If tradition held, in her first hours as the United Kingdom’s new prime minister, Theresa May met with the British defense leadership and received an eye-opening briefing about the nation’s nuclear plans.

Sir Nicholas Houghton, the 61-year-old chief of the defense staff who is due to retire this month to become the constable of the Tower of London, will have walked Prime Minister May through the country’s nuclear plans and the damage that could result in the event of nuclear attack on her country.

Amidst all the public pomp and circumstance of assuming office and determining a course of action for the country following the Brexit vote, one of the first things May was tasked with is perhaps the most grim duty of any head of government in the world: Writing what’s known as a “Letter of Last Resort” — secret instructions, to be remain sealed until after Armageddon, about what the nation’s submarine commanders should do with the U.K.’s nuclear weapons if the country has been destroyed. Actually, she’ll have written four of them — all identical — one to each submarine commander in the U.K. fleet.

The safes will only be accessible to the sub’s commander and deputy, who must decide together when Britain has been entirely destroyed.

Throughout the Cold War, each nuclear power struggled to figure out how it would approach Armageddon. The Soviet Union ultimately built a rocket that could beam launch orders to Soviet silos even after the human chain of command had been destroyed, a “Dead Hand” machine ultimately uncovered by nuclear historian Bruce Blair in 1993 and made famous by journalist David Hoffman's eponymous 2009 book. The United States, meanwhile, built a complex network of planes, trains, ships, communication networks and bunkers that could ensure control over the nation’s nuclear systems even amidst a devastating attack.

The British approached a nuclear holocaust differently, and in an appropriately British fashion. Rather than rely on hi-tech gadgetry, their prime ministers hand-wrote “Letters of Last Resort,” and then locked those letters inside of a safe inside of another safe, and placed them in the control rooms of the nation’s nuclear submarines. The safes will only be accessible to the sub’s commander and deputy, who must decide together when Britain has been entirely destroyed.

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Britain has long charted its own course when it comes to nuclear weapons, so much so that the secrets of one prime minister often surprise the next. As early as 1941, Britain determined that it would need its own nuclear weapons and partnered closely with the United States on the Manhattan Project to develop the first atomic weapon. Yet the tight secrecy around the Manhattan Project — Harry Truman hadn’t even known about it as vice president — undermined that same collaboration after the war: Not understanding Britain’s key role in the nuclear program, the U.S. Congress passed an act in 1946 prohibiting international cooperation over nuclear matters.

That left Britain to push forward on its own: After World War II, Winston Churchill’s successor as prime minister, Clement Attlee, moved ahead with the U.K.’s own nuclear force — a decision considered so vital to the nation’s success that the Labour prime minister enlisted a Tory, Sir John Anderson, as his lead adviser on the project.

As the scale of nuclear devastation began to boggle the imagination, Britain faced a unique threat among the nuclear superpowers.

The decision to build the bomb, British historian Margaret Gowing wrote, was not “a response to an immediate threat but rather something fundamentalist and almost instinctive — a feeling that Britain must possess so climacteric a weapon in order to deter an atomically armed enemy, a feeling that Britain as a great power must acquire all major new weapons, a feeling that the atomic weapons were a manifestation of the scientific and technological superiority on which Britain’s strength, so deficient if measured in sheer numbers of men, must depend.”

At the time, the project was deeply secret — unknown even to the House of Commons. Attlee himself never spoke about it publicly, and it was to Churchill’s great surprise when he retook the office in 1951 that the United Kingdom stood on the verge of becoming an atomic power. In 1952, the United Kingdom became the third country — after the United States in 1945 and the Soviet Union in 1949 — to test a nuclear weapon, exploding weapons in Australia. In the late 1950s, as the Cold War darkened and Sputnik raised fears that the Soviet Union was marching ahead scientifically in space and in ICBM technology, Britain moved ahead with a thermonuclear weapons program.

Yet as the scale of nuclear devastation began to boggle the imagination, Britain faced a unique threat among the nuclear superpowers: Its comparatively tiny island — and its heavily concentrated population and government centers — could be easily obliterated by the power of later generations of atomic and hydrogen bombs. Whereas even a relatively large attack might have left much of the United States or the Soviet Union untouched and allow enough survivors to reconstitute the so-called “National Command Authority,” the military and civilian leaders who can order a nuclear launch, and plan a retaliatory strike, even a small-scale surprise attack from the Soviet Union would have likely destroyed all remnants of Whitehall and the British command chain. Plus, given its geographic proximity to the Soviet Union, Soviet subs, bombers and ICBMs could strike quickly, with little warning and little time to evacuate the nation’s leadership to protective bunkers readied in the English countryside.

And thus was born the tradition of the “Letter of Last Resort.”

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It has become a moment when British leaders must wrestle with the awesome new responsibilities embodied in their nuclear control. One of Houghton’s predecessors, Lord Charles Guthrie, has said that when he briefed incoming Prime Minister Tony Blair, the young Labour leader blanched. “I think quite honestly, like most prime ministers, he hadn’t given a huge amount of thought to what this really meant. And it is actually an awesome responsibility. It really comes home to you that he could, if the circumstances demanded it, create devastation on a huge scale,” Guthrie told a BBC documentary in 2008. “[Blair] went quite quiet.”

