On the southern flank of England, buffeted by the fierce winds of the English Channel, Portsmouth has been a hub of the British Navy – and British power – since the reign of Henry VIII 500 years ago. Without Portsmouth, Britain’s empire could not have become the largest in the world.

Here, bristling with cannons, sits Lord Nelson’s flagship, the HMS Victory, which led to Britain’s triumph in the 1805 Battle of Trafalgar against the French and Spanish and thus to 100 years of British supremacy of the seas. Here, ahead of World War I, Britain built the potent Dreadnoughts that touched off a naval arms race around the world.

“Portsmouth was arguably the biggest, most advanced military industrial complex in the world,” says Jock Gardner, a naval historian with Britain’s Ministry of Defense.

Even today, the docks at Portsmouth reflect the city’s crucial role in powering Britain. An aircraft carrier, destroyer, and frigate line up in port. Portsmouth remains the home of most of the Royal Navy’s surface fleet.

Yet the coastal city has also been subject to recurring military cuts since World War II, giving it today the feel of a living museum more than a bustling Navy town. Now one more part of its seafaring identity is vanishing: In late August the city closed its last shipbuilding facility, leaving England without the ability to produce large advanced warships.

In the grand scheme of all that is Britain, its military, its economy, and its global historic clout, this is a side note. But it’s taken on greater political and symbolic significance in a year in which Britain is questioning its standing in the world – perhaps more than at any time since the end of World War II. Britain faces two major referendums, one of which would physically shrink its population, reduce its landmass, and put key defense capabilities at risk. The other would sever its historical ties to Europe.

Scotland is holding a vote on Sept. 18 on whether it wants to stay part of the United Kingdom, to which it has belonged for 307 years, or whether citizens will forge forward with a smaller but independent nation that secessionists argue will be more aligned with Scottish sensibilities. At the same time, amid a rise in anger at the European Union, Prime Minister David Cameron is moving forward with a referendum on whether Britain – or what remains of it if Scotland goes independent – wants to continue to be a member of the EU, the world’s largest market.

Many argue that a vote to leave, in either case, could mean Britain’s influence in the world is irrevocably diminished, including its “special relationship” with the United States. This had led to apocalyptic pronouncements. George Robertson, the former secretary-general of NATO, said that Scottish independence would be “cataclysmic” for the world. Yet there is also a growing feeling in Britain that fewer foreign entanglements, whether military or economic, after centuries of “punching above its weight,” might be the right role for a mid-size power in the 21st century. Either way, Britain finds itself at a hinge moment, searching for its place in a postimperial, postindustrial, globalized age.

“These are big, big relationships, and they have been fundamental in how the people of these isles have seen themselves,” says Robert Colls, professor of cultural history at De Montfort University in Leicester. “There have been so many shifts over the past couple of generations, probably more than in any commensurate period in our modern history, and I don’t think the British people have quite caught up with it.”

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At its zenith at the turn of the 20th century, the British Empire ruled roughly 25 percent of the world’s population and made up more than a quarter of its land surface. It was a stunning rise given Britain’s late start to empire-building, more than a century and a half after Christopher Columbus had established outposts in the Americas, writes Niall Ferguson in his 2003 book, “Empire: How Britain Made the Modern World.”

But Britain progressively caught up and overpowered its European rivals, through prowess and piracy on the seas, a knack for copying the financial institutions pioneered by the Dutch, and through its position at the heart of the Industrial Revolution, he writes. In 1880 Britain’s share of world manufacturing production was 23 percent. In 1909 the empire stretched 12.7 million square miles across five continents, with India at its heart. “Rule, Britannia!” was the patriotic song that encapsulated the mood of the 18th and 19th centuries.

But as the 20th century dawned, attitudes about Britain’s rule hardened. It began with atrocities committed against civilians during the second Boer War, as Britain scrambled for power in Africa. Mr. Ferguson likens it to what America went through with Vietnam. It gave rise to peace movements at home and fostered unyielding questions about Britain’s aims abroad.

