President Trump’s self-proclaimed “liberation day,” in which he announced across-the-board tariffs on the United States’ trading partners, carries an echo of another moment when an advanced Western economy threw up walls around itself.

Like Brexit, Britain’s fateful vote nearly nine years ago to leave the European Union, Mr. Trump’s tariffs struck a hammer blow at the established order. Pulling the United States out of the global economy is not unlike Britain’s withdrawing from a Europe-wide trading bloc, and in the view of Brexiteers, a comparable act of liberation.

The shock of Mr. Trump’s move is reverberating even more widely, given the larger size of the American economy and its place at the fulcrum of global commerce. Yet as with Brexit, its ultimate impact is unsettled: Mr. Trump could yet reverse himself, chastened by plummeting markets or mollified by one-off deals.

More important, economists say, the rise of free trade may be irreversible, its benefits so powerful that the rest of the world finds a way to keep the system going, even without its central player. For all of the setbacks to trade liberalization, and the grievances expressed in Mr. Trump’s actions, the barriers have kept falling.

The European Union, optimists point out, did not unravel after Britain’s departure. These days, the political talk in London is about ways in which Britain can draw closer to its European neighbors. Still, that sense of possibility has come only after years of turbulence. Economists expect similar chaos to buffet the global trading system as a result of Mr. Trump’s theatrical exit.

“It will not be the end of free trade, but it is certainly a retreat from unfettered free trade, which is the way the world seemed to be going,” said Eswar S. Prasad, a professor of trade policy at Cornell University. “Logically, this would be a time when the rest of the world bands together to promote free trade among themselves,” he said. “The reality is, it’s going to be every country for itself.”

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As entrances go, this one was both unexpected and welcome. That much was clear from the thunderous applause and cheers on Sunday as Pope Francis made his first public appearance since leaving a Rome hospital two weeks ago.

Francis arrived, unannounced, on the dais in St. Peter’s Square near the end of a Mass that was part of a pilgrimage by health care workers and their patients.

The pope said very few words, his voice still strained after a six-week hospital stay for pneumonia and other problems that his doctors said twice brought him close to death.

“Happy Sunday to everyone, many thanks,” Francis said, waving his hands. A nasal cannula was visible in both nostrils; Francis continues to rely on supplemental oxygen, the Vatican has said.

Sitting on a wheelchair pushed by his trusted nurse, Massimiliano Strappetti, Francis moved through groups of pilgrims — including dozens of doctors and nurses from around the world — who had come the Vatican on Sunday for this weekend’s Jubilee of the Sick and Health Care Workers.

“What a wonderful surprise — the pope still has health problems but he wanted to give a gift to the faithful,” said Lamberto Rosa, a businessman who volunteers with the Order of Malta at some Vatican events. “He has a fighting spirit and wants to be present.”

Britain and Israel traded sharp criticisms this weekend after Israel blocked two British lawmakers from entering the country and sent them back to London.

The members of Parliament, Abtisam Mohamed and Yuan Yang, both of whom belong to the governing center-left Labour Party, said in a joint statement on Sunday that they had been “astounded” to be denied entry to Israel the previous day. They described their trip as “an M.P.s’ delegation” to the Israeli-occupied West Bank alongside charities “to visit humanitarian aid projects and communities.”

“We are two, out of scores of M.P.s, who have spoken out in Parliament in recent months on the Israel-Palestine conflict and the importance of complying with international humanitarian law,” they said, adding that lawmakers “should feel free to speak truthfully in the House of Commons, without fear of being targeted.”

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The mischievous posters began appearing all over London in the past two months.

On the side of an East London bus stop, one of them shows Elon Musk, the world’s richest man, emerging from a Tesla’s roof with his hand pointing upward in a straight-armed salute. “Goes from 0 to 1939 in 3 seconds,” the ad reads. “Tesla. The Swasticar.”

Another mock ad shows Mr. Musk and President Trump in front of a red Tesla with the words: “Now With White Power Steering.” In North London, a fake movie billboard blares: “The Fast and the Führer,” with a picture of Mr. Musk saluting beside a Tesla with a DOGE license plate, a reference to the budget-slashing federal agency he currently leads on behalf of Mr. Trump.

“Parental Guidance,” warns the billboard, put up by a group calling itself Overthrow Musk. “Tesla’s CEO is a far-right activist. Don’t give him your money.”

Across the British capital and in several European cities, Mr. Musk’s signature business has become the target of the same kind of political anger that has fueled vandalism of Tesla cars in the United States and sometimes violent protests at his dealerships.

There have been some instances of unruly protests and vandalism in Europe. But much of the anti-Musk sentiment has taken the form of political satire, of the kind that has flourished in Britain since at least the 18th century.

Just outside Berlin, a group called the Center for Political Beauty used high-power lights to project the word “Heil” onto the side of a Tesla factory so that it read “Heil Tesla,” along with a picture of Mr. Musk saluting during a speech in Washington. In Italy, street art depicts Elon Musk taking off a mask to show Adolf Hitler’s face underneath. The words “Elon Mask” appear above the picture.

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Carrying his 6-year-old daughter on his shoulders, Pierre Poilievre, the Conservative Party leader running to lead Canada, was wading through a crowd of well-wishers in his electoral district in Ottawa.

“Hey, there he is, Mr. Alexander!” he said, gripping the hand of Mark Alexander, whose dairy farm the politician had visited 21 years ago on his first run for office.