The British letters, of course, must have some nuance and flexibility to them. The missives are written long before global tensions might escalate and without the knowledge of how nuclear war might break out — which side might start the war, or even, in a world increasingly complicated by nuclear proliferation, who the aggressor might actually be. Post-Cold War prime ministers face a more dynamic geopolitical environment, where the Soviet Union is no longer the sole aggressor.

Those who have worked on the process of the “Letters of Last Resort” say that the options a prime minister considers generally fall into a few broad categories.

How do you craft a letter, to be read after you’re dead, that provides advice on how to respond to a scenario like a rogue state-sponsored terrorist attack on London? Or a suicidal general in Pakistan or supreme leader in Iran?

Those who have worked on the process of the “Letters of Last Resort” say, though, that the options a prime minister considers generally fall into a few broad categories: Should the submarines go ahead and launch their missiles in a retaliatory strike? Turn themselves over to serve another allied nation? Retreat to another country that’s part of the British Commonwealth, like Australia or New Zealand? Or perhaps leave the ultimate decision up to the sub commander himself? What precisely the orders might encompass is known only to the prime ministers who write and address the letters in private.

For the last two decades, the letters have been written to the commanders of the nation’s four remaining Vanguard-class ballistic missile submarines, all based out of the Faslane naval base on the east coast of Scotland: The HMS Vanguard, HMS Victorious, HMS Vigilant and HMS Vengeance. The Vanguard-class submarines, which were begun in the 1980s, never actually patrolled during the Cold War, arriving in the British fleet only in 1994. But ever since the United Kingdom gave up its last operation air-dropped nuclear bombs, the subs’ missiles have been the core of its nuclear program. Today, during normal peacetime operations, only one submarine is generally on patrol at a time — stocked with 40 of the country’s 120 operational nuclear weapons.

The letters, as the name implies, are intended only as a “last resort.” A complex — and secret — process and checklist guides each submarine commander on when to open the safe containing the safe containing the prime minister’s sealed letter. The commander, somewhere out under the world’s oceans, must first determine that London and the British government have been destroyed and that neither the prime minister nor designated successors are still alive — and that Her Majesty’s Government is likely destroyed as well.

Only one prime minister, James Callaghan, who served from 1976 to 1979, has ever spoken publicly about his decision-making.

One of the few details known about the process is that the submarine commander must check whether BBC’s Radio 4 is still broadcasting. According to historian Peter Hennessey, “The failure to pick up the BBC Today program for a few days is regarded as the ultimate test.” Then, convinced Britain no longer exists, the commander, with the deputy alongside, will open the letter and execute the final wishes of the prime minister. It is, in its own way, the ultimate dead hand — a letter from beyond the grave.

So how has each prime minister decided to react to nuclear war? No one knows. The letters are destroyed at the conclusion of each government — meaning that, in all likelihood, the letters that David Cameron have been destroyed, with no human eyes other than his ever glimpsing them.

Only one prime minister, James Callaghan, who served from 1976 to 1979, has ever spoken publicly about his decision-making. He said, years later, that his belief was that the nation should fire its weapons: “If it were to become necessary or vital, it would have meant the deterrent had failed, because the value of the nuclear weapon is frankly only as a deterrent. But if we had got to that point, where it was, I felt, necessary to do it, then I would have done it. I’ve had terrible doubts, of course, about this. I say to you, if I had lived after having pressed that button, I could never, ever have forgiven myself.”

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While most prime ministers have stayed mum, not all would-be leaders have been so circumspect: Jeremy Corbyn, leader of the opposition Labour party, has long been a vocal supporter of nuclear disarmament, and has said that he’d never resort to nuclear weapons, meaning that a Corbyn government — if it ever came to pass — would likely rely on one of the “Last Resort” options that don’t include launching a retaliatory strike.

Such tipping-of-the-hand as a future leader raised eyebrows in Britain last year. General Houghton, who will brief May on the U.K.’s nuclear plans, actually argued publicly with Corbyn, saying he worried that if Corbyn ever became prime minister, his known pacifist leanings would undermine the nation’s 60-year-old nuclear deterrent.

“The whole thing about deterrence rests on the credibility of its use,” Houghton said. “When people say you’re never going to use the deterrent, what I say is you use the deterrent every second of every minute of every day and the purpose of the deterrent is that you don’t have to use it because you successfully deter.”

Whether Prime Minister May agrees, we may never know. Though one might draw some clues from her legislative agenda in the weeks ahead: She’s said she’s eager to push ahead with replacing the aging Vanguard submarines, which will be obsolete in the middle of the next decade. Maintaining the nation’s nuclear deterrence will likely to cost north of $250 billion, but she’s said it’s critical to Britain’s international role post-Brexit. And, in that, predecessors such as Winston Churchill and Clement Attlee would likely agree.

Journalist Garrett M. Graff (@vermontgmg) is the author of The Threat Matrix: The FBI at War, and a former editor of POLITICO Magazine. His next book, Raven Rock, about the U.S. government’s Doomsday plans, will be published in May 2017. He can be reached at garrett.graff@gmail.com.