Iran began to evacuate its military commanders and personnel from Syria on Friday, according to regional officials and three Iranian officials, in a sign of Iran’s inability to help keep President Bashar al-Assad in power as he faces a resurgent rebel offensive.

Among those evacuated to neighboring Iraq and Lebanon were top commanders of Iran’s powerful Quds Forces, the external branch of the Revolutionary Guards Corps, the officials said.

The move signaled a remarkable turn for Mr. al-Assad, whose government Iran has backed throughout Syria’s 13-year civil war, and for Iran, which has used Syria as a key route to supply weapons to Hezbollah in Lebanon.

Guards personnel, some Iranian diplomatic staff, their families, and Iranian civilians were also being evacuated, according to the Iranian officials, two of them members of the Guards, and regional officials. Iranians began to leave Syria on Friday morning, the officials said, speaking on condition of anonymity to discuss a sensitive issue.

Evacuations were ordered at the Iranian Embassy in Damascus, and at bases of the Revolutionary Guards, the Iranian and regional officials said. At least some of the embassy staff has departed.

Some are leaving by plane to Tehran, while others are leaving via land routes to Lebanon, Iraq and the Syrian port of Latakia, the officials said.

“Iran is starting to evacuate its forces and military personnel because we cannot fight as an advisory and support force if Syria’s army itself does not want to fight,” Mehdi Rahmati, a prominent Iranian analyst who advises officials on regional strategy, said in a telephone interview.

“The bottom line,” he added, “is that Iran has realized that it cannot manage the situation in Syria right now with any military operation and this option is off the table.”

Alongside Russia, Iran has been the Syrian government’s most powerful supporter, sending advisers and commanders to bases and the front lines and backing militias.

It also deployed tens of thousands of volunteer fighters, including Iranians, Afghans and Pakistani Shias, to defend the government and to retake territory from the Islamic State terrorist group at the height of Syria’s civil war. Some of Iran’s forces, like the Afghan Fatemiyoun brigade, had remained in Syria at military bases operated by Iran; on Friday, they were also being transferred to Damascus and Latakia, an Assad government stronghold, the Iranian officials said. A video posted on accounts affiliated with the Guards showed the Fatemiyoun in uniform taking refuge at the shrine of Seyed Zainab near Damascus.

The surprise offensive by a rebel coalition has dramatically changed the landscape of the civil war, which Mr. al-Assad had fought to a standstill, and Iran’s control over some of Syria’s territory. In a little over a week, the rebels have swept through major cities like Aleppo and Hama, captured swaths of territory across four provinces, and moved toward Syria’s capital, Damascus.

The Iranian officials said that two top generals of Iran’s Quds forces, deployed to advise the Syrian army, had fled to Iraq as various rebel groups took over Homs and Deir al-Zour on Friday.

“Syria is at the verge of collapse and we are watching calmly,” said Ahmad Naderi, an Iranian Parliament member, in a post on social media on Friday. He added that if Damascus fell, Iran would also lose its sway in Iraq and Lebanon, saying, “I don’t understand the reason for this inaction but whatever it is, it’s not good for our country.”

The rebel offensive came at a moment of relative weakness for three of Syria’s most important supporters. Iran’s ability to help has been curtailed by its conflict with Israel; Russia’s military has been sapped by its invasion of Ukraine; and Hezbollah, which had previously supplied fighters to aid the Assad government’s fight against the Islamic State, has been battered badly by its own war with Israel.

The fall of more territory to rebel forces, which are led by the Islamist group Hayat Tahrir al-Sham, could also threaten Iran’s ability to supply either Mr. al-Assad’s regime or Hezbollah with weapons and advisers.

Iran’s foreign minister, Abbas Araghchi, had traveled to Damascus this week, meeting with Mr. al-Assad and pledging Iran’s full support.

But in Baghdad on Friday, he appeared to make a more ambiguous statement. “We are not fortune tellers,” he said in an interview on Iraqi television. “Whatever is God’s will shall happen, but the resistance will fulfill its duty.”

President Yoon Suk Yeol of South Korea grew increasingly isolated on Friday, after the head of his ruling party came out in support of impeaching the president, and the country’s military said it would not follow any orders to reimpose martial law.

