LONDON -- Britain is under 72 hours away from a once-in-a-lifetime vote on Scottish independence that could break up the 307-year-old United Kingdom, splitting apart one of America’s key global allies. With polls suggesting that a Scottish split from the rest of Britain is a real possibility, lawmakers including Prime Minister David Cameron are making urgent appeals to save Britain its biggest constitutional upheaval since the War of Independence that led to the creation of the United States.

What will be voted on?

More than 4.2 million people in Scotland -- or 97 percent of the adult population -- have registered to vote on whether or not to remain part of the United Kingdom.

Scotland was an independent country until 1707, when the Act of Union with England led to the creation of Great Britain and, ultimately, the United Kingdom -- which also includes Northern Ireland and Wales.

It retained its own separate legal and education systems and was granted its own devolved parliament in Edinburgh in 1999 but control of defense, borders and taxation remain with the U.K. parliament in London’s Westminster and ultimate authority lies with Queen Elizabeth II.

Britain's government agreed to recognize the referendum, gambling that a likely 'No' vote would kill off the issue of Scottish independence for decades. But with polls now putting a 'Yes' vote within reach, the U.K. faces the serious prospect of a break-up.

Why does Scotland’s independence vote matter to America?

The White House says Thursday’s poll is “an internal matter” for Britain, but there are good reasons why the United States is nervous that one of its key global allies could be about to break apart.

Chief among these is the future of the joint U.S. and U.K. nuclear deterrent system. Scotland is home to 58 U.S. Trident II D-5 missiles leased from Washington by the British government, but Scotland’s government wants to ban nuclear weapons on moral grounds within four years of gaining independence. That could force London to relocate the weapons to alternative bases in England or return the weapons to the U.S., costing billions of dollars and leaving NATO without a European nuclear deterrent precisely at a time of heightened security concern.

An independent Scotland would have its own defense force and would likely remain an American ally, but any weakening of Britain’s defense capability would be a worry for the Pentagon. In January, former U.S. defense secretary Robert Gates said cuts in defense spending were already threatening Britain’s “ability to be a full partner.” Almost 30 members of Congress have signed a resolution calling for the U.K. to remain united, saying it was “important for U.S. national security priorities in Europe and around the world.”

Britain’s relationship with the European Union could also be under threat in the event of Scottish independence. Without Scotland’s phalanx of left-leaning Labour lawmakers, Westminster would likely be dominated by David Cameron’s Conservative Party, which favors a referendum on British withdrawal from the E.U.

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How likely is Scotland to become independent?

Within two generations, Scottish independence has gone from an eccentric fringe movement to a dominant political force that is on the cusp of victory.

Until last month, the pro-Union Better Together ‘No’ campaign enjoyed a consistent and comfortable lead in opinion polls but a sudden surge in support for the pro-independence ‘Yes’ side has all but eliminated the gap. A “poll of polls” published by ScotCen on Monday puts ‘No’ at 51 percent and ‘Yes’ just two points behind, at 49 percent. “In a vote where the winning side needs 50% + 1 of votes cast, it is clear that neither side can now be completely confident of victory,” the research organization said.

A major YouGov poll last week gave the ‘Yes’ side a slim majority, spooking the global markets and causing the pound sterling to sink by 1.3 percent on foreign exchange markets before recovering. British lawmakers, who previously vowed to stay out of Scotland’s national debate, have been making urgent visits to Scotland to implore voters to save the United Kingdom.

What is the case for independence?

The ‘Yes’ campaign is led by First Minister Alex Salmond, whose Scottish National Party has governed since 2007. It says Scots should have total control of their own affairs and that revenue from Scotland’s offshore oil fields would sustain the country’s economy. Support for independence was boosted by the election in 2010 of a Conservative British government, angering voters in Scotland where the Conservative party remains deeply unpopular.

What is the case for keeping the United Kingdom?

The ‘No’ campaign says and independent Scotland would be weaker on the world stage and would have to raise taxes to pay for the replication of institutions and services currently shared with England, such as defense forces and state pensions. Many cross-border businesses have warned that they might withdraw from Scotland in the event of independence, threatening jobs.

