Workers dig the foundations of a wall near the “Jungle” migrants camp along the road leading to the harbor of Calais, France, on Sept. 26. (Sylvain Lefevre/Getty Images)

So far, Donald Trump’s plan to build a wall along the Mexican border is all talk. Last week, France and Britain actually began building one along theirs.

Construction started here on a roughly mile-long concrete barrier intended to separate a sprawling migrant camp from the tunnels that offer passage to Britain, the latest attempt in what has become a global effort to throw physical barriers in the way of historic streams of human ­migration.

From a razor-wire-topped border fence in Hungary to the sealed border of Macedonia and Greece to Trump’s proposed wall , polarized societies across the world are finding that they can unite around keeping others at bay.

The “Great Wall of Calais,” as the project is informally known, is considerably shorter than Trump’s proposed partition of the United States and Mexico, but its message is much the same: Keep out. The concrete wall, which will rise to 13 feet, extends a fence near the sprawling Calais migrant camp known as the “Jungle,” where more than 7,000 migrants have been stranded as they seek to enter Britain by all means possible. The concrete will be specially formulated to make it difficult to scale.

The camp here has become one of the most visible symbols of Europe’s migration crisis: a squalid no man’s land nestled between London and Paris, two of Europe’s wealthiest cities. The victorious British campaign to leave the ­European Union was partly fueled by concerns over immigration, while candidates for France’s presidential election next year are already competing over being tough on migrant flows. Opponents of French President ­François Hollande have tried to turn the camp into a symbol of his weakness.

“It’s something scary, this resignation, this lack of authority,” former French president Nicolas Sarkozy said while visiting Calais last week. Sarkozy’s campaign to recapture the presidency has pulled anti-immigrant rhetoric from the surging far-right National Front party, which has vowed to reestablish controls at France’s borders.

So far, the only sign of the wall is an anonymous, unmarked construction site at the edge of the highway, far from the center of this small, working-class city. Late last week, a small squadron of construction workers laid small foundation slabs into ditches cleared alongside the highway as cars and trucks sped by.

The aim is to keep migrants from stowing away on ferries and trucks. The barrier follows portions of the highway leading into the port, which passes directly in front of the encampment’s entrance. Britain has contributed most of the money for the project, which will cost about $2.5 million, according to leaked reports in the British media. The British Home Office refused to confirm the price tag. In total, London has pledged $22 million to France for assistance with border security. 

British Prime Minister Theresa May has taken Britain’s exit referendum as a mandate to toughen immigration policies, working hand-in-hand with French authorities to try to discourage migrants from pooling in Calais. Both nations have taken an increasingly hard-line approach. The moves come as many European countries have pursued national policies to discourage migration, while campaigning against German-led efforts to share asylum seekers among the 28 nations of the E.U.

The wall will “prevent illegals trying to get to the U.K.,” British Home Secretary Amber Rudd told the House of Commons in September , defending the initiative against critics who called it a waste of money.

In France, the wall’s construction comes at a tense political moment, as the country prepares for the 2017 presidential election. The rise of Marine Le Pen — the outspoken leader of the far-right National Front Party — has pulled the political mainstream toward her agenda, which is firmly opposed to France becoming a haven for migrants and refugees. 

In the aftermath of three devastating terrorist attacks in the past two years, contenders in the forthcoming presidential election are increasingly engaged in a debate focused on immigration and Islam rather than economics or domestic policy. The camp in Calais has become a flash point.

“The attacks have considerably changed the climate in France,” said Bruno Cautrès, a political analyst at the Center for Political Research at Paris’s Sciences Po. “The desire for many is to have a president who can bring security back.”

Local authorities in Calais ­oppose Paris’s decision to construct the wall, preferring instead only the dismantling of the Jungle camp, whose existence has placed considerable strain on the town of 126,000.

“The beginning of the solution begins with the demolition of the Jungle,” said Faustine Maliar, the chief of staff for the mayor of Calais, a Sarkozy ally. “The ­moment the Jungle is destroyed, there is no need for a wall,” she said.

Hollande, who is seeking reelection next year, has vowed to demolish the camp by the end of the year.

“From now on our objectives are clear,” Hollande told reporters in Calais on Monday. “To ­guarantee the security of the people of Calais, maintain public order and ensure that conditions for the migrants and refugees are dignified.” 

Migrants who wish to claim asylum in France will be shipped to other shelters across the country, Hollande said. But because the vast majority of Calais’ migrants want to travel onward to Britain, the plan’s chance for success is unclear. Previous attempts to squeeze the size of the camp have failed.

From a practical perspective, the short, easily defendable French wall — combined with the formidable obstacle of the 22-mile-wide English Channel — may be more successful at preventing passage than any Trump-built wall on the long U.S.-Mexico land border, said Alexander Betts, the director of the Refugee Studies Center at the University of Oxford. But the political symbolism is more important than any practical purpose, he said.

“The wall is an idea that public audiences can understand as a tool of exclusion,” he said. 

 With both governments agreeing that they want to deter migrants from coming to Calais, “they can agree on strong deterrence. So if building a wall solves that, or building an asylum center in Paris solves that, then that’s fine, even if it’s not in the best interests of the refugees and migrants in Calais,” he said.

 If the wall is intended to deliver a message of deterrence, so far it does not seem to have accomplished its mission. Few migrants in the camp appear to have heard about its construction — but they were much more worried that they would soon be pushed out of their temporary home.

“The wall is not important,” said Tariq Shinwari, a 26-year-old business administration graduate from Afghanistan who said he had lived in the Jungle for six months. “People in here do not care about the wall — they care about the demolition. We have minors in here. If they demolish the camp, where will they go?”

Birnbaum reported from Brussels.