Britain and France announced a pair of landmark agreements Thursday on nuclear security and immigration, uniting two allies — if occasionally squabbling neighbors — to confront a world of proliferating threats.
Prime Minister Keir Starmer and President Emanuel Macron said that for the first time, their countries would work together to deploy nuclear weapons if allies in Europe came under extreme threat. While each will retain control over its arsenal, both will coordinate on policy and more closely align their nuclear doctrines.
“Our adversaries will know that any extreme threat to this continent would prompt a response from our two nations,” Mr. Starmer said, standing next to Mr. Macron at Northwood, a military headquarters northwest of London. “There is no greater demonstration of the importance of this relationship.”
Mr. Macron, who was wrapping up a three-day state visit to Britain, said that no two other countries had “this closeness on nuclear doctrine,” and that it was a “message our partners must hear, as well as our adversaries.”
Among those is the United States, which under President Trump has left many in Europe questioning whether they can rely on the nuclear umbrella provided by the Americans since World War II. Both leaders said that Europe needed to provide more independently for its security.
Mr. Starmer and Mr. Macron also announced an agreement under which Britain would send back some migrants who illegally cross the English Channel, smoothing a chronic source of friction between the countries and opening what their leaders hailed as a new era of harmony after the acrimony of Brexit.
The twin deals drew a surprisingly substantive end to a hectic state visit that began with pomp and pageantry, showcasing the ancient ties between France and Britain, and reinforcing their common cause in defending the Europe of today as the trans-Atlantic alliance comes into doubt.
They also illustrated vivid differences between the leaders.
Mr. Macron, in the twilight of his presidency and eager to cement his legacy as a statesman, dwelt on the global implications of the nuclear accord; Mr. Starmer, elected only a year ago and politically beleaguered at home, played up the migrant deal as a win on a thorny domestic issue that has bedeviled several of his predecessors.
There were strict limits to both agreements.
Mr. Macron emphasized that the nuclear accord had no connection to the war in Ukraine, where Britain and France are redoubling efforts to marshal a Europe-wide security force, announcing the creation of a headquarters for it in Paris. Analysts described the agreement, known as the Northwood Declaration, as only a first step toward a European nuclear deterrent.
And Mr. Starmer acknowledged that the migrant deal was a “pilot program” that would involve returning only about 50 people a week to France. So far this summer, more than 800 people a week have been landing on British shores after perilous crossings.
Experts noted that the legality of the agreement could be challenged in Europe, where other countries have raised concerns about its wider effects. Still, said Peter Ricketts, a former British ambassador to France, “it’s certainly at the top end of my expectations for this summit.”
Mr. Ricketts had a role in another element of Mr. Macron’s visit. He served as a special envoy for Britain negotiating the lending of the Bayeux Tapestry, which depicts the Norman Conquest of England in 1066, to the British Museum. It is the first time in centuries that the tapestry has returned to Britain from France.
That was the cultural highlight of a visit that included a lavish banquet at Windsor Castle hosted by King Charles III, at which Mr. Macron and his wife, Brigitte, dined with guests who included Elton John, Mick Jagger and the actress Kristin Scott Thomas. The king squired Mr. Macron to the castle in a horse-drawn carriage.
Global security loomed large on the final day of the visit. The leaders traveled to the military headquarters for a video call with other European leaders to try to reinvigorate their proposal for a so-called coalition of the willing to help Ukraine. While the Trump administration has played little role in this effort, Keith Kellogg, the retired general who is Mr. Trump’s adviser on Ukraine, took part in the call.
The shift in nuclear language had been in the works for months. It is the most significant update of the doctrine between Britain and France since 1995, when Britain and France agreed to deepen their nuclear cooperation under John Major, the prime minister, and Jacques Chirac, the French president.
“These things change glacially and only with a lot of careful thought,” Mr. Ricketts said. He said it reflected a debate that Mr. Macron recently initiated about whether there was a role for France’s nuclear arsenal in European deterrence. Britain already plays such a deterrent role through its membership in NATO.
Georgina Wright, a senior fellow and expert on Anglo-British relations at the German Marshall Fund in Paris, said, “It is significant that they have decided to talk about it, and coordinate.” She noted that Britain and France would “always be the ones to decide when to push their respective buttons.”
But raising the idea of coordination, she said, symbolizes how they are reckoning with an aggressive Russia and a United States that is pulling back. Neither Mr. Macron nor Mr. Starmer mentioned Mr. Trump, but Mr. Starmer said Europe needed to “step up” and do more for its “collective defense.”
The deal on migrants, which Mr. Starmer labored to extract from Mr. Macron even as the visit was well underway, is a similar mix of symbolism and substance, according to analysts. Under a “one in, one out” structure, Britain is to send back to France roughly 50 people a week who have crossed the channel illegally. In return, it is to take a comparable number with processed claims to asylum in Britain.
“This is different and significant, in that France is saying for the first time that they accept the principle of taking people back,” said Madeleine Sumption, director of the Migration Observatory at Oxford University. “The big question is: What will the French position be about scaling it up?”
If the number of people returned remains at these modest levels, she said, it would likely not discourage other migrants, who have already taken risks to get to the French coast. Migrants tend to overestimate their odds of winning asylum in Britain, which makes them even readier to take risks.
Still, for Mr. Starmer, nailing down some kind of deal was politically imperative. The flow of migrants, though only a fraction of Britain’s total net immigration, has become a stark illustration of the government’s failure to control the borders, and a weapon in the hands of right-wing populists.
Nigel Farage, the leader of the anti-immigrant party Reform U.K., took to a boat out on the channel on Wednesday to disparage the deal — then still being negotiated — as an example of Mr. Starmer surrendering control to Mr. Macron.
For his part, Mr. Macron argued that Britain’s decision to leave the European Union had worsened the problem of illegal immigration.
“The British people were sold a lie,” he said. “With your government, we’re pragmatic, and for the first time in nine years we are providing a response.”
Mark Landler is the London bureau chief of The Times, covering the United Kingdom, as well as American foreign policy in Europe, Asia and the Middle East. He has been a journalist for more than three decades.
Michael D. Shear is a senior Times correspondent covering British politics and culture, and diplomacy around the world.
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