THIS was meant to be the week when a proud, sovereign nation served notice that it wanted to leave the overbearing, unrepresentative union to which it had long been shackled. And so it was—but not in quite the way that Theresa May had imagined. Britain’s prime minister had planned to trigger Article 50 of the European Union treaty, beginning the two-year process of Britain’s exit from the EU. But she was forced to delay her plans when Scotland’s first minister, Nicola Sturgeon, upstaged her by announcing that she would seek a new referendum on Scottish independence.

The threat of a second constitutional earthquake in as many years is the latest reminder of Brexit’s unintended consequences (see article). The English-led move to leave a 40-year-old union with Europe is pulling at the seams of its 300-year-old union with Scotland. Mrs May’s fundamentalist interpretation of the Brexit referendum—that it requires departure from the EU’s single market and an end to free movement to and from the continent—ignores the concerns of Scots, who voted to remain, and creates an intractable problem for Northern Ireland, which shares a land border with the EU. But the lesson for Scots from Brexit is more complex than Ms Sturgeon suggests. The arguments she puts forward for remaining in the EU highlight the weaknesses in their case for independence.

The Scottish independence referendum of 2014 was billed by nationalists as a “once-in-a-generation opportunity”. But they are right to demand another. Ms Sturgeon’s Scottish National Party (SNP) won an election last year on the promise of a new referendum in the event of a “material change” in circumstances. Brexit is as material as it gets. Mrs May and Britain’s Parliament, the consent of which is needed for another plebiscite, must not deny the Scottish people a second vote.

If at first you don’t secede...

But Mrs May has the power to delay it—and on March 16th she said that there should be no referendum before Britain’s relations with the EU are clear. Ms Sturgeon wants the vote to take place at some point between autumn 2018 and spring 2019, when Brexit negotiations will be entering their final, fraught phase. She suggested this week that this would allow an independent Scotland speedily to rejoin the EU. That is mistaken. There is no prospect of Scotland completing “Scoxit” before Britain leaves the EU (at the time of the referendum in 2014, an exit period for Scotland of 18 months was pencilled in). European officials have made clear that there would be no “fast track” entry process for a country that was previously part of a member state.

What holding a referendum during Brexit negotiations would achieve, as Ms Sturgeon surely knows, is maximum pressure on the British government, which would be incapable of fighting on a second front in Scotland. And it would damage Scotland’s own interests: first by muddying the Brexit talks, in which Scotland has a stake, whether it ends up as part of Britain or not; and second by forcing Scots to vote before it is clear what sort of deal Britain is going to get with the EU.

Whenever the second referendum campaign begins, Brexit will make life trickier for the unionist side. Already Mrs May is finding that her position on the European Union makes it harder to defend the British union. Ms Sturgeon says she wants Scots “to be in control of events and not just at the mercy of them”. How can British ministers disagree, when so many of them urged Britons to “vote Leave, take control” last summer?

Yet Brexit creates problems for the nationalists, too. Just as it sounds unconvincing for Brexiteers now to argue for the union, it is difficult for Ms Sturgeon to beat the drum both for membership of the EU and for exit from Britain. As she has pointed out, it is a bad idea to leave the single market to which you send the lion’s share of your exports. For Scotland, that means Britain. She laments the hardening of Britain’s borders with Europe. Yet an independent Scotland might well mean a harder border with England, particularly if Scotland rejoined the EU. Pro-Europeans have noted that the sovereignty you regain by leaving a union is illusory when it also means losing the clout you get as a member of a more powerful group. So it would be if Scotland left Britain: it would indeed be more sovereign in a pure sense, but at the cost of its seat on the UN Security Council, nuclear weapons, G7 membership and much else that aids true self-determination in the world.

The Scots are in a wretched position. But they should be in no doubt: exit from Britain would compound the mistake Britain is making by leaving the EU. Though Brexit is the main motive for Ms Sturgeon’s renewed independence push, it is also a warning of the perils of going it alone.