When Scotland voted in an independence referendum in September 2014, nationalist leaders pitched it as a once-in-a-generation chance to break a three-century-old bond.

But less than two years after Scots opted to remain in the United Kingdom, the specter of secession again looms over the lush green expanse of the British isles. The trigger this time is another referendum with existential impact: this month’s vote on whether to leave the European Union.

[What’s a ‘Brexit’? A guide to Britain’s E.U. drama for confused non-Europeans.]

If Britain chooses to ditch the E.U. despite a vote to stay from the Euro-friendly Scots, nationalist leaders here say they will revive the push for an independent nation in order to keep Scotland inside Europe. And they think that the second time around, they would win.

“Pulling Scotland out of the European Union against our will would be a change in material circumstances,” said Alex Sal­mond, who led the campaign for independence in 2014 and now represents Scotland in the British Parliament.

In Peterborough, 45-minutes north of London, anti-E.U. sentiment runs strong. A large part of the discontent is the mass influx of Eastern European immigrants that residents say has transformed the ancient market town. (Griff Witte,Jason Aldag/The Washington Post)

In that scenario, he said, there will be “a referendum on Scottish independence within the next two years. And this time, the result would be ‘yes.’ ”

The potential for a British breakup as fallout from the June 23 referendum underscores just how much is at stake when the country decides whether to become the first nation to withdraw from the 28-member E.U.

A shock to the global economy, a rupture in the Western alliance and a change in occupancy at 10 Downing Street are all possible consequences of a British vote to leave — popularly known as “Brexit.”

The very existence of Great Britain could also be on the line.

British Prime Minister David Cameron reluctantly offered the public a direct say over the country’s E.U. membership for much the same reason he acceded to the Scottish call for an independence vote in 2014: He thought it was the only way to settle the fundamental questions at the heart of British identity. Is the United Kingdom part of Europe or not? Is it one nation or more?

[Scottish leader: If Britain leaves the E.U., we’ll leave Britain]

But the potential for a British exit from the E.U. to reawaken the push for Scottish independence reflects just how much Cameron’s strategy may have backfired. Instead of laying the issues to rest, critics say, he may have unleashed the age of the “neverendum” — a prolonged period of turbulence that does not stop until the public votes to take Britain out of Europe and split Scotland from the United Kingdom.

“In order to put these questions to bed for a generation, you need a vote of 60-40,” said Menzies Campbell, a veteran Scottish member of Parliament who supports keeping Scotland in Britain and Britain in the E.U. “If the losing side gets 45 [percent], they’re not going to give up.”

That was what pro-independence Scots won in the 2014 vote. Since then, their side has delivered a pair of electoral thumpings: The Scottish National Party won by huge margins in both the 2015 British parliamentary elections and in the Scottish parliamentary contests last month, suggesting that the appetite for independence has hardly ebbed. Opinion polls show that Scotland would be about evenly divided if the independence vote were re-run today.

If Britain chooses to leave the E.U. — despite Scottish objections — that could tilt the balance in the nationalists’ favor, reinforcing divisions between north and south.

The visceral anti-E.U. sentiment that runs through English politics can hardly be found north of Hadrian’s Wall, the ancient stone fortification that bisected Britain during Roman times. Polls show a decisive advantage for the “in” campaign in Scotland, while England flirts with “out.”

[Immigration backlash at the heart of British push to leave the E.U.]

The reasons for the difference are both historical and contemporary. Scotland has long had a close affiliation with continental Europe, going so far as to side with the French in wars against the English. As citizens of a small nation, Scots see membership in a broader European community as a comfort; the English are more likely to see rival power centers on the continent as a threat.

“There’s an emotional connection between Scotland and Europe,” Campbell said. “We’ve never had the residual antagonism toward Europe that has been maintained in England.”

But perhaps the most important reason for the split in opinion is immigration.

In crowded England — which makes up nearly 85 percent of the U.K. population but only about half the land — many people regard arrivals from elsewhere in Europe under the E.U.’s free-movement rules as an unwelcome burden. In sparsely populated Scotland — the entire population of 5 million is roughly equal to the inner boroughs of London — there is plenty of room for newcomers.

“Scotland is not full up,” Salmond said. “We’re much more like America of 100 years ago than the England of today.”

Scotland is not the only place in the United Kingdom where this month’s referendum threatens to bring politically destabilizing consequences. Welsh leaders, who tend to be pro-E.U., have said a British vote to abandon the union could spark a constitutional crisis. In Northern Ireland, where a tenuous peace has held for nearly two decades, a vote to leave would add a new line of partition to the Emerald Isle, with the Republic of Ireland inside the E.U. and the counties of Northern Ireland outside it.

Analysts have warned that such division could hinder the economy, prompt renewed border controls and revive dangerous levels of sectarianism.

In an echo of the nationalist push in Scotland, Catholic leaders in the generally pro-European north say that if Britain opts to leave the E.U., there should be a referendum on the reunification of Ireland.

Surveys suggest that Protestant voters would block any such move and keep Northern Ireland inside the United Kingdom. The polls in Scotland are far less clear, but the determination of nationalists to hold another referendum is not.

“The nationalists will use any justification to call another vote,” said Ross Thomson, a Conservative member of the Scottish Parliament who is among the few elected officials in Scotland campaigning for Brexit. “It doesn’t have to be the E.U. They’ll just do it when the polls look good.”

Other Brexit advocates who favor keeping Scotland inside the U.K. say they do not think the

E.U. matters enough to Scottish voters to make a difference in an ­independence vote.

“It’s very soft support,” said Robert Malyn, a pro-Brexit campaigner who was handing out fliers one recent afternoon at the central train station in Glasgow, Scotland’s largest city. “The E.U. is not loved enough to be a red line.”

The lack of enthusiasm is reflected in the difference between this campaign and the one in 2014. During the run-up to the independence vote, all of Scotland — from the Gothic back alleys of Edinburgh to the remote valleys of the Highlands — seemed bathed in the dueling paraphernalia of the “no” and “yes” camps. Signs hung from storefronts, buttons peeked out from jacket lapels, and fierce debates erupted nightly in pubs and across dinner tables.

[If Britain votes to leave the E.U., Germany could be one of the biggest losers]

This time, there is virtually no visible evidence that in less than a month, Scotland — and the United Kingdom — will be making such a consequential choice.

“The E.U. is such a big institution, and it seems far away from everybody. It’s a hard thing to get your head around,” said Jonny Ross-Tatam, president of the students association at the University of Edinburgh.

Still, Ross-Tatam has been making the case among his fellow students for why it matters to stay in the E.U. If Britain leaves, he said, research funding would be jeopardized and students could lose their ability to live, work and study across the continent.

“We can go to Sweden, Germany or France and not pay anything in tuition,” he said. “This vote is one of the biggest decisions that our generation is going to have to make.”

That is what campaigners on both sides told Scots in the lead-up to the 2014 referendum. But these days, such monumental decisions are coming often — and there could be another one looming.

Indeed, Salmond said that a second independence referendum will be held sooner or later, regardless of which way Britain votes this month.

“Independence is inevitable,” he said. “We’re just debating time scale now.”

Karla Adam in London contributed to this report.

Read more:

Immigration backlash at the heart of British push to leave the E.U.

Obama’s support for the E.U. is driving some Brits mad

What’s a ‘Brexit’? A guide to Britain’s E.U. drama for confused non-Europeans.

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