For the United States, the pros and cons are naturally about strategic interests. “On the one hand, it is perfectly reasonable for Washington to assert its own interests by continuing a relationship with Westminster it heavily dominates,” said Jonathan Sher, a charity director from North Carolina who has also applied for dual citizenship and has publicly backed the ‘Yes’ side. “On the other hand, it is just as reasonable for Scotland to pursue its own interests in what will inevitably remain a friendly set of connections with America. Profound family, historical, musical, academic and trading ties between my two nations provide a firm foundation upon which we can and will build.”

How star-studded has the campaign been?

Edinburgh-born James Bond actor Sean Connery has been a decades-long supporter of Scottish independence, alongside Gerard Butler ("The Bounty Hunter"), Brian Cox ("The Bourne Supremacy," "Braveheart") and Alan Cumming ("The Good Wife"). Fashion designer Vivienne Westwood has also said a ‘Yes’ vote would be "great.”

Hollywood and Broadway star Alan Cumming joins the Deputy First Minister Nicola Sturgeon and "Yes" campaigners in Glasgow on Sept. 8. Jeremy Sutton-Hibbert / Getty Images Contributor

Harry Potter author J.K. Rowling donated £1 million ($1.6 million) to the ‘No’ campaign, publicly declaring that Scotland would be stronger within the U.K. Other high-profile Better Together backers include actors Helena Bonham Carter, Patrick Stewart and Judi Dench as well as England soccer star David Beckham. The former Manchester United and L.A. Galaxy star said Monday that "what unites us is much greater than what divides us.”

What would an independent Scotland look like?

Scotland’s 5.3 million citizens represent about eight percent of the total U.K. population and would create a new country bigger than Ireland (4.5 million) but smaller than Denmark (5.5 million).

Its global economic position would depend greatly on what share of offshore oil revenue would be negotiated with the remainder of the U.K. – and how long that oil would last. The Scottish National Party believes lower corporate taxes would encourage investment and would allow Scotland to lower its reliance on oil in the longer term.

What would happen to the queen?

The Scottish National Party plans to keep the monarchy allowing Queen Elizabeth II to remain Queen of Scotland just as she is also queen of other British Commonwealth countries such as Canada and Australia.

She has not formally expressed an opinion on independence, but told a member of public on Sunday that she hoped voters “think very carefully about the future” in Thursday’s poll. She was speaking in Scotland, where she spends several weeks of the year at her summer retreat, Balmoral Castle.

Would Scotland immediately become independent on Friday?

No. The referendum has no direct legal power, but the British government has promised to negotiate Scotland’s split from the United Kingdom in the event of a ‘Yes’ victory, with an official split slated for March 2016. Unlike the United States, the U.K. does not have a written constitution - its political institutions and systems have evolved over the centuries – and so the re-creation of an independent Scotland would be uncharted territory. Scotland’s government wants to keep the pound sterling by forming a currency union tied to the Bank of England. However, Britain’s Treasury chief George Osborne has warned that the rest of the U.K. may not agree to such a deal – leaving an independent Scotland to bring in its own currency.

Scotland would also have to renegotiate its membership of the European Union. Its government remains confident that it would still meet all the criteria for membership after March 2016, but it could face tough opposition, most notably from Spain which is seeking to thwart the ambitions of an independence movement in Catalonia.

What happens if Scotland votes ‘No’?

No matter what happens, Scotland’s relationship with the rest of the U.K. will change. In a last-ditch attempt to persuade Scots to vote ‘No’, Cameron last week promised future extra powers for Scotland’s existing devolved parliament if Scots chose to vote 'No'. However, this olive branch may be too late to change Thursday’s result; Cameron had rejected extra powers as a middle-way option on the ballot paper, gambling that most voters would choose 'No' if given a straight Yes/No question on independence. Offering more devolved government at such a late stage has been dismissed by the ‘Yes’ side as a panic measure.

And whatever the outcome, the 18-month campaign has left social divisions as supporters on both sides trading acrimonious barbs over social media using the hashtag #indyref. Thursday’s poll has become a topic to avoid in bars and family gatherings. Fiona Scott, whose father taught Salmond at school, wrote in an open letter to The Herald newspaper on Saturday that the referendum “has succeeded in creating divisions across Scotland that were not there before and will still exit after the referendum, no matter which way the vote goes.” She added: "Relationships between neighbors are now threatened if you indicate which way you are voting."