(CNN) -- On September 18, Scots go to the polls to vote on the future of their country.
It's a vote that could end Scotland's 307-year union with England and Wales as Great Britain -- and see it launch into the world as an independent nation of some 5.3 million people.
When campaigning began, that seemed a far-fetched prospect. But the most recent polls suggest that what many Britons consider unthinkable could happen -- and the United Kingdom as we know it could be torn asunder.
Here's what you need to know about the landmark referendum.
What are the Scottish voting on, and why?
Voters will be presented with a simple yes/no question: Should Scotland be an independent country?
The Scottish government, led by the Scottish National Party, says this is a "once in a generation opportunity" for Scotland's people to take control of the decisions that affect them most. A "yes" vote means that "Scotland's future will be in Scotland's hands," it says, and that life will be better and fairer for its people.
British Prime Minister David Cameron wants Scotland to remain part of an undivided United Kingdom of Great Britain and Northern Ireland. He says that it is a decision solely for the Scottish people -- but that remaining part of the United Kingdom will give them security and strength. "There will be no going back," he warns.
Because the United Kingdom has no written constitution, there's no established law to govern the process. So these are truly uncharted waters.
Why is this significant to the rest of the world?
The question mark over Scotland's future is already having an impact on domestic and international business. Some worry that the breakup of the United Kingdom could undermine London's standing as an international financial capital.
Last month, 130 business leaders published an open letter in which they warned of the impact of uncertainty over issues including currency, regulation, tax, pensions, EU membership and support for Scottish exports. A day later, more than 200 other business leaders signed an open letter backing an independent Scotland.
The British pound sank Monday after the first poll that showed the "yes" vote in the lead, with CNN Money reporting that it reflected uncertainty over the outcome of the referendum and an increased risk of a "messy divorce."
The UK's defense capability could be affected. The Scottish government says it wants to remove nuclear weapons from Scotland as soon as possible -- namely, the UK Trident nuclear submarine fleet based at Faslane. The Scottish government says, "It is our firm position that an independent Scotland should not host nuclear weapons and we would only join NATO on that basis."
Scotland would have to renegotiate its entry to both NATO and the European Union if it votes for independence. EU leaders have signaled they would take a hard line and make Scotland apply to join like any other independent nation. However, the "yes" campaign says it could easily be done through amendments to existing treaties.
If Scotland chooses to split from Britain, it could give other people ideas.
The debate is being closely watched by independence movements in Spain's Catalonia province, Canada's Quebec province and France's Mediterranean island of Corsica.
If Scotland votes to leave, the British Prime Minister will likely come under pressure to resign -- although he has told UK media "emphatically" he will not do so. The major Westminster parties have promised to devolve more powers to Scotland if it chooses to stay in the union.
Who can vote?
Thanks to a bill passed last year extending the vote to 16- and 17-year-olds, almost everyone living in Scotland who is 16 or older on the day of the referendum will be able to vote.
This means English or Welsh citizens who reside in Scotland can take part. But Scots who are living elsewhere in the United Kingdom or overseas will not be entitled to cast a ballot.
It also means that the residents of England, Wales and Northern Ireland get no say on a historic change to the makeup of the United Kingdom.
What's the history behind the vote?
Scotland has long had a testy relationship with its more populous neighbor. The Act of Union in 1707 joined the kingdom of Scotland with England and Wales, but many Scots were unhappy at being yoked to their longtime rival south of the border.
Since 1999, Scotland has had a devolved government, meaning many, but not all, decisions are made at the Scottish Parliament in Holyrood, Edinburgh. In May 2011 the nationalist Scottish National Party, which had campaigned on a promise to hold an independence referendum, surprised many by winning an outright majority in the Scottish Parliament.
In October 2012, the UK and Scottish governments agreed that the referendum would be held, and the question to be put to voters was agreed on early last year.
Dauvit Broun, a professor of Scottish history at the University of Glasgow, said one driving force for the vote was the widening gulf between the policies pursued by the coalition UK government in Westminster, led by the Conservative Party under Cameron since 2010, and what the Scottish people want.
Many Scots are strongly opposed to the current Westminster government's attempts to reform -- or in their eyes dismantle -- the welfare state and say it was not elected by them. Illustrating that sentiment, there's only one Conservative MP in Scotland at present, leading humorists to point out that even giant pandas are better represented (Edinburgh Zoo has two.)
"Since the period of Margaret Thatcher, there has been a growing divide, and a sense that what Scotland feels consensus about ... has become more and more different to England," Broun said.
Looking further back, Scotland and England have been growing apart since the demise of the British Empire, Broun says. The decline of the Presbyterian church in Scotland, which provided a sense of self-government and Scottish identity, has also played a part in fueling the desire for independence, he said.
Who are the main players?
Scotland's First Minister Alex Salmond is the ebullient leader of the pro-independence campaign. Labour MP Alistair Darling, who represents an Edinburgh constituency, heads the pro-union Better Together campaign.
