LONDON — The United Kingdom has 215 nuclear weapons and a permanent seat on the United Nations Security Council, and has been Washington's best friend for decades.
Less than 100 years ago it ruled over Canada, Nigeria, India, Australia and more, covering almost a quarter of the world's territory and population.
Yet in recent months there has been growing alarm that the U.K. is in danger of breaking apart. Nothing like this has happened before — not to a modern democracy with such geopolitical and historical standing.
The U.K. is unusual because it comprises four nations: England, Scotland, Wales and Northern Ireland. Were one of these to leave the U.K., many would see it as a dismal end to centuries of British history, further diminishing its role as a cornerstone of the Western postwar alliance.
"We should be genuinely worried, there's no doubt about that," former Prime Minister Tony Blair told NBC News in an interview.
"It's incredible to me," he said. "The factors at work here have come into play, not through some act of God, but because people created them."
In fact all five of the U.K.'s living former leaders warn that this is no longer some vague hypothetical; the possible roadmap to disintegration is now clear. Scotland or Northern Ireland — or both — could conceivably hold referendums to leave the U.K. within the next five to 10 years.
Its breakup would be cheered by separatists who see the Parliament in London as a colonial throwback deaf to their interests. They point to the recent rise in English nationalism and the prime ministership of Boris Johnson, who has upended constitutional norms during his short time in office.
But at the heart of the turmoil lies Brexit.
The most urgent issue is a curveball few saw coming.
Before the Brexit referendum in 2016, the idea that Northern Ireland could leave the U.K. and reunite with the Irish Republic was not a serious consideration for most.
"It was hardly mentioned five years ago, but everyone seems to be talking about it now," said Tom McParland, 70, a cattle farmer perusing a livestock auction near the Irish border.
Irish agriculture is forecast to be among the industries hit hardest by Brexit, and that anxiety creeps its way into seemingly every conversation here.
"A united Ireland seemed like a ridiculous thing to talk about, but now people are," said McParland, who comes from the nearby town of Newtownhamilton. He considers himself Irish, not British, and believes reunification is perhaps 15 years away.
Polls suggest people are being swayed by the threat of a damaging "no-deal" Brexit. That is, if it can't work out a divorce settlement to leave the European Union, the U.K. will crash out of Europe with no plan at all.
British lawmakers have tried to block Johnson going down this hard-line route, denying him an early election (for now) and passing a law forcing him to ask for an extension to the Brexit deadline of Oct. 31 if necessary.
However, neither a delay nor an election would change the central fact: Without another solution, the default legal position is that the U.K. is headed toward a no-deal Brexit.
The government's own forecasts warn that this extreme scenario could bring economic misery, transport chaos and shortages of food and medicine.
But it could be even worse in Northern Ireland.
Here, there are widespread fears it could reignite decades of sectarian violence most hoped were in the past. Faced with these threats to their economy and security, more people are becoming open to Irish reunification, according to polls.
The stakes are colossal: The British government must hold a referendum in Northern Ireland if it looks likely most there would support reunification. Though the logistics are vague, a simple "yes" vote would pave the way for a united Ireland, ending the U.K. as we know it.
Northern Ireland: If there was a no-deal Brexit, would you prefer to stay in the U.K. or reunify with the Irish Republic?
Observers say these risks should have been obvious to the previous government of Theresa May, the former prime minister.
"Although from time to time she said she would consult with Scotland, Wales and Northern Ireland, in practice she rarely showed any sign of doing that," said Robert Hazell, a politics professor at University College London.
"It was starkly apparent that the Irish border would be one of the main difficulties with Brexit, yet Mrs. May blundered on, seemingly blind to that very obvious, very startling difficulty," Hazell said.
The border is crucial.
For 30 years Northern Ireland was plagued by "the Troubles," a conflict between mainly Catholic republicans wanting to reunify with the Irish Republic and mainly Protestant unionists wanting to remain in the U.K.
Around 3,600 people were killed in violence between paramilitary groups like the Irish Republican Army and the Ulster Volunteer Force, and the British Army. It spilled over onto the U.K. mainland, most notably the IRA's attempt to assassinate then-Prime Minister Margaret Thatcherwith a bomb in 1984.
The conflict ended in 1998 with the Good Friday Agreement, a landmark peace deal that deployed some ingenious ambiguity.
Northern Ireland would remain in the U.K. but its people would be permitted Irish citizenship. The border was opened and armed checkpoints removed, giving citizens of both countries freedom to cross without fear of confrontation.
"The Good Friday Agreement had an elasticity in it, but that ambiguity has almost been totally hammered out as a result of Brexit," said Feargal Cochrane, a professor of conflict analysis at the University of Kent.
That's because any type of hard-line Brexit would rid the North of the shared E.U. regulations that make this flexibility work. It would mean a return to cameras or manned posts at a "hard border" — a painful reminder of the conflict and a target for paramilitaries eager to revive it.
Despite the end of the Troubles more than 20 years ago, violence lingers.