In a surprise about-face, the leader of the governing People Power Party, or P.P.P., called Mr. Yoon unfit to lead South Korea after his short-lived martial law decree earlier this week catapulted the democracy into turmoil.

“I think that President Yoon Suk Yeol should be suspended from office as soon as possible,” Han Dong-hoon said during an emergency party leadership meeting.

It was not clear how many members of the party shared Mr. Han’s view. But his comments increased the likelihood that the impeachment bill against Mr. Yoon would gain enough support to pass through the National Assembly.

Mr. Han joined opposition lawmakers in warning that if Mr. Yoon stays in office, he might try to impose martial law again in a desperate attempt to hold onto power. In a powerful rebuttal to Mr. Yoon, the defense ministry said that even if he did, the military would not follow. And the country’s special forces command, which joined Mr. Yoon’s ill-fated martial law, said it would not join another.

“I want to make it clear that it will never happen,” Lieutenant General Kwak Jong-geun said during an interview he gave to two opposition lawmakers that was broadcast live on YouTube on Friday, where he was asked about the possibility of another martial law attempt. “Even if I get such an order, I will reject it.”

A broader closed-door meeting of the P.P.P.’s 108 lawmakers took place inside the National Assembly but adjourned without reaching consensus on Mr. Yoon’s fate. Mr. Yoon, though expected at the meeting, didn’t show up. National Assembly Speaker Woo Won-shik said he could not guarantee the president’s safety as angry opposition members rallied at the Assembly’s entrance, chanting “Arrest Yoon Suk Yeol!”

An impeachment vote is scheduled for Saturday evening. At least eight votes from Mr. Yoon’s governing camp are needed to impeach the president.

If impeached, Mr. Yoon would be suspended from office until the Constitutional Court decides whether to reinstate or remove him.

The opposition proposed impeaching Mr. Yoon this week, arguing that he had committed “insurgency” and other anti-constitutional crimes when he declared martial law on Tuesday night. He banned all political activities and sent troops to take over the National Assembly. Legislation on martial law banned such acts, the opposition said in its impeachment bill formally submitted on Thursday.

Mr. Yoon’s surprise declaration of martial law lasted only six hours, until early Wednesday. He was forced to lift it after a vote in the Assembly to repeal the military rule. The short-lived episode, which the opposition likened to a failed “palace coup” by an extremely unpopular leader, triggered outrage across South Korea. Even if briefly, it also exposed the fragility of the hard-won democracy South Koreans have been proud of.

On Thursday, Mr. Han had said he opposed impeaching Mr. Yoon for fear of creating more national confusion. But he said he changed his mind as more details emerged about what happened in the hours after Mr. Yoon declared martial law, especially indications that Mr. Yoon sought to arrest key critics of his government, including Mr. Han himself.

On Friday he said, “There is fear that if President Yoon stays in office, he may repeat extreme actions like martial law.”

“If that happens, South Korea and its people will fall into a bigger crisis,” he said.

Hong Jang-won, a deputy chief of the National Intelligence Service, told lawmakers on Friday that after declaring martial law, Mr. Yoon told him to work with the military’s Defense Counterintelligence Command to “drag them all in,” according to the opposition lawmaker Kim Byung-kee, who attended the closed-door meeting.

According to Mr. Kim, Mr. Hong told lawmakers that Lt. Gen. Yeo In-hyong, the counterintelligence commander, had shared with Mr. Hong a list of people to locate and detain. They included: Mr. Woo, the speaker of the National Assembly; Lee Jae-myung, the leader of the biggest opposition party; and Mr. Han, the head of Mr. Yoon’s own governing party.

Mr. Hong also told the lawmakers that agents from counterintelligence command had planned to take these politicians to their headquarters in Gwacheon south of Seoul.

On Friday, the P.P.P.’s Mr. Han said he also had similar intelligence that the counterintelligence command had planned to detain key politicians in the name of rounding up “anti-state forces.” But when Mr. Han met with Mr. Yoon on Friday to discuss the crisis, the president denied having had such plans, his office said.

In the YouTube interview, Lt. Gen. Kwak, the special forces commander, said his units were not running arrest squads.

