Correction: A previous version of this article misstated the number of Trident II D-5 missiles in Britain’s nuclear arsenal. Britain has 58 Trident missiles and 160 deployed nuclear warheads, not 160 Trident missiles as previously stated. This version has been corrected.

Nuclear submarine HMS Vanguard arrives back at the naval base in Faslane, Scotland following a patrol. (Courtesy of Ministry of Defense)

For decades, Britain’s contribution to the threat of global Armageddon has found a home on the tranquil shores of Gare Loch, where soaring green mountains plunge into murky gray waters plied by sporty kayakers, weekend yachtsmen — and nuclear-armed submarines.

The subs slip past this garrison town as quietly as sea monsters. Their dark hulls breach the water’s surface on their way from base out to the deepest oceans, where British naval crews spend months poised to unleash the doomsday payload.

But if Scotland votes “yes” in an independence referendum next month, the submarines could ­become nuclear-armed nomads, without a port to call home. Washington’s closest and most important ally could, in turn, be left without the ultimate deterrent, even as Europe’s borders are being rattled anew by a resurgent Russia.

Former NATO secretary general George Robertson, a Scotsman, said in a speech in Washington earlier this year that a vote for independence would be “cataclysmic” for Western security, and that ejecting the nuclear submarines from Scotland would amount to “disarming the remainder of the United Kingdom.”

The pro-independence campaign promptly accused Robertson of hyperbolic scaremongering. But the possibility that Britain could become the only permanent member of the U.N. Security Council without a nuclear deterrent underscores just how much is at stake far beyond these silent bays and verdant ridgelines when Scotland’s 5 million residents go to the polls Sept. 18.

“The loss of Scotland would be a massive thing for the U.K. — far bigger than the 10 percent of the population that Scotland represents,” said Phillips O’Brien, who directs the Scottish Center for War Studies at the University of Glasgow. “It would have a profound effect on both the external and internal view of what the U.K. represents. And the nukes are a big part of that.”

Leaders of Scotland’s secessionist movement say their independent nation would be a nuclear-free zone within four years of breaking off from Great Britain.

The vow is a popular one among the movement’s left-leaning voters, and the campaign has distributed fliers with instructions for “how to disarm a nuclear bomb” that begin and end with voting for independence.

At the moment, that argument is losing out to those who advocate sticking with the United Kingdom — and with nuclear weapons. Polls show an approximate 10-point advantage for the unionist camp.

Andrew Nisbet is a local leader is Helensburgh's unionist campaign and supports keeping Scotland in the United Kingdom. (Griff Witte/The Washington Post)

But with a substantial share of voters undecided, U.K. officials remain nervous that Scotland could bolt — and that the nuclear program could be a casualty.

The possibility provides an uncomfortable backdrop for the NATO summit that Britain will host in Wales on Sept. 4 and 5.

With Britain’s arsenal of 58 Trident II D-5 missiles and 160 deployed nuclear warheads based in Scotland, a “yes” vote would leave the remnants of the United Kingdom to find a new home for the weapons and the four Vanguard-class submarines that can be used to launch them.

But no such home exists. Building suitable bases to house the missiles and dock the subs in England would take at least a decade, experts say, and cost billions of dollars that the government doesn’t have. O’Brien said it’s likely that Britain would decide to scrap its nuclear program rather than make painful cuts elsewhere.

Even without the challenge of relocating Trident, there are plans to slash 30,000 troops from Britain’s armed forces by 2020, and former U.S. defense secretary Robert M. Gates earlier this year questioned whether Britain could be “a full partner” to Washington given the scale of the cuts.

Those doubts were reinforced this month when President Obama announced airstrikes on Islamic State militants in Iraq, and Britain declined to join.

But as much as Britain may be receding militarily, the process would probably be accelerated if the United Kingdom is torn in two, three centuries after Scotland and England joined forces.

The Faslane Naval Base in Scotland is home to Britain's fleet of nuclear-armed submarines. (Griff Witte/The Washington Post)

An independence vote would be followed by years of negotiation and transition in which bases, hardware and personnel would be divided, said Malcolm Chalmers, research director at the Royal United Services Institute, a think tank. Scotland would have to build autonomous defense forces, while Britain would be “distracted” by tough choices over what it can afford to do without.

“The U.K. might be absent from action for a significant number of years,” Chalmers said.

If British officials did decide to keep Trident, he said, it would be because they wouldn’t want “the additional humiliation of being forced to disarm” after losing a third of the U.K. land mass.

Britain, of course, is not the first nuclear power to face fragmentation. When the Soviet Union tore apart in 1991, nuclear weapons were spread across the republics. But Russia had its share and didn’t need to relocate facilities to retain its deterrence.

Britain, which has had nuclear weapons since the mid-1950s, originally employed warheads that could be dropped from Royal Air Force bombers.

But that program was replaced in the late 1960s with a submarine-based system nestled in this secluded inlet at the mouth of the River Clyde. At the time, Scottish independence was a fantasy.

Scotland’s drive for nuclear freedom.

For many residents in Helensburgh, a neat and prosperous town of 20,000 just south of Faslane naval base, it still is.

“The base is the biggest single-site employer in all of Scotland. It’s huge,” said Jackie Baillie, who represents the area in the Scottish parliament. “Trident’s removal would be devastating to the local economy.”

Baillie said that as many as 11,000 jobs — many of them for highly skilled, well-paid workers — hinge on the nuclear program staying in Scotland.

But for a town in which seemingly everyone has a link to the base, there’s a surprisingly large well of support for independence. The “yes” camp has festooned a headquarters in the heart of Helensburgh with its blue-and-white banners, and its volunteers are active in the streets.

For them, the end of Trident could be a new beginning for the town. Helensburgh, they say, has grown steadily apart from Faslane as the base has expanded to include vast residential compounds, soccer fields, meal halls and a shopping center — all wrapped in the multiple cordons of barbed wire and security cameras that one would expect of a nuclear weapons facility.

“The base may be in the community, but it’s not part of the community,” said Graeme McCormick, a white-maned “yes” activist and businessman.

Graeme McCormick is a leader of the pro-independence campaign in Helensburgh, Scotland, which is near the mouth of the River Clyde, just south of the Faslane Naval Base, home to Britain's nuclear-armed submarines. (Griff Witte/The Washington Post)

McCormick said most of the service personnel come from “down south” — England — and commute home to their families on the weekends.

That wouldn’t be a problem, he said, if Scotland secedes and the new government in Edinburgh makes Faslane the headquarters for all of Scotland’s armed forces, as its prospective leaders have promised to do.

Vivien Dance is among those from England who moved north of the border because of the base.

But 44 years later, Dance and her husband, a retired submarine navigator, are still here — and both support independence, even if it means the end of the nuclear program that was once their livelihood.

Vivien Dance is a Helensburgh, Scotland, councillor whose husband was a navigator on the nuclear-armed subs at Faslane Naval Base. (Griff Witte/The Washington Post)

“There’s no doubt that the U.K. has enjoyed peace for 60 years, and part of that is down to this deterrent. But the world is changing,” said Dance, now a member of the local council. “I have a confidence in Scotland. Independence is a risk, but that’s true of everything in life. We should take the chance.”

Karla Adam in London contributed to this report.