In April, the Northern Irish journalist Lyra McKee was shot dead, with the dissident group the New IRA reportedly claiming she had been caught in crossfire aimed at police. This summer, the Police Service of Northern Ireland, or PSNI, says it has seen an uptick in activity, including several bombs planted with the intention of killing its officers.
"There's a latent tension that's building," Cochrane said. "The cork is going to pop out of the bottle once Brexit happens."
The U.K. and E.U. have tried to address the threat by negotiating something called the "backstop." This is an insurance policy that says, if nothing else is sorted out, some E.U. rules will be kept in place to prevent the need for a border.
It's the crucial sticking point in negotiations, and hated by many Brexiteers because they fear it risks tying them up in Europe's regulatory web. Johnson's own plan was immediately dismissed by Europe, but negotiations are continuing.
At the livestock market near the town of Markethill, owner and auctioneer Hampton Hewitt, 57, said he fears Brexit's devastating impact on the dairy and lamb business, which he thinks "could simply cease to exist in Northern Ireland."
But he believes reunification is unlikely because of the Irish Republic itself.
Officially, the Republic supports reunification in its Constitution. Under the 1998 peace deal, it would hold its own referendum if there were ever a move toward a united Ireland. But Hewitt is not alone in believing the Republic may be reluctant to annex his volatile homeland.
"They couldn't handle us," Hewitt said. "We're a hot potato that nobody wants."
His son, James, 21, disagrees, and says the door to reunification is at least ajar. "People who have been through the Troubles, it's like you've got an enemy and you pick your side," he said. "But people my age would be more worried about the economy and stability, rather than a unionist or nationalist point of view."
Those in favor of remaining in the U.K. used to point to Dublin's conservative Roman Catholicism and weak economy.
But today, Ireland is Europe's fastest growing economy and "a very cosmopolitan, secular country," Cochrane said. "If the U.K. economy falls through the floor after Brexit, then that's another rationale for the union gone. If Scotland jumps overboard, its whole raison d'etre starts to fall apart."
Over the centuries, the Irish, Welsh and Scottish have fought countless battles against the English. But Scotland's final capitulation came in far less heroic circumstances.
In the 1600s, the Kingdom of Scotland lost an estimated one-fifth of its cash reserves in a disastrous attempt to establish a trading route in Panama. England offered to pay its debt as part of a deal for Scotland to join its realm.
Despite widespread riots on the streets, the Scottish government agreed. And in 1707 Great Britain was born.
Today, proud Scots are more likely to invoke Robert the Bruce and William Wallace, who fought off the English in the 13th and 14th centuries, rather than the more recent financial folly that helped seal their fate.
"I don't feel British at all — at all," said Annie Jenkins, 75, a retired health worker. "When you fly somewhere and have to fill in those forms, I always cross out 'British' and write 'Scottish.'"
Jenkins was among an estimated 12,000 people who flooded the streets for a pro-independence rally in the oil town of Aberdeen in August. She struggled to be heard above bloodcurdling cries of, "Let's take our country back!" as the city was engulfed in a tide of tartan, Saltair flags and woad-like face paint.
"I've been coming on these marches for years and I can feel the difference now," said Helen Armett, 53, who writes a community newsletter on the island of Orkney. "Independence is happening."
Spearheaded by the Scottish National Party, the separatists say they are being held back by the government in London, 400 miles south of Aberdeen.
"We're a remarkable wee country," Armett said.
Income distribution is a big part of the discussion. The British government spends far more money per person in Scotland than it does across the rest of the U.K. This is effectively a subsidy from the rest of the country, particularly the affluent southeast of England and London.
Some economists say independence would make Scotland poorer because spending this much on its own — without these effective subsidies from south of the border — would create a crippling black hole in its budget.
Separatists argue that their books would be in better shape if they were allowed to run their own affairs.
"How can we remove the foreign invaders if they control our system?" said Gary Kelly, 44, organizer of the Aberdeen rally. "Why are we still asking permission? We are a sovereign people."
Not all bridges would be burned. The SNP has proposed keeping the British pound for currency, and even retaining the queen as its head of state, as is the case in Canada and Australia.
But they do want full control of the oil reserves off Scotland's northeast coast, as well as their world-famous Scotch whisky and tourism industries. They oppose the U.K.'s submarine-based nuclear weapons, which are based in Scotland and cost the taxpayer some $45 billion a year.
For unionists such as Tony Blair, Scottish independence would severely diminish the U.K. in its role as Washington's bridge to Europe. He is a true believer in this "special relationship," including his well-known support for President George W. Bush’s invasion of Iraq in 2003.
Blair sees this battle as part of a wider struggle, one for global influence against an authoritarian China, a rising India and an illiberal Russia.
"The 21st century is going to see a competition between East and West over things like technology and trade and, to a degree, military power," Blair said. "It's important those Western countries, and particularly that transatlantic alliance between America and Europe, stick together."
Staying in the U.K. was previously sold as the only way to guarantee E.U. membership, but independence might now be Scotland's only route back in. Like in Northern Ireland, most Scots voted against Brexit but were outnumbered by those in England.
The debate on the E.U. has also allowed separatists a makeover. Before their cause was denegrated as backward, inward-looking and nationalistic. It has now been rebranded as pro-European, progressive and welcoming.
