Nicola Sturgeon’s announcement of her resignation brought some lavish tributes, but perhaps the highest praise came from Donald Trump. “Good riddance to failed woke extremist Nicola Sturgeon of Scotland!” the former president thundered. “The wonderful people of Scotland are much better off without Sturgeon in office!” Naturally, condemnation from Trump is a badge of honour. But the fact that he put out a statement at all was proof that the outgoing first minister has built a serious profile. Back in the 1990s, White House officials would marvel that the tiny population of Northern Ireland had somehow produced a clutch of world-class politicians, singling out the likes of John Hume, Martin McGuinness and loyalist leader David Ervine. “What are they putting in the water over there?” one Washington hand once asked me. For the last 20 years, you could ask a similar question of Scotland and the SNP. In his pomp, Alex Salmond was the most effective political leader in the UK – and, as if lightning had struck twice, the same was true of his successor. The end of the Salmond-Sturgeon era puts a question mark over the fate of the cause that once united them: the quest to take Scotland out of the United Kingdom. Less obvious is that that battle has become intertwined with another epic contest over a union of nations. As the sage of Strathclyde, John Curtice, puts it: “This is all part of the Brexit story.” To be clear, that’s not why Sturgeon quit. A combination of the personal and the political explains that. In the first category, there’s the empty-tank exhaustion Sturgeon described on Wednesday. In the second, the darkening clouds in the SNP sky: an ongoing police investigation into the party’s finances and a row over gender recognition legislation that, in the words of Mark Diffley, an Edinburgh-based pollster who has worked for both the pro- and anti-independence sides over the years, saw Sturgeon “significantly out of step with public opinion” in Scotland. Above all, Sturgeon faced a strategic impasse over the SNP’s defining issue. Polls show support for independence struggling to break through the 50% barrier (and recently falling well short of it), while there is no SNP consensus on how or when to secure a second referendum. The departing leader’s idea of using the next Westminster election as a de facto plebiscite faced stiff internal resistance. Still, none of that obscures the centrality of Brexit in the Scottish debate. For one thing, it is Brexit that provides the justification for having another ballot on independence so relatively soon after the first one in 2014: leaving the EU was the “material change” that merits a second go. These days, the first argument Sturgeon and others make for Scotland exiting the UK is the chance to rejoin the European Union. That makes sense in a country that voted by two to one to remain. Indeed, it fits with a broader trend. In 2016, there were remain voters who had said no to independence and leave voters who had said yes. Since then, and especially since the fevered Brexit year of 2019, there has been a process of sorting – as previously unionist remainers shifted to yes, and previously nationalist leavers defected to no. For a while, those movements seemed to cancel each other out. But given there are twice as many remainers as leavers in Scotland, there are more defectors from no to yes than the other way around – which is why average support for independence rose from 45% to 49%. It’s a Brexit effect. Or as Curtice told me: “Decline in support for the union is attributable to Brexit.” It leaves Scotland in a curious state of deadlock, stuck in a statistical dead heat between yes and no. Breaking that stalemate will require a choice, which boils down to which union, which single market, Scottish voters want to be part of. Do they want to be in the UK, separated by a border from the EU – or in the EU, separated by an equivalent border from England? After Brexit, there is no borderless option. Scots cannot be in a single market with both the UK and EU, as they used to be. Brexit has forced them to choose. The result is a deep paradox. Independence looks like the obvious anti-Brexit position, offering a route to rejoining the EU. And yet, even though independence is backed by remainers appalled by the clear disaster of Brexit, the most potent argument against them is … the clear disaster of Brexit. Unionists can say: “Don’t repeat the Brexiters’ mistake, breaking from a proven union in pursuit of some abstract ideal of sovereignty. We’ve all seen the damage that weakening ties with your nearest neighbour and trading partner can do.” In reply, the yes campaign will be left sounding like the very Brexiters they abhor: “We may take an economic hit, but at least we’ll be free.” Seasoned SNP hands are all too aware of the danger. One tells me that the independence case has to be presented as “the antithesis of Brexit”: full, detailed and honest about the difficulties and trade-offs, rather than offering the slogans and hollow promises of the Vote Leave crowd. But that will require a credible answer to one of the toughest questions of all: how can nationalists be absolutely sure the EU will allow an independent Scotland to rejoin? Without that guarantee, they’d be selling the Scottish people a Boris Johnson- or Nigel Farage-style leap in the dark. With Sturgeon’s exit, it becomes ever clearer that while Brexit was a boost to the cause of Scottish independence, its power as a cautionary tale could also make it a great drag. It is not the referendum of 2014 that should haunt independence campaigners’ dreams, but the fateful one that came two years later. Jonathan Freedland is a Guardian columnist