President Trump has never had much love for the NATO alliance, which he thinks is overdependent on American largess, and in his first term, talked about abandoning the collective defense pact.

In his second term, Mr. Trump and his senior officials have made it clear that the security of Europe is no longer the first priority of the United States, which wants to concentrate resources on its own border and the Indo-Pacific, where China has become a peer rival.

On Thursday, Mr. Trump suggested that the United States might not protect NATO members that he believed were not paying enough for their own defense, calling it “common sense.”

But what would Europeans need to do to replace the enormous American contribution to NATO?

The answer comes down to money, personnel, time and cooperation with Washington, said Ivo Daalder, a former American ambassador to NATO and a coauthor of a recent report from Harvard’s Belfer Center on how to create “a strong European pillar” in the alliance.

The central problem is that NATO was built as an American-dominated alliance, intentionally dependent on American leadership, sophisticated weaponry, intelligence and airlift. The current NATO command structure is essentially owned and operated by the United States, led by Gen. Christopher G. Cavoli.

“The United States is the linchpin of the alliance so we could control our allies and get them to do what we want,” Mr. Daalder said. More practically, the United States military is the skeleton of NATO, and “if you suddenly pull out the skeleton, the body dies.”

Money

Relative to other challenges, money is the easiest part of the European quandary. The question, as ever, is political will and commitment to spending larger sums — and the trade-offs and political costs it will entail.

Prime Minister Donald Tusk of Poland put it simply last week, saying: “500 million Europeans ask 300 million Americans to protect them from 140 million Russians.” What Europe lacks, he said, is “the belief that we are truly a global force.”

Friedrich Merz, who is on course to be the next chancellor of Germany, offered a bold response this week to the new pressures on Europe, proposing to spend nearly 1 trillion euros, or $1.07 trillion, on the military and infrastructure over the next 10 years.

Britain, Belgium, Poland and Denmark have also recently said they would spend more. On Thursday, E.U. leaders agreed to boost military spending outside normal debt limits. But overall, European nations remain far shy of the spending that experts say they will need to replace the American commitment.

Those estimates vary, but could mean a hike of €250 billion a year, or about 1.5 percent of the European Union’s gross domestic product, according to a study from two research institutions, Bruegel and the Kiel Institute for the World Economy.

The study recommends that European nations spend at least 3.5 percent of G.D.P. per year on the military; currently just five of 32 NATO members, including the United States, spend above 3 percent.

The Europeans possess a lot, but also need a lot more sophisticated weaponry, now largely provided by the United States. The biggest gaps are in integrated air and missile defense and long-range precision strike capability, said Ben Hodges, a former commander of the U.S. Army in Europe.

Europe also lacks “strategic enablers,” including transport aircraft, sophisticated drones and satellites — crucial systems for intelligence, surveillance and reconnaissance.

It has the capacity to produce more of its own weapons, he said. But that would require better coordination among nations to invest in the right industries and purchase jointly.

Ideally, Europe should have sufficient stocks of ammunition and missiles to fight a high-intensity war for at least six months, but those are badly depleted from the war in Ukraine.

A study by the International Institute for Strategic Studies found that even fundamentals like the number of combat battalions and in-service battle tanks have remained static or fallen since 2014, despite Russia’s annexation of Crimea.

Political will matters here, too — to overcome bureaucratic constraints and requirements on manufacturers.

The European Investment Bank is prohibited from providing loans to make weapons, though the European Union is rethinking the rule.

And German law requires that weapons makers have direct orders from the government before production can even start. That makes production for potential future sales impossible even if they have excess capacity now.

Personnel

Right now, there are only about 100,000 U.S. troops in Europe — a number that could fit into the University of Michigan’s football stadium — said Mr. Hodges, who used to command them.

Yet it seems almost impossible that the Europeans, even if they increase military spending, could quickly replace the bulk of them, let alone fight on their own for any extended period.

Of the U.S. troops, 20,000 were sent to Europe after Russia invaded Ukraine three years ago, to shore up NATO deterrence. Analysts expect the Trump administration to pull out those troops before too long.

Another 40,000 of the U.S. troops, Mr. Hodges said, are in Europe on expensive rotations, which Mr. Trump is also likely to want to stop.

There are multiple problems for Europe in replacing such numbers.