The human and financial cost of two world wars in the 50 years to follow reinforced those shifts in sentiment – both domestically and within the colonies themselves – and rendered Britain unable to pay for its vast network overseas. Domestic concerns, including building a comprehensive welfare state as other postwar European nations did, took precedence. Before World War I, the world consisted of 59 independent nations. By 1950 it was 89. By 1995 it was 192.

While Britain no longer can claim superpower status, it is still one of the world’s most influential countries. It has the world’s fifth largest military, is one of five permanent members of the United Nations Security Council, and has nuclear capability. It boasts the sixth largest economy. When the US needs an ally, it turns first to Britain.

There have been many moments of British unity and pride in the past half century, from the victory in the Falklands war off the coast of Argentina in 1982 to hosting the 2012 London Olympics 30 years later. But “declinism” has also been a recurring theme, from deindustrialization that began in the 1970s and turned Britain’s economy from a manufacturing economy to a services economy, to the aftermath of the 2008 global financial crisis.

Through it all, Britain’s leaders have continued to tout the nation’s clout in world affairs. Former British Prime Minister Edward Heath dubbed Britain “a medium power of the first rank” in December 1970. Former Foreign Secretary Douglas Hurd said in 1992 that Britain “punched above her weight in the world.” In November 2010, Mr. Cameron rejected the notion of decline altogether. “What I have seen in my first six months as prime minister is a nation at the center of all the big discussions,” he said.

But since then he’s had to contend with secessionist sentiments that threaten to put Britain one step further from the center of world affairs.

• • •

The Scottish Parliament, a sleek steel, oak, and granite structure, sits in the shadow of Edinburgh’s Salisbury Crags at the end of Royal Mile. On this day the glass-paneled chamber is host to a fiery discussion on welfare reform. But if supporters of independence for Scotland have their way on Sept. 18, it could become the legislature of a separate nation.

The Kingdom of Scotland signed the Act of Union with England in 1707. Scotland has increasingly been granted more powers, getting this parliament in 1999. It has also historically had its own education and legal systems.

But the Scottish National Party (SNP), led by Alex Salmond, says that conservative Westminster is increasingly out of touch with left-leaning Scotland. And only full independence, including control of North Sea oil wealth that would turn it into one of the richest, and most equitable, nations in the world, will enable the kind of society that Scotland aspires to.

Polls have consistently put the “no” vote in the lead, but the gap has been narrowing and the race is now too close to call. One recent YouGov poll put the "yes" vote slightly ahead. Their latest survey, published days later, revealed that 48 percent of Scots would vote for independence, while 52 percent would not. “Many understand this as the biggest choice we will make in our lives about the country we live in,” says Angus Robertson, the SNP’s campaign director.

Independence would bring modern Scotland new opportunities and new uncertainties. It would have to work out prosaic things such as diplomatic postings, passport controls, and when to hold an independence day holiday. But there would also be bigger questions to resolve about currencies and membership in the EU and NATO.

Britain has much at stake, too. Without Scotland, it would lose 5 million citizens and 32 percent of its landmass. It could potentially lose some of its key defense capabilities, since Britain’s only operational nuclear arsenal – four submarines armed with Trident nuclear warheads – dock in Scotland (the SNP has promised to eventually remove them if they win). The same is true for warship manufacturing, since Britain’s lone yard to build large naval vessels lies in Scotland.

“As a nation we depend on trade by sea,” says Gerald Vernon-Jackson, a city council member in Portsmouth. “If Scotland votes for independence, we would have to go to a foreign country to build warships. That’s like America saying we can’t build warships, so let’s go to China.”

Scottish Labour parliamentarian Jackie Baillie, who represents Scotland’s “Better Together” campaign, says that being with the UK has brought Scotland stability – or as she puts it, “300 years of what is probably the most stable economic, political, and social union in the world. Why would you want to break that up?”

