Theresa May’s speech in Stirling on Thursday spoke volumes. It was not the content that was especially striking, although there was much there that was interesting, including a review under Lord Dunlop of the role of Whitehall departments in the devolved nations. But the fact and timing of the speech signified more. Mrs May is playing out time in Downing Street. For her to devote one of her last visits to the public pulpit as prime minister to the importance of the UK union is a significant choice. It tells us that she is scared the union will not survive under her successor.

Mrs May’s message was that whoever takes over from her will have among their “first and greatest duties” the need to strengthen the union. This will come as something of a surprise to May watchers. Her three years at the helm have not been marked by sustained sensitivity to the union, by fresh initiatives or by persuasive evidence that she has any great feeling for it. On the contrary, she has been a centraliser, and the Dunlop review – which she says both Boris Johnson and Jeremy Hunt support – appears designed to claim a bigger role, involving greater spending for Whitehall over Scotland and Wales.

The lack of engagement has been disabling, both in Northern Ireland and Scotland. In the former, Mrs May presided over a continuing period of political stagnation. For the majority of her premiership, devolved institutions have been in abeyance. There has been no sense that she sees restoring them as a priority. Quite the reverse. Her pact with the DUP has implied that their view of the union and Brexit is also hers. Her speech in Belfast last year was a low point of her premiership.

With Scotland, Mrs May has been equally cautious. She made an early visit to Edinburgh in 2016, but she has done nothing sustained to reach out to Scottish concerns. Though her speech blamed the SNP for refusing to cooperate, she seemed to decide early on that there was nothing to be gained by working with a resourceful and implacable foe on Brexit. The upshot was not just that Mrs May appeared unresponsive but also that she was unresponsive. As a result, the SNP dominates Scotland’s Brexit debate and is back in the driving seat on independence too.

All of this comes back to Brexit. Northern Ireland and Scotland are the two component parts of the UK that voted to remain in the EU. In both, hostility to Brexit has increased since 2016. They are also the two, unlike Wales, in which the union is under most immediate stress. The connection is umbilical. Mrs May tried to deliver a Brexit that Northern Ireland and Scotland do not want, did it incompetently, in an increasingly extreme form, and without addressing Irish or Scottish concerns honestly, or at all.

The result is that Brexit is disabling the Tory party in Scotland and threatening to undermine the cohesion and viability of the British state. Few now think that the Scottish Tories will improve on their 2016 and 2017 electoral successes; large losses are more likely. An early general election could give the SNP a mandate to demand a second referendum. The narrowing of the polls suggests that this time Scotland might secede. The impact on the UK would be lasting in every way. The future in it of Northern Ireland and even Wales would come into question. The thought has UK military chiefs in mounting panic.

The prospect of Mr Johnson as Mrs May’s successor means that Christmas may come early for the SNP. Mr Johnson embodies a thoughtless, arrogant and disrespectful Englishness that offers rich pickings. It is hard to see any way that he can now avoid becoming the SNP’s recruiting sergeant. But this is not ultimately about personalities. This would not all go away if Mr Hunt prevails. This is about Brexit. And it is about the way that an asymmetric union of four very different nations has failed to ensure enough mutual respect.

Just as she prepares to depart, Mrs May has uttered an anguished plea to think about the union. So she should. But her last-minute anxiety is pretty rich. She should have addressed this from the start, not at the end. The damage has been done on her watch, and as a result of her approach. It may well be true that her successor will do even more damage. But Mrs May’s own role in this disaster cannot be ignored.