The pair have taken part in two TV debates, with Darling widely judged to have come out on top in the first, by a narrow margin, and Salmond to have done significantly better in the second.
David Cameron has also spoken strongly in favor of Scotland remaining part of the United Kingdom.
It's not just UK politicians who are getting involved. Singer David Bowie, in a Brit Awards acceptance speech delivered by supermodel Kate Moss, pleaded, "Scotland, stay with us."
Former Manchester United football club manager Alex Ferguson also opposes a split and has backed the "Better Together" campaign. "800,000 Scots, like me, live and work in other parts of the United Kingdom. We don't live in a foreign country; we are just in another part of the family of the UK," he is quoted as saying.
Harry Potter author J.K. Rowling, who lives in Scotland, made a hefty donation to the Better Together campaign, and Beatles star Paul McCartney has signed a letter urging Scottish voters to stick with Britain.
The Yes campaign has its own celebrity backers -- including former James Bond star Sean Connery, actor Brian Cox, and comedian Frankie Boyle.
Actor Alan Cumming launched the Twitter campaign #goforitscotland. "What's happening now in Scotland is the most exciting political and social discourse that will forever change our destiny. Check it out!" he tweeted as the vote nears.
Even U.S President Barack Obama has gotten in on the act. He acknowledged it was a decision for the people of Scotland, but added: "We obviously have a deep interest in making sure that one of the closest allies we will ever have remains a strong, robust, united and effective partner."
What are the key issues?
Questions over the economy have dominated the debate.
The Scottish government argues the country would be better off after independence, largely based on its taking control of revenues from North Sea oil and gas found in Scottish waters. It says it would manage the energy industry better, invest to boost production, and create a wealth fund, similar to Norway's oil fund, to benefit future generations.
But not everyone agrees with the Scottish government's rosy assessment. A report by the Institute for Fiscal Studies in March said the latest figures showed Scotland's budget deficit had worsened relative to the rest of the United Kingdom, thanks to falling North Sea revenues and higher public spending north of the border. It also warned of the dangers of relying too heavily on a volatile and ultimately finite income source.
The Scottish government says the economy is diverse, with other key elements including food and drink, tourism, creative industries, universities, financial services and manufacturing.
Another big issue is what currency an independent Scotland would have.
Scottish First Minister Alex Salmond has said he wants Scotland to continue to use the pound in a currency union with the rest of the United Kingdom, and that it has the right to do so.
But the three main parties in Westminster -- David Cameron's Conservatives, their coalition partners the Liberal Democrats, and Labour -- have all said this won't be an option. The Scottish government responded that this was "bullying" from Westminster.
It's unclear what would happen to Scotland's share of UK debt if it's not part of a currency union.
What's the mood?
A series of opinion polls in past months has given the pro-union camp a lead. However, the most recent polls have shown that shrinking or disappearing altogether.
A YouGov poll conducted for The Sunday Times and released on September 7 caused waves when it showed the "yes" vote narrowly in the lead for the first time, excluding undecided voters. YouGov President Peter Kellner said it indicated support for the Better Together campaign had fallen "at an astonishing rate."
Of course, it's just one poll among many.
The latest poll of polls by ScotCen, an independent research center, shows the "no" camp hanging on to a narrow lead over the "yes" camp, excluding undecided voters.
Many in the Yes campaign feel they have a momentum of support that has built toward the September 18 vote.
But the No campaigners are confident they represent the silent majority who, after considering all the factors involved, will decide against independence.
What would happen if Scotland votes Yes?
Should Scots defy Westminster's expectation and vote "yes," there will be a flurry of activity to ensure everything is in place for Salmond's projected independence date of 24 March, 2016.
Upon confirmation of a victory, the Yes Scotland leader will put together his "Team Scotland" negotiating team. It is expected to include his deputy, Nicola Sturgeon, alongside a broad, cross-party group.
Cameron -- if he hasn't been forced to resign after presiding over the breakup of the UK's 300-year-old union -- will need to form his own negotiating side.
Chief among the matters up for negotiation are the currency union and Scotland's share of the UK's national debt, the relocation of the Trident fleet and even potential border controls. The "yes" campaign has said it intends for Scotland to remain part of the Common Travel Area, which allows free movement for citizens of the United Kingdom, the Republic of Ireland, the Isle of Man and the Channel Islands.
Cameron will have to move quickly to avoid financial instability by giving a definitive answer to the currency question, and there has been suggestion that the next UK general election -- scheduled for May 2015 -- might be postponed until after Scotland has exited the union.
The Scottish government would have to set in motion a process to produce a written constitution. Queen Elizabeth II would remain head of state.
An independent Scotland would also have to negotiate paths to membership for both NATO and the European Union, two international organizations that the "yes" campaign says are in Scotland's future.
CNN's Euan McKirdy, Susannah Cullinane and Richard Allen Greene contributed to this report.