And unlike the anti-immigration sentiment thriving in England, Scotland needs more people to compensate for its brain-drain.
Then there's the prime minister, Johnson, who is disliked more in Scotland than anywhere else in Britain. He talks about leading an "awesome foursome" but was booed on every leg of his inaugural trip to the U.K. nations.
He was elected by fewer than 100,000 Conservative Party members, most of whom are English. So far in his short premiership he has triggered uproar by asking the queen to suspend Parliament, something deemed unlawful by the U.K.'s Supreme Court and called "a dark day for democracy" by SNP leader Nicola Sturgeon and others.
"English nationalists are the ones that sealed the United Kingdom's fate. We can feel it sinking as we stand," said Sammuel Cook, 23, a car insurance adviser at the Aberdeen rally.
Most experts believe an early U.K. election is inevitable sooner or later. But the union would appear no safer were opposition Labour Party leader Jeremy Corbyn to gain power. Corbyn has for decades supported Irish reunification, and says he would not block a second Scottish referendum if there was sufficient support.
Even avid unionists accept that any government would struggle to deny Scotland a second independence vote were the needle to shift demonstrably. At least one poll suggests it already has.
Scotland: How would you vote in a second referendum on independence?
"Technically it is in Westminster's gift to grant a referendum, so they can always say no," said Kevin Hague, director of the pro-unionist nonprofit These Islands. "But the truth is, you can't hold back that tide, I don't think."
Though fiercely proud of its culture, ancient language and marauding rugby team, Wales is not seen as a hotbed of separatist passions.
When given the chance to create its own Parliament in a 1997 referendum, the nation said yes only by a margin of 6,721 among some 1 million Welsh voters.
Fast forward 22 years and the mood is undoubtedly different. Fresh debates in Scotland and Ireland have coincided with a renewed zeal in Wales.
Wales has seen its own slew of pro-independence marches. And although most polls show support is in single-figure percentages, one bombshell survey last month said one third of the Welsh supported the cause.
Wales: If it meant staying in the E.U., would you vote for independence?
Even many who aren't sure say they are "indycurious."
"The use of the word 'indycurious' is kind of indicative, isn't it? It comes from the LGBT context of bicurious," said Adam Price, leader of the pro-independence party Plaid Cymru.
"People are 'coming out' for Welsh independence," said Price, who is gay. "Much like those Pride marches from years ago, people are gaining confidence by marching together in an unabashed, unashamed and unapologetic way."
It's not just on the streets. An ecosystem of pro-independence meme accounts have sprung up on social media. One of these, "Fiery Welsh Memes for Feisty Independent Dreams," has quadrupled its audience in the past eight months, according to Joe and Haf Williams, two of the students who run it.
"What our page does is take Welsh politics, a usually dull or ignored subject for young people, and presents it in a way that's accessible and funny," said Joe, 22.
"The main focus, though, is comedy," Haf, 19, added. "If you can make people laugh, you can make them listen."
Price, the pro-independence leader, traces his secessionism back to the 1980s, when Thatcher's Conservative government closed down coal mines in Wales and elsewhere.
Rich natural resources made Wales one of the centers of the industrial world in the 19th century. Today it is one of the poorest parts of Europe, with almost 30 percent of children in poverty and a falling life expectancy.
Some economists say because Wales is heavily subsidized by the government in London, it is too small and has too little taxable income to go it alone. But like his comrades in Scotland, Price flips this equation on its head.
"We're poor because we're not independent," he argued. "Independent countries do something about their poverty because they are incentivized to do so."
"In the U.K. you've got a union which is vastly over-dominated by one constituent country: England," he added. "And therein lies the source of all our problems. The only way to resolve that is to through independence."
One part of the U.K. that almost certainly isn't going anywhere is England. It's the dominant family member, in terms of population, land mass and money. Go to any sports event between England and one of its neighbors and you'll leave with ears ringing from the profane insults against the de facto seat of power.
Its influence is so great that foreigners often treat England and the U.K. interchangeably, like when President Donald Trump mistakenly tweeted that he had met the "Queen of England (U.K.)." This is maddening for the other nations, but it's not new. The 1888 Encyclopedia Britannica read, "For Wales, see England."
The key to the U.K.'s success has been England's ability to downplay this dominance so the union feels more balanced, said Hazell, the professor at University College London.
Unlike the other three nations, England does not have its own devolved Parliament. And, because of the quirks of how U.K. funding is distributed, English citizens actually receive less government spending per head than anywhere else.
Brexit has provided a conduit for this simmering frustration by unleashing — or being unleashed by — a resurgence in English nationalism, Hazel said.
"I think English nationalism is the genie that has half been let out of the bottle by Brexit," he said. "If English nationalism is really allowed to let rip, that's dangerous for the U.K."
What astounds Blair is that so many Conservatives seem blasé about the union — especially given that their party's full name is the Conservative and Unionist Party.
"The Conservative Party was always the protector of the union — I mean it's in their DNA!" he said. "But if you actually look at the polls of Conservative Party members, they are indifferent to whether we lose Northern Ireland and we lose Scotland."