The British Prime Minister angrily denounced Sturgeon for threatening "huge uncertainty," and Downing Street was forced to deny
that it had delayed plans to fire the starting gun on Britain's departure from the European Union as a result.
But while Sturgeon's bold move complicates the British government's Brexit plan, it also threatens a political and legal headache for Europe.
If Scotland did vote to split from the UK, it might embolden separatist movements elsewhere, in places such as Catalonia and the Basque country in Spain; and Flanders, the Dutch-speaking region in the north of Belgium.
And if Scotland tried argue that it should remain in the EU even as the rest of the UK left, European diplomats would be faced with an unprecedented quandry.
Sturgeon's strategy: a headache for the EU
Sturgeon announced she would seek a referendum between autumn 2018 and the spring of 2019, before Britain leaves the European Union. By holding the vote before Brexit, Sturgeon may hope to argue that Scotland could remain part of the EU -- even as the rest of the UK leaves. This would present a novel situation for EU lawyers: could Scotland "inherit" the UK's EU membership card?
"The problem with Sturgeon's plan is that she appears to assume that if there's a referendum on Scottish independence before Brexit, then Scotland can remain a member of the European Union," Vincenzo Scarpetta, a senior policy analyst at think-tank Open Europe, told CNN. "This is a legal headache for Europe. Clearly the European Commission's position is that in cases of secession, the newly created state would have to apply from scratch."
That's what former European Commission President José Manuel Barroso said in a 2012 interview
, when the issue was last raised. "For European Union purposes, from a legal point of view, it is certainly a new state. If a country becomes independent it is a new state and has to negotiate with the EU."
But that was before Brexit.
In any case, Scotland can't hold a binding referendum until the UK government agrees to the vote. And experts say that May is unlikely to sign off on the timetable Sturgeon laid out.
"I don't think it's likely Theresa May will give her consent so that this second referendum could happen before the end of the Brexit negotiations, and possibly not even until the next general election in 2020," Scarpetta said. "It would be incredibly difficult for May to manage the two things...Brexit and the referendum."
May's calculation is that she has more of a chance of keeping the UK together if an independence referendum takes place after Brexit. Secession might not look so attractive if it would mean years negotiating re-entry into the European Union.
Secessionist fever could be catching
Scotland's last referendum was watched keenly across Europe, but perhaps nowhere more so than in Spain's Catalonia province -- home to a vocal independence movement. Nearly 2 million Catalans voted to secede from Spain in a symbolic referendum in 2014
, defying decisions by Spain's highest court that it was illegal to cast a ballot on independence. The government has repeatedly insisted that a referendum would be unconstitutional.
The tense situation in Catalonia has made Spain especially wary of other secessionist movements in Europe. In 2014, Spanish Prime Minister Mariano Rajoy warned that Scotland's independence referendum would "torpedo" the foundations of Europe. He expanded on this position last year, saying he would oppose any negotiations on potential EU membership with Scotland.
"I want to be very clear...Scotland does not have the competence to negotiate with the European Union. Spain opposes any negotiation by anyone other than the government of United Kingdom," Rajoy told a press conference following a summit of European leaders in Brussels, Reuters reported
"I am extremely against it, the treaties are extremely against it and I believe everyone is extremely against it. If the United Kingdom leaves... Scotland leaves," he added.
A new future for Ireland?
Scottish independence could also have a knock-on effect in Northern Ireland.
Just hours after Sturgeon demanded a new independence vote, Northern Ireland's largest Irish nationalist party Sinn Fein called for a referendum on splitting from the UK and uniting with the Republic of Ireland "as soon as possible."
"Brexit will be a disaster for the economy, and a disaster for the people of Ireland. A referendum on Irish unity has to happen as soon as possible," Michelle O'Neill, Sinn Fein's new leader in Northern Ireland, told reporters in Belfast on Monday
Under the 1998 peace deal, which ended 30 years of sectarian violence in Northern Ireland, the British government can call a referendum if it appears there is support for a united Ireland. Under the same agreement, a parallel referendum would also need to be held in the Republic of Ireland.
Like Scotland, the majority of Northern Ireland voted to remain in the EU in the referendum last June. But, historically, support for leaving the UK has not been strong. An Ipsos MORI poll
from September showed that 63% wanted to stay in the UK, while just 22% of voters supported a united Ireland.
What does it mean for Europe?
Sturgeon's move comes at a time when nationalist movements in member states are creating a palpable anxiety over the future of Europe.
"The EU is in a difficult place at the moment," Richard Whitman, a visiting senior fellow with the Europe program at Chatham House, a London-based think tank, told CNN. "There isn't really a consensus on what it should look like in the future. The system is already creaking over the Eurozone, the migration crisis...Scotland's call for independence plays on this anxiety about what the EU should become."
Whitman cited the Netherland's upcoming election as an example of Europe's fraught political climate.
"We can see when we look at countries like the Netherlands, for example, that electorates are really attracted to the idea of nationalism and sovereignty, and that presents a problem for the European Union," Whitman said.
As calls for a "Nexit" in the Netherlands and "Frexit" in France are bandied about, Scotland represents a paradox. The Scottish National Party has been slammed by May for its 'tunnel-vision nationalism,' as it seeks another referendum, while aiming to remain in the EU.
"It's the modern nationalist paradox," Whitman said. "On the one hand you have a desire for more autonomy, but to do that through a supranational container."