Mr. Poilievre recalled sitting at the farmer’s kitchen table. He recalled, too, that Mr. Alexander did not get his Conservative membership card because “the party messed up.”

“Then I came and milked cows with you,” Mr. Poilievre went on, as the farmer’s wife, Lynn, chimed in, “Yes, you did!”

After asking about four family members — by name — Mr. Poilievre moved on. “He really cares about the people,” said Ms. Alexander, who was planning to vote for Mr. Poilievre. “And he has an amazing memory. Like he’ll remember those very little details of the first time we met.”

Mr. Poilievre, 45 — who is aiming to defeat Prime Minister Mark Carney, 60, and end a decade of Liberal rule in Canada’s general election on April 28 — is considered one of Canada’s most dexterous campaigners, communicators and politicians.

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Russia bombarded Ukraine with ballistic missiles and drones on Sunday that killed one person and wounded at least seven others, the latest in a series of deadly attacks that President Volodymyr Zelensky of Ukraine has said makes clear Moscow has little real interest in cease-fire negotiations.

While Russian drone and missile attacks have been unrelenting throughout more than three years of war, they have intensified in recent weeks amid ongoing peace talks led by the Trump administration.

The Ukrainian authorities said the barrage on Sunday killed one man, damaged buildings and started fires in three neighborhoods of Kyiv, the capital. Damage and injuries were also reported elsewhere in Ukraine, as the country declared a day of mourning for a deadly strike on Friday in the city of Kryvyi Rih in central Ukraine.

A missile strike on a residential neighborhood there killed 19 people, including nine children, and wounded 74 others. It damaged the courtyard of an apartment block, and emergency medical workers found some of the wounded in a playground, videos released by Ukraine’s emergency services showed. Russia’s ministry of defense said the missile hit a gathering of Ukrainian and foreign military personnel.

But a U.N. team that visited the site said, citing witnesses, that a meeting of beauticians, not military personnel, had been underway at a nearby restaurant when the missile struck.

Most of the children died while playing in a park, the United Nations said in a statement by the High Commissioner for Human Rights, Volker Turk. It was the deadliest single strike for children of the war, said Mr. Turk, who called the attack with a cluster munition warhead “an unimaginable horror.”

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More than 100 chemical weapons sites are suspected to remain in Syria, left behind after the fall of the longtime president, Bashar al-Assad, according to the leading international organization that tracks these weapons.

That number is the first estimate of its kind as the group, the Organization for the Prohibition of Chemical Weapons, seeks to enter Syria to assess what remains of Mr. al-Assad’s notorious military program. The figure is far higher than any that Mr. al-Assad has ever acknowledged.

The sites are suspected to have been involved in the research, manufacturing and storage of chemical weapons. Mr. al-Assad used weapons like sarin and chlorine gas against rebel fighters and Syrian civilians during more than a decade of civil war.

The number of sites, and whether they are secured, has been a mystery since rebels toppled Mr. al-Assad last year. Now, the chemicals represent a major test for the caretaker government, which is led by the group Hayat Tahrir al-Sham. The group is designated as a terrorist organization by the United States, but it has renounced its links to Al Qaeda.

The stakes are high because of how deadly the weapons are, particularly when used in densely populated areas. Sarin, a nerve agent, can kill within minutes. Chlorine and mustard gas, weapons made infamous in World War I, burn the eyes and skin and fill the lungs with fluid, seemingly drowning people on land.

Vietnam’s top leader, To Lam, has asked President Trump to delay the imposition of tariffs for at least 45 days so the two sides can avert a move that would devastate the Vietnamese economy and raise prices for American consumers.

The 46 percent tariff rate the United States has said it will impose on Vietnam is among the highest any country faces. The prospect of such a steep tariff has left Vietnam with a sense of whiplash and deep apprehension. It also presents a sharp contrast to Washington’s recent embrace of Hanoi as an important bulwark against China and a manufacturing destination for many American apparel brands.

Mr. Lam’s proposal to President Trump was laid out in a letter dated Saturday, according to a copy obtained by The New York Times. In the letter, Mr. Lam called on Mr. Trump to appoint a U.S. representative to lead negotiations with Ho Duc Phoc, a Vietnamese deputy prime minister, “with the goal of reaching an agreement as soon as possible.”

Mr. Lam had been one of the first world leaders to reach out to Mr. Trump after the tariffs were announced. In a phone call, he offered to reduce tariffs on U.S. imports to zero, and urged Mr. Trump to do the same, according to the Vietnamese government. Vietnam has said its tariffs on U.S. goods is an average of 9.4 percent.

Mr. Trump later described the call as “very productive.”

In his letter, Mr. Lam asked Mr. Trump to meet him in person in Washington at the end of May “to jointly come to an agreement on this important matter, for the benefit of both our peoples and to contribute to peace, stability and development in the region and the world.”

Vietnam’s Foreign Ministry did not respond to a request for comment.

Vietnam, which faces punishingly high tariffs along with China, Cambodia and Laos, would be the hardest-hit economy in Asia if the tariffs are imposed as planned on Wednesday, according to economists. The United States is Vietnam’s largest export market, accounting for about 30 percent of the country’s total exports. A 46 percent tariff rate would put 5.5 percent of Vietnam’s gross domestic product at risk, according to ING, a Dutch financial services company.

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