He said that Defense Minister Kim Yong-hyun had instructed him to remove lawmakers from the Assembly hall, where the vote on repealing the martial law order was taking place. But he said he ignored the order.

“I knew I could be punished for disobedience,” Lt. Gen. Kwak said. “But I also thought such an order had legal problems and I told my troops not to go in there.”

Mr. Kim resigned after the martial law collapsed. He has since been barred by prosecutors from leaving the country.

On Friday​, the Defense Ministry said it suspended three officers, including Lt. Gen. Kwak and Lt. Gen. Yeo, from their jobs because of their roles in Mr. Yoon’s martial law.

Opposition lawmakers asked the police to investigate Mr. Yoon and others involved in the declaration of martial law on charges of insurgency. If convicted on those charges, the person the court considers the mastermind of the insurgency faces either the death penalty or life imprisonment.

Mr. Yoon has made no public appearances since withdrawing his martial law decree. A survey by Gallup Korea showed Mr. Yoon’s approval ratings plummeted to a record low of 13 percent in the wake of his martial law decision, dragging down his party’s popularity as well.

The impeachment of Mr. Yoon would strengthen the position of the opposition, which has long accused him of incompetence and abuse of power. But it would thrust his already divided party and Mr. Han’s own political future into deeper uncertainty. Political commentators have said the relationship between Mr. Yoon and Mr. Han has irreparably soured in recent months.

Both Mr. Yoon and Mr. Han were star prosecutors before entering politics. Mr. Yoon appointed Mr. Han as his justice minister and helped make him the chairman of the P.P.P. But Mr. Han, once the most trusted ally of Mr. Yoon, has become one of his most bitter enemies since he started bickering with the president over how to handle allegations of corruption involving the first lady, Kim Keon Hee. Recently, they have often ignored each other even when they sat nearby in public events.

The collection of rebel groups fighting to depose President Bashar al-Assad of Syria pushed farther south on Friday toward a major city en route to the capital, as the government’s chief patron, Iran, moved to evacuate military commanders and other personnel from the country.

The rebels’ stunningly rapid gains spread alarm to neighboring countries, prompting border closures to guard against the prospect of further chaos as Mr. al-Assad’s authoritarian government lost more of its grip over swaths of the country.

And in another sign of the government’s loosening control, a Kurdish-led force backed by the United States, which is separate from the rebels advancing on Homs, said it had deployed in the eastern city of Deir al-Zour, which the government had previously held.

Taken together, the battlefield gains present the most serious challenge in years to Mr. al-Assad’s power.

But perhaps most significant was the withdrawal of Iranian personnel after more than a decade of staunch support for Mr. al-Assad. Those evacuated included top commanders of Iran’s powerful Quds Forces, the external branch of the Revolutionary Guards Corps, according to Iranian and regional officials.

Evacuations were ordered at the Iranian Embassy in Damascus, and at bases of the Revolutionary Guards, Iranian and regional officials said. Iranians began to leave Syria early Friday, the officials said, heading toward Lebanon and Iraq.

U.S. officials, speaking on condition of anonymity, said Damascus could soon be under threat. A senior State Department official said Mr. al-Assad needed ground forces and that Iran would be hesitant to provide any.

The U.S. Embassy in Damascus on Friday urged Americans “to depart Syria now while commercial options remain available in Damascus.”

“The bottom line,” said Mehdi Rahmati, a prominent Iranian analyst, “is that Iran has realized that it cannot manage the situation in Syria right now with any military operation and this option is off the table.”

Despite being largely overshadowed by the wars in Gaza and Lebanon, the Syrian civil war never ended and instead fell into a protracted stalemate. Diplomatic efforts to find a political solution have been stagnant for years.

The coalition of advancing rebels is led by Hayat Tahrir al-Sham, a group previously affiliated with Al Qaeda. Although it split with Al Qaeda in 2016 and has attempted to gain international legitimacy, it is still designated as a terrorist organization by the United States and the United Nations.

The group’s leader, Abu Mohammad al-Jolani, said in an interview with The New York Times this week that his goal was to “liberate Syria from this oppressive regime.”

The reactions on Friday of Syria’s neighbors appeared to reflect the deep concerns about the spiraling and unpredictable war.

