of track and field athletes in Kenya is now “the best in the world,” Mr. Clothier said.

Unlike Russia, whose state-sponsored doping program offered advantages to elite athletes and corrupted dozens of international sporting events before it was revealed, doping in Kenya rises organically from the ground up and is complicated by a mix of economic uncertainty and intense competition.

Nearly all the best runners, Mr. Clothier and Kenyan athletics officials said, hail from three counties in the northern valleys, far from the capital Nairobi. In those counties, running is one of the few proven ways to escape grinding poverty.

“It’s like organized crime,” Mr. Clothier said of the effort to control and profit from a pool of talented middle- and long-distance runners that is deeper than anywhere else in the world. “Every wannabe criminal sees them as a moneymaking opportunity.”

Those conditions have attracted not just Kenyans but also foreign nationals who seek out talented athletes and offer them the chance to secure riches that can be not only life changing but also community changing.

“Runners are walking around with flashing dollar signs, and there are people telling them, ‘We can make you run faster,’” Mr. Clothier said.

The most lucrative and commercial sport is marathon running, and Kenyans happen to be the best in the world at it. Scores of major cities around the world host annual marathons, and each one represents an opportunity for a payday.

Winning a major marathon can be worth $100,000 or more, and even minor ones offer prizes of $5,000 to $10,000 for winning. In Kenya, even those smaller purses can represent the equivalent of a year’s wages. An endorsement contract with a shoe company can bring in yet more income.

That contrast between that money and poverty “creates a huge risk of doping,” Mr. Clothier said.

But the same depth of talent among Kenyan runners means the very top ones are always in fear of being knocked off their perch by those just below.

To reduce the risk that elite runners will turn to doping to protect their status, and their incomes, the number of named athletes in Kenya’s drug-testing pool was increased to 300 from 30. The results have been encouraging for the testers: The number of international doping cases — runners caught cheating — involving Kenyans went from just one in 2017 to 38 in 2022.

“On Kenya, the jury’s out,” Mr. Clothier, a naturally cautious Australian lawyer, said when asked if that represented the end of Kenya’s doping problem. “We’ve got off to a good start.”

But that early success was why track officials, and their antidoping surrogates, have worked to persuade the Kenyan government to reaffirm its commitment to overhaul a system that had fallen prey to unchecked corruption.

Just before the Rio de Janeiro Olympics in 2016, the head of Kenya’s track and field squad was caught in an undercover sting offering to tip off athletes about the timing of drug tests in return for thousands of dollars.

“Everyone knew there was a lot of cheating in Kenya,” Mr. Clothier said. “For our sport, Kenya is the No. 1 country in our sport, with the No. 1 doping problem. So we decided we are just doing it.”

The A.I.U. has had a semi-permanent presence in Kenya for years, sending some of its top staff to East Africa for several months at a time. It has increased its intelligence-gathering capability in the country and worked to make remote training camps targets for random testing. It also has recruited a new generation of local staff members with promises of higher pay, which, in some cases, was as much as five times higher than in the past.

The response was immediate. The local caseload increased, with sophisticated drugs detected and prosecuted, and the number of elite athletes caught by the A.I.U., which is responsible for pursuing the very best runners, started to fall just as the total caught by the new local antidoping agency crept higher. In 2023, the Kenyan antidoping agency opened cases against 60 athletes, and another 29 were investigated by global bodies.

The Kenyan antidoping agency’s additional resources have also helped catch some of the biggest stars and expose elaborate efforts to avoid testers, including faking car crashes and sometimes working with senior medical doctors to fake records.

Its most recent success story took two years to complete. Lawrence Cherono, a winner of the Boston and Chicago marathons, claimed he had inadvertently ingested heart medication that had been prescribed to his wife. Demands for official documents proving the medication was his wife’s were stymied for months, eventually requiring a court subpoena and the use of private investigators to track the athlete as he visited the clinic.

Eventually, Cherono’s story fell apart, and in July, the 37-year-old was banned for seven years.

All of those success stories, and the others that have taken out some of Kenya’s biggest stars, have hurt, said Mr. Korir, the Kenyan athletics commission chief executive. But he also said the “bad apples” must not overshadow decades of athletic success.

“I want to tell the world that most of the Kenyan athletes from the past have been running clean,” he said.