Mr. Robertson counters that many Scots understand it would be leaving a legacy of power, but that ultimately it is in the best interest of the Scottish people. The SNP opposes Britain’s nuclear ambitions. He says the “illegal” invasion of Iraq by Britain is also not the kind of “clout” that Scotland seeks. Instead, he sees Scotland, like its Scandinavian neighbors, asserting itself on the world stage through peacekeeping, and environmental and other issues.

Politicians in London don’t want Scotland to leave, which they made clear when they came together in rare unity to say Scotland cannot continue using the pound if it becomes independent. If Scotland chooses independence, the balance of power would shift to conservatives in the House of Commons. Scots are also more pro-EU than their English counterparts, meaning the “out” vote in any EU referendum would gain momentum without their participation.

Many British citizens say they don’t want to see Scotland leave, either. A poll from the Institute for Public Policy Research last year showed that 49 percent of those surveyed feel that Scotland should remain part of the UK, compared with 30 percent who supported independence.

Perhaps no residents are more anxious about potential change than those on the Scottish-English border. Gail French is walking her dog in Berwick-Upon-Tweed, an English town with medieval stone walls and Elizabethan ramparts that has changed hands between Scotland and England for centuries. Ms. French, like many residents here, was born in Scotland but ended up on the English side of the border.

For her a “yes” vote would be a logistical disaster – and possibly a financial one. She works in the nuclear facility on the Scottish side. “I think Scotland is far better off in the union than out,” she says.

Even if Scots do vote to stay part of the UK, Britain may have to contend with emboldened regions that reject the consolidated power of Westminster. Former Labour Prime Minister Gordon Brown, in a Guardian op-ed piece in June, put it this way: “The Scots have already sunk without trace the old idea of Britain as a unitary state.”

• • •

Even as the UK wants to keep Scotland part of its union, a growing number in Britain want the nation to leave the EU. In a bid to placate growing Euroskepticism, Cameron promised to renegotiate Britain’s relationship with the EU and then hold a referendum on membership in 2017, if he wins reelection next year.

In an Opinium/Observer poll in June, 48 percent of respondents said they would definitely or probably vote to leave the EU under its current arrangement, while 37 percent said they would definitely or probably vote to stay in.

Britons have always been on the periphery of the bloc, both physically as an island and politically, since they have often felt closer to the US than to continental Europe. Britain only joined the European community in 1973 and never gave up the pound for the euro. In a YouGov poll in May, only 17 percent of British respondents said they felt strongly European, compared with 33 percent of French and 43 percent of Germans.

Anti-European sentiment has been rising since the financial crisis in 2008. One group that has exploited the mood is the United Kingdom Independence Party (UKIP), which took the highest percentage of votes, 27.5 percent, in European parliamentary elections in May.

Younger Britons are far more supportive of the EU than older ones. So are the better educated and well off. Tellingly, in London, it’s hard to find those who wish to cut ties.

“I say in my own life, if you want to go somewhere fast, go alone; if you want to go far, do it together,” says resident Colin Easby, who opposes both Scottish independence and Britain severing ties with the EU.

Some of UKIP’s supporters are conservatives who believe that Britain would be much stronger economically without the constraints of the EU. But UKIP also appeals to those who feel “left behind” by globalization – particularly in Britain’s postindustrial heartland, where workers are angry about joblessness, low wages, and high numbers of immigrants coming to the UK, which they blame in part on the EU.

The town of Blyth, on the North Sea, sits in Britain’s rust belt. A former shipbuilding and coal mining town, it is now rife with unemployment. A group of old friends from the coal mining days sit in the plaza, slipping into Geordie, the dialect from the Tyneside region, as they commiserate.

Michael Rutherford began working in the coal mines at 17, and when the industry declined in the 1970s, he got a factory job. Later he worked in a department store warehouse, until a year ago, when that closed. “I’m 61. Who is going to employ me?” he asks.

Brian Baxter started working at the shipyards when he was 15. Then he went into the coal mines and eventually the factories. “Back then, everyone was working,” says Mr. Baxter.