Lebanon announced on Friday that it was closing all land borders with Syria except for one that links Beirut with Damascus. Israel said it would reinforce “aerial and ground forces” in the Golan Heights, which Israel seized from Syria after the Arab-Israeli War of 1967.

Jordan closed a border crossing with Syria on Friday after insurgents captured the area on the Syrian side, Jordan’s Interior Ministry said in a statement.

And beyond the main rebel advances, the Assad government appeared to be losing other pockets of territory. The Syrian Observatory for Human Rights, a British-based war monitoring group, said that the city of Sweida, south of Damascus, was no longer under government control.

The Damascus embassy of Russia — one of Syria’s most important allies — issued a statement warning Russians of “the difficult military and political situation in Syria.” The embassy reminded Russians “of the opportunity of leaving the country on commercial flights through operating airports.”

The sudden rebel advance, launched last week, has abruptly changed the landscape of the 13-year civil war.

The rebels have swept through major cities like Aleppo and Hama and captured a significant amount of territory across four provinces, while government forces seemed to put up little resistance.

U.S. officials have been surprised by the rebels’ progress so far, and had not assessed that the Assad government’s control on Aleppo was so weak. They said the rebels appear to have quickly taken advantage of the chaos created by their offensive, whose success likely exceeded their own expectations.

If the rebels gain control of Homs, it would be a significant turn in fortunes for Mr. al-Assad. The city sits at the crossroads of major highways, including one that leads to Damascus. Without Homs, the government would lose a key buffer between rebel-held areas in the northwest and Damascus farther south.

Early in the civil war, parts of Homs fell to the rebels. Over the years, government forces besieged and bombarded rebel-held areas to wrest them back, devastating parts of the city.

The opposition fighters heading toward Homs on Friday came out of Hama, the city they breached just a day earlier.

While the rebels say their goal is to oust Mr. al-Assad, it is not clear what would happen if he were to fall. Many in the international community had come to grudgingly accept him as Syria’s leader, even after he violently crushed his country’s opposition and used internationally banned chemical weapons.

For them, Mr. al-Assad offered a semblance of control, while a rebel takeover threatened more uncertainties in a region already in upheaval. Some Arab states last year normalized diplomatic relations with the Assad regime after shunning his government for years.

Publicly, American officials have been cautious about Hayat Tahrir al-Sham. But inside the U.S. government, some officials said they believe the group’s turn to a more pragmatic approach was genuine, adding that its leaders know they cannot realize aspirations to join or lead the Syrian government if the group is seen as a jihadist organization.

The group has retained its identity as a conservative Islamist organization, but it has shown itself to be pragmatic while governing Idlib, in northwestern Syria, U.S. officials said. In areas it has taken over, officials said, it has quickly focused on providing services to civilians.

In a briefing for reporters this week, Mouaz Moustafa, the executive director of the Syrian Emergency Task Force, an opposition group that works with the U.S. military, praised the group for turning on Aleppo’s electricity soon after it occupied the city.

Mr. Moustafa said it was wrong to tie the group to its jihadist roots, insisting that it was not affiliated with terror groups. He urged the United States to give its full-throated support to the push against Mr. al-Assad, saying the offensive was in U.S. national interests.

“Everyone should see this as Iran losing in Syria,” he said.

The rebel advance struck at a moment of weakness for Mr. al-Assad’s allies: Iran’s power has been curtailed by its conflict with Israel, and Russia’s military sapped by its invasion of Ukraine.

The Syrian government has relied on those countries and the Lebanese militant group Hezbollah — now battered by its own war with Israel — to fight the rebels.

Russian airstrikes to try to slow the recent rebel advance have been relatively sparse, in what analysts see as a sign of Russia’s limited ability to aid Mr. al-Assad.

The Syrian Observatory said Friday that a Russian convoy had fled Deir al-Zour, the eastern city where Kurdish forces were advancing, and was headed to Damascus.

On Friday, the Syrian military struck rebels and their vehicles both north and south of Hama with artillery, missiles and airstrikes, assisted by Russia. Dozens of opposition fighters were killed and wounded, according to Syrian state media.

The Syrian military, after withdrawing from Hama on Thursday following several days of fighting, issued an unusual statement explaining its pullback, saying that it was seeking to avoid battles that would endanger civilians.

