Within days of entering office, every British prime minister must carry out a grim task: writing letters of “last resort” to the commanders of the country’s nuclear-armed submarines. The letters are written out in long hand and placed in sealed safes in each of the nation’s four Vanguard-class submarines, all home-ported on the River Clyde in Scotland. The missives spell out the prime minister’s orders in the case that he and other government officials have been killed in a nuclear attack on Britain.

But after a majority of Brits voted last month to leave the European Union, those letters — and with them the U.K.’s nuclear arsenal — may become a relic of the past.

In Scotland, a whopping 62 percent voted to stay in the EU. As a result, the Brexit referendum has given fresh legs to another referendum on Scottish independence. The last time the Scots tried — in 2014 — their bid for independence was defeated. But resentment of the British bolt from the EU runs deep in Scotland, and a desire to remain in Europe is bolstering support for independence.

And Scottish nationalists have made clear that if Scotland secedes, it will no longer play host to Britain’s nuclear-armed submarines at Faslane on the River Clyde.

Since 1969, Britain has maintained one ballistic missile submarine on patrol at all times. The silent subs each carry eight Trident missiles with a range of 7,500 miles and 40 atomic warheads that could wipe out another country’s cities. The Trident “deterrent” is supposed to protect Britain by discouraging an enemy from attempting a nuclear attack or nuclear blackmail through the threat of catastrophic retaliation. The U.K. joined the nuclear weapons club in 1952, making Britain just the third member after the United States and USSR. And since 1998, the subs have been Britain’s sole nuclear deterrent.

But the four submarines that make up the nuclear arsenal — the HMS Vanguard, Victorious, Vengeance, and Vigilant — all operate out of Faslane, and there is currently no alternative base to host the subs if Scotland chooses to kick them out.

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A 2012 parliamentary report concluded that it could take up to 20 years and cost 3.5 billion pounds to build alternative port facilities to accommodate the submarines south of the Scottish border.

That presents a dilemma for London and NATO with a resurgent Russia stirring up trouble in Ukraine, the Baltics, and the eastern Mediterranean. Britain’s submarine force plays a crucial role inside transatlantic alliance, guarding the so-called “GIUK gap,” the stretch of the North Atlantic from Greenland and Iceland to the United Kingdom itself — and the Russian Navy’s potential entry point into the Atlantic Ocean. British submarines are considered a “first line of defense” against potentially hostile Russian runs through the passage, according to Lisa Sawyer Samp, who served as the director of NATO and European strategic affairs at the National Security Council until last year.

Seeking to defuse speculation that the U.K. might have to relinquish its nuclear force, British Defense Secretary Michael Fallon told Parliament last month that the country’s allies “can rest assured that our commitment to NATO, and our commitment as a nuclear power to NATO, is not altered by the result of the referendum.”

Scottish secession would not necessarily mean scrapping the U.K.’s nuclear arsenal, said Mark Fitzpatrick, a former senior U.S. diplomat now with the International Institute for Strategic Studies.

“There are options. They’re expensive,” Fitzpatrick told Foreign Policy. “It wouldn’t mean the end of Britain’s nuclear deterrence. It would mean that it would become even more costly at a time when Great Britain would be a diminished economic power.”

If London chose to construct a new base in England for the submarines, the vessels would need to be docked elsewhere while the work was carried out. That would mean negotiating a deal with Scotland to keep the subs in place for a number of years or to possibly park them at a naval base in another country.

One option could be the East Coast of the United States. Britain is the only country with which the U.S. shares nuclear technology and has enjoyed, since World War II, a “special relationship” with Washington. British subs could potentially dock at a U.S. submarine base in Kings Bay, Georgia. Or the sprawling U.S. Navy base in Norfolk, Virginia, the world’s largest, “could provide a temporary holding ground” for the British subs until London built another facility, said Samp.

But if the U.K. were to give up its deterrent, that would leave the United States as the sole nuclear guarantor for the NATO alliance.

“More than anything, I think more of the burden would shift to the United States, so it’s exactly the wrong message at the most inopportune time,” given the pattern of increasingly aggressive behavior by Russia, Samp said.

The Obama administration has said it hopes the U.K. retains its Trident submarines but has downplayed the possibility of Britain losing its base at Faslane.

“The United States values the longstanding role of the United Kingdom’s nuclear deterrent as a contribution to the overall deterrence and security of the NATO alliance,” State Department spokesman John Kirby told FP.

“It would not be appropriate to speculate on the outcome of a hypothetical Scottish referendum,” he added.

Even before the Brexit vote, the fate of the nuclear deterrent had become the subject of intense debate in Britain. The current Vanguard-class submarines will need to be replaced sometime in the late 2020s, and funding needs to be approved this year to start work on a replacement fleet. The successor program would cost an estimated $22 billion to $30 billion, though critics insist the price tag would be far higher. For years, political parties have been postponing a decision on the issue.

The Brexit vote, by raising questions about the U.K.’s place in the world, has prompted the British government to double down on its support for new nuclear-armed subs. London appears determined to show it is not retreating from the NATO alliance or the international arena. Defense Secretary Fallon told MPs last month the government is “committed in our manifesto to replacing the four Trident submarines.”

At a NATO summit on Saturday in Poland, British Prime Minister David Cameron — who campaigned unsuccessfully for Britain to remain in the EU — called the nuclear deterrent the “ultimate insurance policy” and announced a parliamentary vote on the issue on July 18.

“The nuclear deterrent remains essential in my view – not just to Britain’s security but – as our allies have acknowledged here today – to the overall security of the alliance,” said the leader of the ruling Conservative party, who has promised to step down in October due to the Brexit result.

Thousands of jobs are tied up in the Trident program. But opposition Labour Party leader Jeremy Corbyn has called for rejecting funding for the new subs, questioning the high price tag, as well as the whole idea of planning for a nuclear doomsday in which Whitehall would order the destruction of dozens of cities.

The uncertainty over the future of Britain’s nuclear deterrent is matched only by uncertainty over its true utility, both as a weapon and as a geopolitical tool.

During the Cold War, the nuclear force was supposed to dissuade the Soviets from launching an attack on Britain, even if Moscow could somehow take out the United States. And from Britain’s perspective, the arsenal has helped buttress the country’s claim of being one of the world’s major powers and helped justify its seat as a permanent, veto-wielding member of the U.N. Security Council.

Skeptics say nuclear arms are a devalued currency in the battle for global prestige, especially in an era of insidious threats from Islamic State extremists. For Washington, the British nuclear force was never absolutely essential, as the United States has designed its own arsenal and strategy to safeguard both itself and its NATO allies, said former U.S. diplomat James Dobbins, a senior fellow at the Rand Corp. Germany, he said, has no nukes yet is considered the most important country in Europe.

“I think it clearly would have some impact on Britain’s image of itself as a world power,” but not on perceptions outside the country, he said.

Yet others argue that Britain could be more vulnerable to blackmail from a rogue state or terrorists if it lost its own arsenal. And Britain’s clout in the world, as it still clings to major-power status, would also take a hit if it lost its atomic punch, according to the pro-Trident camp.

In an increasingly unstable world, with threats seemingly proliferating from the Arctic to the South Pacific, Britain’s four nuclear hulls still play an outsized role.

“The deterrent has strategic value for the U.K., for NATO, and for the U.S.,” said Elbridge Colby, a former senior advisor on arms control at the U.S. Defense Department who is now at the Center for a New American Security.

He added: “It says to a potential opponent, if you are able to cut out the Americans, you still have to deal with London.”

FP staff writer Paul McLeary contributed to this article.

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