The UK Supreme Court on Wednesday rejected an argument that the Scottish government can pass legislation allowing it to hold a second independence referendum next year, leaving nationalists searching for another route to a new vote.
In a referendum in 2014, Scots voted 55%-45% to remain in the United Kingdom, but the independence movement argues that the vote for the UK to leave the European Union two years later changed everything.
Here is the history of the push for Scottish independence and how another vote could happen:
ACT OF UNION
The nations of Britain have shared the same monarch since 1603, when King James VI of Scotland became James I of England. In 1707, a formal union created the Kingdom of Great Britain.
Now, the United Kingdom of Great Britain and Northern Ireland binds England, Scotland, Wales and Northern Ireland and has an overall population of about 68 million, of which Scots make up some 5.5 million.
In 1998, the then Labour government passed the Scotland Act which created the Scottish parliament and devolved some powers from Westminster.
ONCE-IN-A-GENERATION VOTE
Both sides agreed at the time of the 2014 plebiscite that it should be a once-in-a-generation poll. However, Scottish nationalists say Brexit means they should be allowed a second vote. While the United Kingdom as a whole voted in favour of leaving the European Union in 2016, a clear majority in Scotland voted to stay in the bloc.
Independence supporters say one of the main arguments put forward in 2014 by opponents of a break-up was that an independent Scotland could not join the EU.
The left-wing, nationalist Scottish National Party (SNP), which has run Scotland's devolved assembly since 2007, also argues that the UK government has pursued policies with which the vast majority of Scots disagree.
In the last national election for the UK parliament in 2019, the SNP won 45% of votes cast and 48 of the 59 Scottish seats, while Britain's governing right-wing Conservative Party captured just six.
POLITICAL PRESSURE
The Scottish First Minister Nicola Sturgeon said on Wednesday she would now turn the next general election due in two years into a de facto referendum to ramp up the pressure on the British government to grant another vote.
Sturgeon said she would ask her party's national executive committee to discuss and agree the detail of how they would use the next election as a de facto referendum.
She said her party will now "launch and mobilise a major campaign in defence of Scottish democracy".
James Mitchell, a professor of politics at Edinburgh University, said a decision to call for the next election to be a de facto referendum would be a "major gamble".
Even if the SNP won more than 50% of the vote in Scotland it was still unlikely the British government would agree to negotiate a new referendum deal and Sturgeon would have very little international support, Mitchell said.
In their best ever election result in 2015, the SNP fell just short of 50% of the votes.
If the SNP fails to win more than half of Scots' votes they may have to accept that voters have made their decision and the issue would be settled for a generation, he said.
"It is difficult to see her continuing in office" if that happened, he said. "Internal dissent has been building for some time and it seems most likely that the gamble of a 'de facto' referendum - very much out of character for a very cautious politician - is her last throw of the dice."
KINGMAKER ROLE
At the next UK general election, if the main opposition Labour Party is the largest party in the London parliament, but falls short of a majority, the SNP could support a minority government in return for being given permission to allow Scotland to hold another independence referendum.
Labour has ruled out any deal with the SNP after the next election that must be held by January 2025, saying this scenario is talked up by political opponents to damage the party.
Michael Keating, a professor of politics at the University of Aberdeen, said such a deal is unlikely because Labour "would be accused of putting the unity of the United Kingdom at risk for short term gain".
"Then they would look pretty vulnerable," he said. "They would be more likely to say we are forming a minority government" and challenge the SNP to vote to bring the government down, he said.
UNAUTHORISED REFERENDUM
Sturgeon has previously said she would only seek to secede from the United Kingdom through a legally agreed referendum.
Sturgeon is under pressure from some activists frustrated with her gradualist strategy to call a referendum without the British parliament's permission. But unionists could boycott this vote and claim the result lacks legitimacy.
If the independence movement was to hold a referendum without the consent of the British government, it could also struggle to gain international recognition if it won.
That would mirror the situation in Spain over Catalonia four years ago, when the regional government held an independence referendum that the central government said was illegal.
(Reporting by Andrew MacAskill and Michael Holden; Editing by Toby Chopra)