Only a handful of European nations still have conscription forces. And attracting the right recruits is hard; pay scales and career prospects are better in the civilian world. Even once soldiers are trained, especially in high-tech warfare or “back office” jobs like engineering or mechanics, it is hard to retain them.

Recent suggestions that Europeans put troops into Ukraine to secure a potential peace deal would put extra strain on personnel, potentially for the long term.

NATO is already pressing members to meet requirements for a new force model. Under that agreement, more than 300,000 troops would need to be available within 30 days to reinforce the alliance’s eastern flank against Russia in the event of a crisis.

For now, there are simply not enough soldiers, logistics specialists and intelligence officers to go around.

“European armies are too small to handle even the arms that they’ve got now,” said Jim Townsend, a former U.S. deputy assistant secretary of defense who is now at the Center for a New American Security.

“The British and the Danes, to pick two examples, are good militaries, but they would not be able to sustain intense combat for more than a couple of weeks,” he said. “It doesn’t matter how good you are if there aren’t enough of you.”

Time

The scale of what the Americans now handle for NATO is too big to replace quickly. To buy or produce the necessary equipment and recruit and train the necessary troops will simply take time.

In normal times, it would take a decade for Europe to catch up, said Camille Grand, a former NATO assistant secretary general, who wrote a detailed report about the problem for the European Council on Foreign Relations last year.

Today’s accelerated sense of urgency might help the Europeans do it a little sooner. But critics argue that Europe has waited far too long to respond to clear signaling from Mr. Trump’s first presidency, let alone to his Russia-friendly comments during the campaign.

American officials from both parties have been urging European allies to do more for their own defense for 50 years, and President Emmanuel Macron of France’s warnings in 2019 about fading American commitment to NATO were heard but largely unheeded.

The Europeans are finally trying to address the money problem. But they cannot magically reduce the time needed to make a transition from American domination of NATO in a way that would not damage their security sufficiently to tempt Russia to test the alliance.

Most importantly, it would require that the United States aid the transition and synchronize its withdrawals with the European buildup.

Cooperation

To move from a U.S.-dominated conventional defense of Europe to a European one could be very dangerous without American cooperation.

A sudden American withdrawal would be tremendously tempting for President Vladimir V. Putin of Russia, who wants to undermine NATO. The United States must be willing to wait to withdraw key capabilities until Europeans are ready to take them over, Mr. Daalder said.

Even in Ukraine, with its 900,000 soldiers helping to pin down the Russian army, a European commitment of even 30,000 to 40,000 peacekeeping troops could undercut NATO’s ability to deter Russia from testing the alliance in the Baltics, for example.

That has led some experts to suggest that a European force in Ukraine should be a NATO force, without U.S. troops on the ground, something Mr. Trump has in any case ruled out. But a NATO force, at least, could use existing NATO assets, like surveillance planes and intelligence capabilities, within the NATO command structure.

Others, like Max Bergmann of the Center for Strategic and International Studies, think it’s time to revisit the concept of a European army, which had long been opposed by the United States.

A unified army, he suggests, would go a long way to ending duplication and making spending more efficient. But who would command such an army, and under what political authority, are difficult questions to answer.

A standing European army, he argues, need not replace the United States in each capacity but could be integrated into NATO and be robust enough to do its main job: to deter Russia from invading member states.

After all, he notes, “Europe on paper has nearly 2 million personnel in uniform and spends roughly $338 billion per year on defense, more than enough to deter Russia and enough to make Europe collectively a military power.”

Violence has erupted in Syria’s coastal region, a longtime bastion of support for Bashar al-Assad, the ousted president.

At least 70 people have been killed in clashes between government security forces and gunmen loyal to the Assad regime. Thousands of protesters have flooded the streets in the first wide-scale demonstrations against the new government. Residents have been ordered to stay indoors as security forces scramble to contain the turmoil.

This unrest is one of the most serious challenges yet to Syria’s new rulers, who swept to power in December after a lightning advance led by Hayat Tahrir al-Sham.

Where are the clashes?

The violence is centered on Syria’s Mediterranean coast, the heartland of the country’s Alawite minority. About 10 percent of Syrians belong to the sect, an offshoot of Shiite Islam. The Assad family, which governed Syria with an iron fist for five decades, are Alawites, and the sect dominated the ruling class and upper ranks of the military.

