Brexit was always a threat to the territorial integrity of the UK. The Leave campaign airily dismissed such talk as scaremongering. It was not, as Nicola Sturgeon, Scotland’s first minister, made plain on Monday.

Ms Sturgeon’s demand for a second referendum on Scottish independence comes as the UK government prepares to trigger Article 50, setting in motion divorce proceedings with the EU. The Brexit negotiations will be hard enough. Simultaneously fighting a Scottish referendum campaign would be a nightmare for Theresa May. It would also be against Scotland’s interests to make an irrevocable decision without knowing the terms of Brexit — an irony which will not be lost on Britain’s voters.

Yet Ms Sturgeon can argue legitimately that there has been a material change in political circumstances since the Scottish referendum in 2014. It may be a mistake for Scotland to demand a second vote, and voting to go might leave Scotland poorer and weaker; but Brexit changes everything.