But sacrificing Hama also enables the Assad government to shift its limited military forces to areas it regards as more important, such as Homs.

President Recep Tayyip Erdogan of Turkey, who has backed some of the Syrian opposition, including rebel factions in the current offensive, said on Friday that he had been frustrated with Mr. al-Assad over his unwillingness to negotiate over Syria’s future. He issued a qualified approval of the rebel advance.

“Idlib, Hama, Homs, and the target, of course, is Damascus,” Mr. Erdogan told reporters following Friday prayers in Istanbul, according to Turkish state media. “The opposition’s march continues. Our wish is that this march in Syria continues without incident.”

But Mr. Erdogan also seemed to express concern about the rebel advances, calling them “problematic,” according to the Reuters news agency.

“These problematic advances continuing as a whole in the region are not in a manner we desire, our heart does not want these,” he said. “Unfortunately, the region is in a bind.”

Aaron Boxerman in Jerusalem, Ronen Bergman in Tel Aviv, Anton Troianovski in Berlin, Jacob Roubai in Beirut and Julian Barnes, Eric Schmitt and Adam Entous in Washington contributed reporting.

Geopolitics can often be best understood as an equilibrium — of expectations, of strength and of capabilities. When something upsets that balance, it can lead to chaos.

That’s been a frequent theme of this column over the last few months, and the examples keep adding up. It’s now clear that a number of “frozen conflicts” — wars that had become stuck in stalemates for years because neither side was strong enough to make substantial gains — have suddenly heated up.

Five years ago, there were frozen conflicts in eastern Ukraine, Gaza and Lebanon. All have since flared into open conflicts that shattered the status quos.

These conflicts are all different, with their own separate causes. But they also share many of the same players, which means that ripple effects of events like Russia’s invasion of Ukraine and Hamas’s attack on Israel are now being felt around the world.

The most notable recent example is Syria, where the frozen civil war reignited in dramatic fashion when the rebel group Hayat Tahrir al-Sham launched a surprise attack last week, capturing a large swath of territory, as seen in this map by my colleague Samuel Granados:

The rebels have taken Aleppo, Syria’s second-largest city, and Hama, a longtime government stronghold. They are pushing south toward the major city of Homs, and could even threaten the capital of Damascus.

What upended the equilibrium in Syria? And does it share any factors with other frozen conflicts that have thawed, with bloody results, in the last few years?

Distant ripples lead to big changes

First, let’s define what we’re talking about here. Conflicts freeze because they come into equilibrium, with no side strong enough to win or weak enough to lose. Violence may not cease entirely, with isolated incidents of airstrikes and other clashes, but the front lines and strategic calculus do not change much.

Any material change in global affairs — even far from the conflict itself — can throw things off balance, and turn protracted stalemates into all-out war. The repercussions can rapidly ricochet around the globe.

Russia and Ukraine were arguably in a frozen conflict before 2022: Russia had annexed Crimea and backed separatists in Eastern Ukraine, who had fought to a standstill against the Ukrainian military. That freeze ended when Russia invaded Ukraine — with ripples that are still being felt around the world.

When Hamas carried out its bloody attack on Israel on Oct. 7, 2023, that not only triggered a devastating war with Israel, but also led to yet another war between Israel and Hezbollah in Lebanon. It brought the covert war between Israel and Iran — which backs Hamas and Hezbollah — out into the open. And it indirectly contributed to the rebel offensive in Syria, because the Assad government also relied on support from Hezbollah and Iran.

In Syria, changes to the equilibrium have played out slowly and by degrees — until last week, when the Hayat Tahrir al-Sham rebels began their lightning offensive.

Russia has supported the government of Bashar al-Assad since 2015, and maintains a major military presence in Syria.

But since its invasion of Ukraine, Russia has withdrawn a lot of its forces, including special operations soldiers that were supporting the Syrian army, said Nicole Grajewski, a researcher at the Harvard Kennedy School and the author of the forthcoming book “Russia and Iran: Partners in Defiance from Syria to Ukraine.” That drawdown was compounded by the recent weakening of Hezbollah, the Iranian-backed militant group, which had provided a lot of the ground support for the Russian air campaigns in Syria, Grajewski said.