Since Syria’s new Islamist rulers swept to power, many Alawites have grown unnerved.

Syrians are demanding accountability for crimes committed under the Assad government, and the country’s interim president, Ahmed al-Shara, has pledged to hunt down and prosecute senior regime figures. Mr. al-Shara has promised stability and to safeguard the rights of ordinary Syrians from all sects. But the Alawite-dominated region has experienced low-level violence in recent months, often as a result of security forces trying to arrest former officers.

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Traffic ground to a halt at one of France’s busiest train stations on Friday after an unexploded World War II bomb weighing more than 1,000 pounds was uncovered just north of Paris, causing travel chaos that extended across the English Channel.

The bomb was discovered in the Saint-Denis suburb during overnight work on tracks that lead into the Gare du Nord, a major Parisian transit hub that serves northern France and other parts of Europe, including Britain.

“Traffic has been totally interrupted since this morning to and from Gare du Nord” at the request of the Paris police, France’s national railway company S.N.C.F. said in a statement, adding that trains would start running again once the police completed demining operations.

The bomb was discovered around 3:30 a.m. about a mile and a half from the Gare du Nord, the company said. Workers were landscaping at a bridge renovation site when an earth-moving machine revealed the bomb, which had been buried about six and a half feet underground.

It is about three feet long and includes more than 400 pounds of explosive material, the company said, adding in a travel notice that “extensive earthworks” were necessary to safely defuse it.

Mine-sweeping crews established a large security perimeter, which included sections of a nearby road and a highway that the police closed off.

Traffic on high-speed and commuter railways was stopped for hours. All Eurostar trains that connect Paris to London, Brussels and Amsterdam were canceled, stranding travelers at the Gare du Nord, which served more than 226 million travelers in 2023.

Across the English Channel at London St. Pancras International Station, crowds of bewildered Eurostar passengers were met with a closed barrier. One of them, Katie Jones, was planning to celebrate her 40th birthday at Disneyland Paris. Her plans for an earlier trip to Paris in 2020 had fallen victim to the Covid-19 pandemic that sent much of the world into lockdown.

“I’m scared of flying, and that’s why we chose the Eurostar,” said Ms. Jones, who sobbed against a pillar as she called friends to relay the news. “The irony.”

It is not uncommon in Europe for construction workers to come across unexploded ordnance from the conflicts that tore the continent apart during the 20th century, and the French railway company said that the area in which the latest bomb was found was “well known for its World War II remains.”

Last month, more than 175 practice bombs that had been used for training in the Second World War were found under a children’s playground in northern England. In 2018, thousands of people were evacuated in Berlin after an 1,100-pound World War II-era bomb was uncovered.

“It doesn’t happen every day, but it happens,” said Fabien Villedieu, a leader of Sud-Rail, a union of national railway workers, who shared a picture of the bulky, cylindrical, rock-encrusted bomb on X.

Mr. Villedieu, who said he had received the picture through an internal information-sharing system for railway workers, noted that the unexploded device was found about 600 feet from a busy express road that circles Paris.

“It’s in the middle of an urban zone,” he said.

The local authorities in Saint-Denis said in a statement that nearby houses with windows overlooking the area had been ordered to evacuate during the demining process, which was expected to last about an hour. Six schools and a retirement home were also put on lockdown as a precaution.

Philippe Tabarot, France’s transportation minister, told reporters in Paris that the authorities were taking all the necessary steps to safely remove the bomb and that train traffic would resume as soon as possible.

That was little solace to the passengers at St. Pancras, whose travel plans were turned upside down by a long-forgotten implement of war.

Some took it in stride. Clara Thompson, 52, who is scheduled to run the Paris Half Marathon on Sunday, quickly booked a flight for Friday evening amid the chaos. “It’s an inconvenience, but I get it,” she said. “They found a bomb and they can’t do anything about it.”

But Allison and Jon Heine, who had just arrived on a red-eye flight from Houston, suddenly had three inconsolable children — ages 7, 9 and 13 — on their hands. Their vacation in Paris had been months in the planning.

“It’s not looking good,” Ms. Heine said. “There aren’t any flights. We heard there might be a bus, but it’s eight hours.”

