At first blush, the Scottish independence referendum due on September 18 might seem to be an event on the margins of Europe. Entertaining to watch, perhaps, just like Mel Gibson’s Braveheart, but surely nothing that might have significant implications for the EU, whichever way the vote goes.

Well, think again. Usually, it’s the eccentricity of a case that renders it relatively unimportant. Here, it’s the opposite.

The Scottish Nationalists (SNP), who are the first political party of Scotland, are pitting themselves against the three mainstream UK parties: the Conservatives, Labour and the Liberal Democrats. We may be used to regional parties pitting themselves against the entire political establishment but such parties are usually either right wing or anti-EU (or both). The SNP, however, is neither.

It’s a centre-left party, bona fide social democrat, which is pro-EU. The SNP wants to pull Scotland out of the United Kingdom but keep it firmly anchored within the EU. Indeed, it wants to take Scotland deeper into Europe than the UK will ever go, envisaging entry into the eurozone within 18 months of independence. And this is where the rhetorical somersaults begin.

David Cameron, the UK Prime Minister, has said that the SNP leader, Alex Salmond, is selling smoke since an independent Scotland would be, legally speaking, a new State and, therefore, would need to reapply for EU membership.

Let’s pause here to appreciate the back flip. Cameron is famously a politician who thinks that too many national powers of decision-making have been granted to Brussels. That, and Eurosceptic pressures in England and on his Conservative Party, have seen him commit to a UK referendum, in two or three years, on renegotiating the conditions of the UK’s EU membership.

But, in the Scottish case, he is wagging his finger that independence would cost the Scots the EU membership that they so value. (Scotland is markedly more pro-EU than England is.) Strictly speaking, Cameron isn’t contradicting himself but he is showing some chutzpah.

Cameron appears to have roped in the help of the outgoing president of the European Commission, José Manoel Barroso, who has confirmed that Scotland would need to reapply for membership as a new State.

Barroso has also added that he thinks it unlikely that all EU members would approve Scottish entry. He seemed to be alluding to a State like Spain, which would want to discourage breakaway movements (such as Catalan separatism) within its own boundaries.

Barroso’s assessment has been challenged. Spain has clarified that it considers the Scottish and Catalan cases to be completely different.

And it has been pointed out that the implications of stripping all Scots of their existing rights as EU citizens need to be thought through in terms of EU law and their provisions, particularly since it would seem that the Scots were being punished for exercising national self-determination.

Even at this stage, therefore, the case is raising issues that go to the heart of the EU’s identity.

On these terms alone, the proponents of Scottish independence are right to point out that rejecting Scotland’s EU membership would complicate the EU’s external policy on the eastern front. Its arguments in favour of Ukraine’s self-determination and choice of international alliances would suddenly become trickier to make.

How things pan out in Scotland and the UK will mean a lot for what kind of EU we’ll belong to in five years

But there’s more. It isn’t possible to dissociate the Scottish referendum from the one that has been promised by Cameron, to be held in the next Parliament if he wins the next general election, on the terms of the UK’s EU membership.

It all goes back to the different attitudes that England and Scotland have towards the EU.

Should the vote for Scottish independence pass, then the UK referendum will take place with Scottish voters out of the picture, which would tilt the UK vote further in the direction of the Eurosceptics.

In such a context, Cameron, who wants the UK to remain in the EU (even if on different terms), would have a harder time riding the tiger of Euroscepticism.

However, if Scotland votes to remain within the UK (as it currently appears it will do), another possibility arises.

Suppose the UK referendum leads to a considerable weakening of ties with the EU. It can only do so on the strength of a predominantly English vote outweighing a generally pro-EU Scottish vote.

It is difficult to see Scotland accepting a redefinition of its relationship to the EU essentially because of an English vote. The case for independence would be resuscitated.

Does the irony need to be spelled out? The threat to British Unionism doesn’t just come from the Scottish left. It also comes from the British right – the champions of UK exceptionalism and anti-EU sentiment, like the United Kingdom Independence Party. Ukip’s wildest dream – withdrawal from the EU – could trigger its nightmare, the breakup of the UK.

For Cameron, the irony is another. It is unlikely that he can remain in control of his party, and its core vote, unless he throws the Eurosceptic segment some red meat to fend off the threat from Ukip.

But he also needs the staunch pro-EU vote in the UK to keep the Eurosceptic vote in check. Otherwise, he would end up as the most unforgiveable of Tory prime ministers – the one who presided over the breakup of the (British) Union.

The situation is very different from that of two decades ago. Then, it was possible for thoughtful British Tory Eurosceptics, like William Waldegrave, to countenance a UK withdrawal from the EU, accepting a trade-off between influence and UK independence, thinking that the UK would become a kind of Canada.

At the time, the interests of identifying with one Union – the EU – could be judged as coming at the expense of identification with another Union – the UK.

Not any more. It now seems that identifying with one Union depends on identifying with the other.

This isn’t just a drama of British identity. How things pan out, first in the Scottish referendum, then in the UK one, will mean a great deal for what kind of EU we’ll belong to in five years.

ranierfsadni@europe.com