Does Britain’s nuclear deterrent have a Trump-shaped problem?

Sir Keir Starmer’s announcement on February 25th that Britain will raise its defence spending from 2.3% of GDP to 2.5% by 2027, with the aim of hitting 3% after 2030 to meet a “generational” security challenge, was meant to grab attention—and not just in Britain. The prime minister was due to meet Donald Trump in the White House 48 hours later. He would now at least have something to show him.

Sir Keir is also hoping that he can reclaim Britain’s role as a security heavyweight on the continent while signalling new purpose to voters at home. A group of European leaders are to meet in London on March 2nd to discuss defence. Whether the extra money, which has been found by ransacking the foreign-aid budget, will be enough to address the problems of what critics have described as Britain’s “hollowed out” armed forces is doubtful. But it is a start.

It may not satisfy Mr Trump, who has called on Europe to hike military spending to 5% of GDP. Britain has a uniquely close security relationship with America. The second coming of Mr Trump has cast doubt on once unassailable assumptions about the solidity of that relationship.

Nothing symbolises more powerfully how militarily joined at the hip the two countries are than the help America provides in keeping Britain in the nuclear-deterrent business. The British deterrent has also acquired a new significance for European allies fearful that Mr Trump could fold the American nuclear umbrella that has shielded them for more than 70 years. Before becoming Germany’s presumptive next chancellor Friedrich Merz mused about Britain and France “sharing” their nuclear forces if America can no longer be relied upon.

Unlike France’s nuclear deterrent, which is a wholly national endeavour, Britain’s forces, while “operationally independent”, rely on a 67-year-old agreement under which America shares its nuclear technology and infrastructure with Britain. What if Mr Trump concluded that the deal no longer met American interests?

The founding documents of the nuclear relationship are the 1958 Mutual Defence Agreement (MDA) and the 1963 Polaris Sales Agreement through which Britain was able to acquire the new American submarine-launched ballistic missile. While Britain would build and design its own submarines and warheads, it would lean heavily on American technology. Still, Britain’s nuclear forces would be under the sole control of the British prime minister, who could order the missiles to be fired against targets in Russia without the permission of the American president.

It was clear what was in it for Britain. It would remain a nuclear power more cheaply than otherwise possible. For the Americans the benefits were more nuanced. Britain could spend more on its conventional forces in Europe and would make some contribution to America’s costs. Sir Lawrence Freedman, a British strategist, says the Americans also appreciated the ingenuity of British nuclear scientists who would sometimes find cheaper ways of doing things. A further rationale was that a second nuclear decision-making centre would strengthen deterrence if the Russians (wrongly) questioned America’s nuclear commitment to Europe.

Britain became somewhat more dependent on the Americans in the 1980s when the more powerful Trident missile succeeded Polaris. The Thatcher government decided that because the 58 Trident missiles that Britain was purchasing the rights to could stay in their launch tubes for longer than Polaris, they did not have to be stored in Britain as before. They would become part of a pool of missiles kept at the Kings Bay submarine base in Georgia.

Concerns about dependence were batted away. Margaret Thatcher’s cabinet secretary, Robert Armstrong, told her that “in the inconceivable case of a future US government deciding to cut off supplies of components”, there would be “no sudden effect”, though over “two or three years” the deterrent’s effectiveness would “begin to fall off”. In 2006 Commodore Tim Hare, a former nuclear-policy chief at the Ministry of Defence, told the Commons Defence Committee: “If, over a very long period, we became deeply estranged from the Americans and they decide to rat on their agreements, we would be in…great difficulty.” But the risk was “very low”.

Operational independence endures. James Jinks, author of “A Very British Bomb”, to be published next year, says: “There is nothing in the Trident system, for example a kill switch or software, which would prevent a UK submarine commander launching missiles following authorisation from a British prime minister.” Nor can the Americans turn off guidance systems, which are self-contained.

Britain has also recently taken some steps to grant itself more agency. As part of the renewal of the deterrent and the AUKUS nuclear-submarine deal with Australia and America, Britain announced a reinvigoration of its nuclear industrial base last year. In 2021 Britain took back control of the Atomic Weapons Establishment, which designs and manufactures the warheads, from a consortium led by Lockheed Martin, an American defence firm.

However, the British nuclear enterprise remains deeply entwined with America’s. The missiles come from the pool at Kings Bay; missile testing is carried out under American supervision at Cape Canaveral; British submarines and warheads share designs with their American counterparts and rely on spare parts made in America (Britain is also believed to send parts in the other direction). The MDA used to be renewable every ten years, but was amended last year to be an “enduring” agreement. Whether it could endure a decision by Mr Trump to end it is another matter.

Sir Lawrence reckons that were that to happen there would not be a “cliff-edge” loss of effectiveness, “but over time we would be scrambling to keep it going”. The biggest problems would be replacing the Kings Bay missile storage and running out of critical spare parts.

In the past such thoughts would not have been on the mind of a British prime minister visiting the White House. Ten years ago Malcolm Chalmers of RUSI, a think-tank, said: “If the US were to cut off nuclear aid now—after almost 60 years—it would be such an antagonistic act as to throw the wider alliance relationship into question. I see no prospect that this will happen.” Those were different times. ■

We’re hiring: The Economist is looking to hire a Britain economics writer, based in London. Journalistic experience is not necessary. The ability to write clearly and entertainingly is crucial. So is knowledge of economics and an ability to work with data. For details go to economist.com/britain-econ-job

For more expert analysis of the biggest stories in Britain, sign up to Blighty, our weekly subscriber-only newsletter.