“They’re super disappointed,” she said, pointing to her children. “Tons of tears.”

Rome had spent months prepping up for the Roman Catholic Church Jubilee. It built tunnels, opened up squares and scrubbed moldy fountains. Now thousands of pilgrims were flocking from all over the world for the occasion — a year of penance and forgiveness that takes place every quarter century.

The only thing missing was the pope.

For the past three weeks, Pope Francis, leader of the world’s nearly 1.4 billion Catholics, has been in seclusion on the 10th floor of the Gemelli hospital, where he is being treated for pneumonia and other infections. He has missed his regular Wednesday general audiences, where he greets visitors and pilgrims, and his Sunday Angelus prayer, and on Ash Wednesday he entrusted his homily to a cardinal.

A brief audio recording of the pope faintly giving a blessing in Spanish was all the faithful received on Thursday, as they gathered for a rosary for Francis in St. Peter’s square.

“We were hoping we could see him,” said Dinora Ramirez, a pilgrim from Honduras, who prepared to cross St. Peter’s basilica’s holy door as tears filled her eyes. “Our hearts are aching.”

Doctors speak of a “guarded prognosis” for the pope. They have offered mostly terse reports about his condition, which has alternated between crises and stable moments, leaving ample room for conspiracy theories to flourish about his health and his intentions to resign, and even false reports of his death.

But the one undisputed reality is the pope’s prolonged absence.

It is especially notable for a pope who has made a point of being among the people, frequently venturing into crowds, embracing the faithful and engaging in impromptu conversations. And it is even more deeply felt as thousands of pilgrims have been coming to Rome hoping to catch a glimpse of Francis, and as Roman Catholics prepared for Easter celebrations.

On Ash Wednesday, the first day of Lent, about a dozen cardinals in red biretta caps and shoulder capes walked in a procession among olive trees and umbrella pines along Rome’s Aventine Hill, then entered the Santa Sabina basilica for Mass. As the warm afternoon light filtered through the windows, Pope Francis’ homily — prepared, a Vatican official said, before his illness — echoed in the basilica.

But Francis did not deliver it. An Italian cardinal, Angelo De Donatis, read the pope’s words, imposed ashes on the friars’ balding heads and swung the censer as the choir chanted.

“The pope is not here and you can feel it,” said Mario Maurer, 27, a theology student at the Mass. He had come to Rome on a pilgrimage for the Jubilee.

“There is a cardinal who can read the text, and that’s OK,” Mr. Maurer said. “But the pope is a whole other category.”

Standing outside Santa Sabina as he paused his tour of Rome’s infinite churches, Mr. Maurer said the pope’s absence was palpable not only during the Mass but also in the city, of which he is the bishop. “It’s this void that is here, but also in Rome itself,” he said.

In his sermon, Pope Francis offered a message in keeping with his long-established themes, lamenting “the return of old identity ideologies that theorize the exclusion of others, the exploitation of the earth’s resources, violence in all its forms, and war between peoples.”

For some of the faithful who attended, that, at least, was a consolation.

“He made us feel his presence through the homily, which was in the spirit of Pope Francis, about peace, pollution,” said Giuseppina De Palma, 67.

The Vatican official did not rule out that going forward, Francis might hold off preparing texts if he is aware that he is not going to deliver them, saying it might not make sense to do so.

For some Catholics, the pope’s absence at a moment when the global order is under exceptional pressure was particularly disorienting.

“I wish he would tell us more about how to proceed,” said Luz Viviana Flores Maciel, 21, who is originally from Mexico. “The world is upside down, and we are like a nation without a leader.”

For three weeks, the Vatican has also not shared any photos of Francis.

In the past, before the advent of technology and mass media, the physical appearance of popes was much less familiar, even if that did nothing to diminish their authority. But now it is highly unusual for the world to go for weeks without seeing images of the church leader (although there has been a flurry of AI-generated counterfeits circulating on social media).

To try to feel Francis’ presence, some pilgrims have hiked to the Gemelli hospital to pray beneath his windows. Others have chosen not to dwell on his absence.

“Pope Francis would agree that we should focus on the presence of Jesus, rather than anyone of us,” said Jonah Berger, 24, a Dutch Catholic who attended the Ash Wednesday Mass.