Over the last few months, Israel has wiped out much of Hezbollah’s leadership and severely degraded its military capability in Lebanon — a war triggered by the Oct. 7, 2023 attacks led by Hamas, an ally of Hezbollah within Iran’s ‘Axis of Resistance.’ Israel also targeted Hezbollah forces in Syria, leaving them weaker there.

Last week, when Hayat Tahrir al-Sham began its offensive, Syrian defenses collapsed immediately, allowing the rebel group to seize the cities of Aleppo and Hama in a matter of days.

In the background, a deeper shift

But there is also another shift, harder to measure but perhaps ultimately more important, that may have played a role in thawing these conflicts: the weakening of longstanding norms of the post-Cold War order.

Since World War II, and particularly since the end of the Cold War, there have been strong international norms discouraging the use of military force, especially on foreign soil, and of respect for the borders of sovereign states.

There are many exceptions, including the U.S. invasion of Iraq in 2003. But there is increasing evidence that those norms have started to crumble, and the world has changed yet again.

Ukraine is an instructive example. When Russia annexed Crimea in 2014, it hid the fact that the armed men who seized the territory were Russian troops, a subterfuge that paid lip service to the prevailing norms even if it fooled no one. By 2022, there was no such illusion: The Russian military openly invaded and attempted to annex a sovereign state.

“There’s clearly no longer a feeling of deterrence,” said Thomas de Waal, a senior fellow at Carnegie Europe who co-authored a book on frozen conflicts. “So I think we’re living in a more Darwinian world.”

The embers of Notre-Dame Cathedral in Paris were still hot when President Emmanuel Macron vowed that France would rebuild it to be “more beautiful than ever.”

Then the French leader gave a deadline so ambitious it took many aback: “I want this to be finished in five years,” Mr. Macron said.

It was April 2019, and flames had just torn through the 860-year-old Gothic monument, obliterating its ancient wood and lead roofing and sending the tip of its spire crashing through the stone vaults below. Some called the deadline feasible. Others said it was wildly unrealistic.

Now, after five and a half years and about $900 million in donations, France is on the verge of success.

Renovations to the cathedral’s exterior will continue, but the bulk of what was destroyed has been restored. Excitement is mounting just days before Notre-Dame reopens for millions of tourists and pilgrims.

The tight deadline was “necessary,” Philippe Jost, the head of the reconstruction task force, told us during a tour of the work in progress in June. The daunting goal served to unite about 250 companies and 2,000 workers and artisans from all over France who knew the world was watching, and drove them to give their all for the project of a lifetime.

The day of the tour, after taking a clanging elevator to the top of a maze of scaffolding, we saw roofers installing new lead sheets and crested ornaments on the roof, whistling, drilling, hammering and soldering as birds squawked above. Perched on the new spire were a cross and a gilded copper rooster that contains relics of saints and now, a scroll naming all of the restoration workers.

Those workers overcame Covid-19 lockdowns, toiled under stringent safety measures to avoid exposure to toxic lead dust, and coped with the death last year of Mr. Jost’s predecessor, Gen. Jean-Louis Georgelin, who prided himself on keeping the project on track.

“At the beginning — especially in the beginning — most people didn’t think that it was possible,” Mr. Jost said. “And he was very clear in his head, and with others, that we will do it.”

Stabilizing the cathedral

In 2019, when the flames were finally extinguished just before midnight after a five-hour battle, emergency workers were able to enter the cathedral and assess the damage. They found reason for both heartbreak and relief.

The bell towers, stained-glass rose windows and precious artworks were mostly intact. But the structure had come dangerously close to collapsing.

Notre-Dame’s limestone, scorched and then drenched by tens of thousands of gallons of water from firefighters, was coated with ash and lead dust. Gables and statues were threatening to fall. The building had to be secured before any repairs could proceed, a process that ended in 2021.

“Our teams had to intervene within a very short time-frame, but with infinite precaution,” said Julien Le Bras, the chief executive of Le Bras Frères. His company, which specializes in restoring historical monuments, was working on Notre-Dame when the fire hit, and helped scramble to stabilize it. “It really was a very delicate piece of engineering work,” he said.