Blyth is a Labour Party stronghold. But UKIP is making inroads – its vote in EU elections in the Northeast region was up by 13.8 percent from the last EU elections. Both Mr. Rutherford and Baxter agree with some of UKIP’s core messages, even if they still vote Labour. “People are coming in and taking our jobs,” says Baxter.

That kind of sentiment, heard in many places outside London, is driving Britain closer to leaving the EU, which many say could have grave repercussions. “The elections this year were remarkable in how they demonstrated how impervious London was to UKIP’s appeal,” says Martin Farr, a professor of contemporary British history at Newcastle University. “If Scotland leaves the union and the remnants of [Britain] leave the EU, there would be a diminution in Britain’s status and influence in the world, however you measure it.”

• • •

Nor would the ramifications be felt in Britain alone. They would reverberate throughout Europe and even across the Atlantic. No country in Europe wants to see Britain leave, especially powerhouses Germany and France, which have long turned to Britain for hard power and, depending on the issue at hand, to bolster their positions.

For Hans Kundnani, research director at the European Council on Foreign Relations in London, Europe could lose as much from a “Brexit,” as it’s called, as Britain.

“It could begin the complete breakup of Europe. It is a moment of disintegration,” he says. “Europe has succeeded until now in part because of a careful balance of power” between France, Germany, and Britain.

Europe has already been frustrated by Britain’s turn inward. “There has been a steady erosion in the UK’s capacity to be a global leader,” says Fredrik Erixon, director of the European Center for International Political Economy (ECIPE) in Brussels. “It’s a country which is increasingly occupied with smaller identity politics, like immigration, a country which has less appetite or less understanding for the principle of international cooperation.”

The Scottish vote for independence could also have repercussions beyond the EU. “The UK has been a very stabilizing force in the world,” says Tom Gallagher, a political analyst in Edinburgh. “And I think the UK, if it breaks up, would give a very ominous signal to other parts of the world, where there are restive minorities.”

Still, while allies want London to continue asserting itself in world affairs, many in Britain are saying that it’s time for the UK to focus on itself. A telling moment came last August, after the British Parliament voted against a possible joint military strike with the US on Syria despite Cameron’s support. Some saw it as the clearest signal yet of British isolationism. Others called it a welcome reality check.

“If that is a consequence of this vote, then I will be absolutely delighted that we relieve ourselves of some of this imperial pretension that a country of our size can seek to be involved in every conceivable conflict that’s going on around the world,” conservative lawmaker Crispin Blunt told the BBC. “If it makes British foreign and defense policy rather more limited and rather more sustainable with our own resources and our own size, so much the better.”

• • •

At the historic shipyard in Portsmouth, Jane Skinner stands on the deck of the HMS Warrior, the world’s first iron-hulled, armor-plated warship, which revolutionized naval warfare. “She was so big and so fast no one could touch her,” says Ms. Skinner, the ship’s business manager. Not unlike the British Empire itself? “Exactly,” she says.

Keri Charlesworth, who was out of a job for three years before recently finding work as a truck driver, sits in front of the ship eating chips with his wife, Sara, and daughter, Amy. The end of British supremacy is in many ways “a shame,” he says. And yet, “it’s finished, and it’s time to focus on our own. We are always poking our nose into everyone’s business.”

His opposition to more engagement with the world is a view ascendant in many countries. It manifests itself in different ways – as anti-immigrant sentiment, as secessionist movements, as people forming political alliances around their own interests.

“With the acceleration of globalization over the past 20 years, you begin to look for the particular cultural trait that makes you different from other societies,” says Mr. Erixon of ECIPE.

But it’s worrisome to many who say integration is the best way forward – for Britain and the globe. Portsmouth arose and thrived because of ties forged with the rest of the world.

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“Scotland should stay in Britain, and Britain should stay in the EU,” says Mr. Vernon-Jackson from the Portsmouth City Council. “And we should belong to NATO. We should belong to everything.”

In 1624, English poet “John Donne wrote that ‘no man is an island,’ ” Vernon-Jackson adds. “That is certainly still true today.” ρ