Nicola Sturgeon has handed Theresa May a lifeline with her call for a new referendum on Scottish independence

Nicola Sturgeon has announced plans to hold a new referendum on Scottish independence. The first minister told the Scottish parliament she will soon introduce legislation to prepare for another vote by 2021 if Scotland is taken out of the EU.

I write this just a stone’s throw away from Holyrood and a short walk from the Edinburgh polling place where I voted in 2014’s historic independence referendum. But now, five years on, everything has changed – or so Sturgeon wants us to believe.

During her speech, Sturgeon suggested that the case for independence is "even stronger now", given the "profound changes" that have occurred since 2014.

She told Holyrood: "With all of our assets and talents, Scotland should be a thriving and a driving force within Europe. Instead we face being forced to the margins, sidelined with the UK."

Whatever your political persuasion, it is hard to argue with the diagnosis that Brexit has materially changed the UK’s political landscape. On the cusp of leaving the EU, the Britain which 55 per cent of Scots voted to remain a part of in 2014 no longer exists.

Brexit has been a political gift to the SNP. 62 per cent of Scots rejected it, in every single region – the most resounding Remain vote of the UK’s four nations. A year on from a barnstorming performance in 2015’s General Election, the referendum result gave the SNP a clear mandate: oppose Brexit at all costs.

Throughout the Brexit process, the Scottish Parliament has found itself ignored and sidelined, despite frequent attempts to engage in dialogue with Theresa May. The Brexit process has also highlighted empty promises that Scots were fed in the run up to 2014’s independence referendum, like the unionist campaign line that “Scotland shouldn’t leave the UK, it should lead the UK”. Aged most badly are the frequent assertions by Better Together, the main unionist campaign group, that the only way to protect Scotland’s EU membership was to reject independence.

Brexit has also complicated the case for independence, however. The day after 2016’s Brexit vote, Sturgeon announced that a new referendum would be on the horizon. This was seized upon by her opponents, particularly Scottish Conservatives leader Ruth Davidson, who claimed the SNP are “obsessed” with independence and Sturgeon should “stick to the day job”.

The backlash against Sturgeon’s eagerness to hold a new vote resulted in a bloody nose for the SNP in 2017’s General Election. The party lost a third of its seats, while the pro-union Tories had their best performance in decades.

In terms of the future independence argument, key questions remain. People weren’t convinced by economic factors like currency in 2014, so will they be swayed this time? A million Scots also voted for Brexit in 2016, presumably including people who also voted Yes in 2014. If they are given the choice, which union will these voters choose?

Having been to the polls in two general elections, two referendums and a Scottish Parliament election in the last five years, the Scottish electorate has become highly astute; it is also traditionally more cautious than in other parts of the UK. Given how difficult, or impossible, the Brexit process has often seemed, will Scots fear causing more uncertainty and division with a vote for independence?

Still, Sturgeon does appear to have learned lessons from the 2014 and 2016 referendums. Her speech yesterday stressed the need for a respectful, wide-ranging conversation, imploring her opponents to voice their arguments too. She also proposed that the Scottish government set up a "Citizen's Assembly" to consider "what kind of country we are seeking to build". Drawn from countries like Ireland, which have used such methods, this sort of participative democracy shows genuine commitment to a more inclusive debate.

Fundamentally, if Sturgeon does decide to hold another vote, victory for the independence camp is likely. Complaints that she is using Brexit as an excuse to re-run the 2014 poll have quietened now that the Brexit process has become such an excruciating international embarrassment. Despite the electoral reverses of 2017, Sturgeon has emerged largely unscathed and is often seen as a voice of reason, frequently attempting to guide May towards a softer Brexit compromise. She also backs a People’s Vote, which is supported by a majority of Scots.

The first minister’s argument is clear: independence is a route to “avoiding the worst of the damage Brexit will do.” In the current climate, this feels like a winning message.

The independence movement was just a whisker from victory in 2014, starting from a poll position of 30 points behind. If the campaign had continued another few weeks, the close final result may well have been different. With new polls putting both sides neck-and-neck, while economic uncertainty and a lurch to the right seem certain within the UK, who would bet against Sturgeon’s success?

Westminster is certainly worried, which is why Downing Street has quickly stated that it will refuse to give approval to Sturgeon’s plan. The UK government’s approval is, under the current framework, essential - though it's worth recalling what Ruth Davidson said in 2015: “I actually don’t think, in the longer term, Westminster saying ‘No you cannae’ will play well in Scotland, and I think that it would damage the unionist cause.”

Such a move would probably draw international condemnation and protests like we have seen in Spain over Catalonian independence. A potential embarrassment like this, generating criticism from our newly scorned European neighbours, would surely be avoided at all costs.

Plus, who knows who’ll be prime minister this time next year? It is unlikely that Jeremy Corbyn, the bookies favourite to be next PM and a stickler for proper democratic process, would overrule the Scottish Parliament’s request to hold another vote.

There is a clear caveat to the debate though. According to Sturgeon’s announcement, a new independence referendum will take place only if Scotland is dragged out of the EU by 2021. In the strongest position she has occupied since becoming Scotland’s first minister, Sturgeon is giving May’s government and Westminster an ultimatum: abandon Brexit, or lose Scotland. Remain in the European Union, or risk ending a union that has existed for 416 years.

Of course, the SNP will always be a party that pursues independence - that is practically its raison d'être. But if Brexit doesn’t happen, the argument that circumstances have “materially changed” will carry less sway. It will also be much tougher to ask Scots to back a move which causes uncertainty and division after the disaster of Brexit has been narrowly avoided.

Sturgeon’s pro-independence majority in the Scottish parliament, comprised of the SNP and Greens, may well be lost in 2021’s Holyrood election, eliminating her ability to call another vote without unionist defectors.

Now is Sturgeon’s moment – and she plans to seize it. She is gambling that, when presented with the choice, May and the Tories will choose their own ambitions and Brexit dreams over the security of the United Kingdom. Given May’s track record of tunnel vision and relentless Tory infighting over Europe, it’s hard to argue with her logic.

But if May can break with tradition, Sturgeon’s very credible threat may have just provided her with a lifeline to save the two unions of which the UK remains a part. The question is: will she take it?