Rope workers pulled tarpaulin coverings across gaping holes to shield the interior from rain; later, a permanent sliding “umbrella” was created.

Sensors placed throughout the cathedral monitored every structural shift. A forest of scaffolding over 80 feet high was assembled inside, allowing architects to assess normally inaccessible sections and helping prop up weakened vaults with wooden arches.

Workers also consolidated Notre-Dame’s 28 flying buttresses, a Middle Ages innovation that allowed for thinner walls and taller cathedrals. With the weight of the roof mostly gone, the buttresses were bolstered with eight-ton arches to prevent the walls from collapsing.

One of the biggest concerns was the 220-ton tangle of 40,000 steel scaffolding tubes that the flames had fused together — remnants of renovation on the spire that predated the fire and that were now threatening the structure. They had to be stabilized, then painstakingly dismantled or cut up and removed by crane.

A deep cleaning

Notre-Dame was a mess.

Remote-controlled robots pulled out charred beams, fallen stones and other fragments that were carefully sorted and classified because they had archaeological and scientific value or could be reused.

Inside, workers removed dust and centuries of accumulated grime using high-powered vacuums, a special strip-away latex coating and damp sponges, returning hundreds of thousands of square feet of limestone to its original brilliance.

Restorers spruced up stained-glass windows, fixed railings, and cleared grit from murals and painted decorations in the choir and nave chapels, revealing vivid pigments and gilding.

“We were able to witness all the other craftsmen at work,” Charlotte Phelouzat, who worked with 13 other independent painting restorers, said after showing a newly cleaned depiction of St. George slaying the dragon, as the noise of buzz saws and beeping forklifts echoed off the cathedral’s brightly lit stonework.

“It was quite exceptional,” she said.

The cathedral’s great organ — one of the largest in France, with over 100 stops and about 8,000 pipes — was not burned or damaged by water, but it did have to be cleansed of lead dust.

Parts of the organ that were too big or too fragile to be moved were cleaned or replaced on site; the rest was dismantled and sent to three workshops in the Hérault, Corrèze and Vaucluse areas of southern France, where restorers carefully dusted pipes, cleaned windchests that control the organ’s air flow, and redid its electric and pneumatic transmission system.

Once the organ was reassembled, specialists harmonized it at nighttime — which proved challenging when scaffolding was altering the acoustics.

“It was very meticulous work,” said Bertrand Cattiaux, a retired organ specialist who had worked on previous restorations of the organ and who contributed to this one with his successor. “Hours of very small, very sensitive gestures that can considerably change the sound.”

Finding oaks and limestone

Suggestions that Notre-Dame should get a modern update were quickly shelved. In a country where preserving architectural heritage and centuries-old skills is paramount, the idea never gained popular traction. After all, many argued, the cathedral’s stone and woodwork had stood the test of time for over 800 years.

So using the right material was critical.

“At the end of the day, we are as close as possible to what existed before the fire,” said Jean-Louis Bidet, the technical director of Ateliers Perrault, a company in the Loire Valley that specializes in historical monuments, after ducking into the new wooden attic his company had helped build.

Some doubted that France had the right trees or enough skilled carpenters to rebuild the medieval attic — a lattice of ancient beams known as “the forest,” made primarily of rows of giant triangular trusses — and the 19th-century spire, a complex assembly of about 1,000 oak pieces that culminates more than 300 feet above the ground.

Those fears proved unfounded.

Over two thousand oak trees were donated by private and public forests across France, especially in the east and north. The National Forests Office and France Bois Forêt, a timber trade group, coordinated the selection process, using original 19th-century architectural plans and a trove of archaeological and digital data.

Walking around forests, “Notre-Dame was on everybody’s mind,” said Michel Druilhe, the president of France Bois Forêt from 2018 to 2021. “We looked at the trees and worked with architects to see how they could be used, how they could be placed, how they should be sawed.”

Logs — some over 60 feet long — were transported to sawmills around France, cut into beams, dried naturally to lose humidity and sap, then sent to carpentry workshops.

For the stonework, scientists and engineers studied blocks recovered from the cathedral and identified suitable limestone quarries to provide replacements. Over 45,000 cubic feet of stone were sourced from quarries in the Oise and Aisne areas, north of Paris, to rebuild collapsed vaults, plug walls and adorn gables with new sculpture.

