Scotland's First Minister Alex Salmond (R) welcomes British Prime Minister David Cameron (L) outside at St Andrews House in Edinburgh, Scotland, on October 15, 2012. (Andy Buchanan/AFP/Getty Images)

Britain and Scotland signed an accord Monday clearing the way for a historic vote on independence that could see this island’s northern lands stand alone for the first time in three centuries as Europe’s newest sovereign state.

Monday’s accord effectively launches a critical two-year independence campaign during which the Scottish National Party — whose surprise victory in regional elections last year propelled the push for a referendum — will go toe-to-toe against those fiercely opposed to rupturing modern Britain.

The deal agreeing on the terms of a Scottish referendum, to be held by fall 2014, comes at a time when independence movements are rapidly gaining strength in Spain and Belgium amid Europe’s brutal debt crisis. But the vote on independence for Scotland sets up the possibility that Washington’s closest strategic ally could be torn asunder.

“This marks the beginning of an important chapter in Scotland's story and allows the real debate to begin,” British Prime Minister David Cameron said after signing the deal with Scotland’s First Minister Alex Salmond, the National Party leader. “It paves the way so that the biggest question of all can be settled: a separate Scotland or a United Kingdom? I will be making a very positive argument for our United Kingdom.”

After centuries of bloody battles with the English, Scotland signed away its sovereignty in the early 1700s. By the late 1990s, however, it had won the right to a “devolved” Parliament, and it now has sweeping powers over its judicial system and public spending.

Full independence would give the ruling National Party the authority to fulfill a host of pledges, including the expulsion of the British nuclear fleet from Scottish waters, withdrawal from NATO and the removal of Scottish regiments from Britain’s military forces overseas. It would also give politicians in Edinburgh the freedom to vote separately from — and perhaps counter to — Britain in world bodies such as the United Nations and the International Monetary Fund.

The deal signed Monday means the Scots will be able to stage their 2014 referendum amid the emotional 700th anniversary of the Battle of Bannockburn, in which Robert the Bruce led Scottish troops to victory over the English invaders.

Recent polls have shown Scottish support for independence — which has hovered around a third to a quarter of the population — waning a bit since late last year. A solid majority of 55 percent are opposed to breaking away. But Salmond remains wildly popular in Scotland, and analysts say it would be wrong to discount his ability to mobilize a successful yes vote.

“At the moment, it appears a yes vote will be difficult, but you would be unwise to bet against Alex Salmond,” said Ben Page, chief executive of the London-based Ipsos MORI polling firm. “He will be thinking about the symbolism of the anniversary of the Battle of Bannockburn and looking to a Scottish generation that came of age with [the movie] ‘Braveheart.’ ”

A move toward independence would also lock Edinburgh and London in fierce negotiations over the cash cow that is North Sea oil — control of which is seen as essential to National Party dreams of Scotland emerging as a wealthy and progressive nation, mirroring energy-rich Norway. Independence for Scotland could rob the British state of a massive source of revenue, draining the coffers of a nation undergoing painful waves of austerity to cut its deficit and bring down the national debt.

Salmond on Monday vowed to run a “positive” campaign focused on arguing that Scotland would be “economically and socially” better off alone.

“The Scottish government has an ambitious vision for Scotland as a prosperous and successful European country . . . a Scotland with a new place in the world, as an independent nation,” Salmond said.

Sensing the force of Tartan pride, Cameron and the “no” campaign appear set to focus not on the economic question but on the other reasons for Scots to remain in the union, including the weight of history and the outsize influence Britain as a whole enjoys on the world stage.

Cameron, a Conservative, said he would “passionately” campaign to keep Scotland in the union, despite the fact that the opposition Labor Party — which has a strong following in Scotland — would stand to lose the most from independence and would find it much harder to win enough support to return to power in a smaller United Kingdom.

Regardless of the outcome of the referendum, Salmond’s party has promised to keep the British monarch as Scotland’s head of state, mirroring decisions by Australia, Canada and other Commonwealth nations.

Monday’s agreement amounted to a compromise. To win legal recognition of the vote from the British Parliament, Salmond gave up his insistence that the vote contain a second question asking Scots if they wanted even more autonomy from Britain as an alternative to independence.

Cameron also conceded several major points. He had called for a speedier timetable and for holding the referendum before 2014, given that experts say the current double-dip recession may spook Scots into voting to stay within the union, which also includes England, Wales and Northern Ireland.

In addition, Cameron bowed to Salmond’s calls to allow 16- and 17-year-olds to vote in the referendum, with the agreement recognizing the power of the Scottish Parliament to decide the question. Although polling has been somewhat inconclusive on their preferences, there is a perception in Scotland that younger voters who grew up with a devolved Scottish regional Parliament would be more willing to support independence. Although the agreement applies only to the independence referendum, the concession appeared set to touch off a broader campaign to lower the voting age for a host of national and local elections.

“It would have been unfair to have a vote on the future of Scotland in which the future of Scotland could not take part,” said David Linden, 22, head of the Young Scots for Independence. “We’ve grown up in a Scotland with its own Parliament and don’t want to see any Scottish laws made by a Parliament in London. We are ready to take the next step.”