Skills from across France

The renovation tapped a constellation of workshops, craftsmen, and companies around the country who combined ancestral techniques with modern engineering.

“If we are able to succeed and move forward, it’s because the skills are there,” said Mr. Jost, the head of the reconstruction task force. “You can’t rebuild a cathedral with good feelings alone.”

Instead of entrusting it all to one company and its subcontractors, the task force split the project into more than 140 lots and opened up bids for each one. Some of the 250 businesses involved were nationally recognized; others were tiny workshops with niche expertise. Often they joined forces.

The apse cross, for instance, was restored in rural Normandy by Fer Art Forge, a small metalworking shop on an old farm, in coordination with UTB, a construction and renovation firm of over 1,000 employees that did the choir covering.

“No matter the size of the company, it’s people’s skills that count,” Julien Soccard, the operations manager for UTB, said in May at the workshop, as cows grazed in apple orchards near the 40-foot cross — an ornate mix of steel, lead, brass and copper that fell during the fire.

Attention to detail was deep. Carpenters who hand-hewed beams used 60 or so long-handled axes and broadaxes that were forged manually in the eastern Alsace region with detailed specifications, down to the kind of markings the blades had to leave.

“We were struck by the emotion of carpenters bonding with their axes,” said Soumia Luquet, who runs the forge with her husband. “That is the epitome for a blacksmith.”

Her workshop and four others assembled for an “unprecedented challenge,” she said. It was a rare opportunity to show their skills. Over four months, 10 or so ax makers slept and ate in her home and toiled at the forge, welding steel and puddled iron together at over 1,800 degrees Fahrenheit.

“This profession disappeared almost entirely during the 20th century with industrialization,” said Martin Claudel, one of the ax-makers, who trekked back and forth from his own workshop in Brittany. “Notre-Dame shined a big spotlight on it.”

It all came together in Paris

Notre-Dame buzzed with increasing intensity as the deadline approached and reconstruction started in earnest. Many companies were used to working on historical monuments, but not simultaneously on a cramped island in the heart of the French capital.

The choreography was complex, requiring years of advance planning to ensure that the long list of those involved — architects, engineers, carpenters, masons, steeplejacks, archaeologists, crane operators, roofers and more — did not step on one another’s toes or cause delays.

At the peak, 600 workers were on site daily, entering through locker rooms with two sides connected by showers, leaving street clothes at one end and putting on construction garb at the other to prevent lead contamination.

“The space was small, the deadline was tight, and there were exceptional security requirements,” said Mr. Le Bras, whose company also handled most of the scaffolding and, with three others — Cruard Charpente, Métiers du Bois and Asselin — worked on the spire. “But everyone involved on the project, without exception, was passionate above all.”

“It’s almost more of a mission than a construction project,” he said.

Much of the cathedral’s new wooden roofing was dry-fitted — in Lorraine for the spire, in Normandy for the nave, and in the Loire Valley for the choir — then brought to Paris, reassembled section by section on giant platforms, and hoisted up by crane.

Workers also installed fire protections that were lacking in 2019, including misting devices, massive firewalls, thermal cameras and thicker roof boards that burn more slowly. A recovery system to treat rainwater running off the lead roof before it enters the sewers will also be tested.

Collapsed vaults were rebuilt, stones weakened by the scorching blaze were replaced, and a new checkered marble platform was installed for the altar.

On our visit, we were led onto temporary walkways though the new “forest,” where we saw sawdust-coated white tarps stretched across the vaults and electric cables snaking along the pathways

But one day soon, Notre-Dame will again be a working cathedral, not a construction site, and architectural features like the forest will recede into hiding.

The fire and the reconstruction that followed afforded Parisians a rare, if distressing, firsthand glimpse into the heart of a beloved landmark. So last year, when gigantic solid-oak trusses were delivered by barge and lifted by crane, a crowd quickly gathered on the banks of the Seine to watch.

Loïc Baril, 75, rushed over as soon as he heard the news, though the summer heat was blazing.

“We’ll only see this once in our lifetime,” he explained.

Produced by Josephine Sedgwick and Tala Safie