There was a moment in modern British history when Arlene Foster seemed to control the course of events.
This was in early December 2017. Theresa May, the Prime Minister of the UK was in Brussels to sign off on an agreement on Britain's withdrawal from the European Union. Had that meeting proceeded smoothly, the successive three years of wrangling and angst might have been avoided. We might never have heard the tiresome word "backstop".
But it didn't. At lunchtime, May took a call from Arlene Foster. The DUP was in a "confidence and supply" arrangement with the Conservative Party to keep it in power. And Arlene Foster made plain that this deal with Brussels was not acceptable to the DUP. There would be no more backing for Tories in Parliament if May signed it. Mrs Foster had the unprecedented power, for a unionist leader, to call Mrs May back home and insist that she think again.
What was wrong with the agreement was that it established "regulatory alignment" between Northern Ireland and the European Union. May had realised that this was the only way to get out of the European Union without creating a hard border in Ireland.
The deadlock was broken days later with a form of words that enabled negotiations to proceed. If Britain and the EU had not reached a full free trade agreement by December 2020, the UK would have to come up with a good idea to avert a hard border, or Northern Ireland would continue to trade under EU rules. That held open the option of an alternative to the regulatory alignment - if anyone could think of one.
When the EU came back in February 2018 with a renewed clarity in its language about the backstop, May baulked. She was back under pressure to concede what the DUP refused to allow and she had to find another way.
That other way could only be a backstop that applied to the whole of the UK, diluting the whole concept of Brexit. This appalled the hard Brexiteers in the Conservative Party, but - crucially - the DUP opposed it, despite the fact that it had been contrived to meet their demand that there should be no difference between Northern Ireland and the rest of the UK in relations with the EU. "We leave as one" was Arlene Foster's core principle. And Brexit had to be Brexit.
For three years, the British Parliament would anguish over how to leave the European Union without a hard border in Ireland and without taking rules from the EU. And it would come back in the end to special arrangements for Northern Ireland as the only solution.
By then, the DUP would have lost its stranglehold over the Conservative Party. Boris Johnson would be Prime Minister and no phone call from Arlene Foster was going to rein him in.
Now, no one talks about the backstop, but the intention of avoiding a hard border in Ireland by keeping Northern Ireland aligned with the EU has been met. Even in the last weeks of negotiations in 2020, Boris Johnson threatened to undermine his own agreement to this - and to break international law to do so.
In the end, an announcement came in December, three years, almost to the day, after May's summons from Arlene Foster, that Michael Gove had agreed a "protocol" on Northern Ireland and much of the agonising in between seemed to have been a waste of breath.
Now the graffiti is calling for Arlene Foster to go, because her campaign to secure Brexit has led to what both Britain under May and the EU both foresaw: an Irish Sea border.
The bizarre thing about the DUP's determination to back Brexit is that it was not in their own interest. They appear not to have sat down and thought about how it would work out.
Belatedly, they thought up the argument that a division of the UK - a border in the Irish Sea - would be as much a compromise of the spirit of the Good Friday Agreement as customs checkpoints on the roads to Dundalk and Muff and about 300 other roads between them.
The real problem about the land border was that it would be impossible to control, would open up massive smuggling and would ultimately impose a need on the Republic to leave the single market.
There was no need for the argument, much deployed, that such a border would bring back the Troubles; that it would also rile the rabble and send them burrowing for their guns.
There may have been some substance to that case, but it ultimately isn't a good look to say that something has to be done because otherwise terrorists will get twitchy.
Having played that fear so adamantly has created the conviction in some loyalists that we only have an Irish Sea border because we needed to appease militant republicanism.
That's the argument of Jamie Bryson in his book Brexit Betrayed. He can cast up at unionists their defunct "red lines"; their insistence that there would be no Irish Sea border. Oh, how they swore it would never happen.
But he can also attack nationalism and what he quaintly calls the "latte-drinking liberal elite" for raising the threat of violence and suggesting that affronting nationalists is a greater breach of the Good Friday Agreement than affronting unionists would ever be.
Anyway, the damage is done. The Secretary of State, Brandon Lewis, has the luxury of being able to say that the sea border does not actually exist, because he does not rely for votes on those he seeks to befuddle. Arlene, on the other hand, lives here and leads here and can lose her party and her people.
This is the big weakness in the whole point of having a Secretary of State. It is written into the terms of their employment that, being from neighbouring island constituencies, they don't have to pay for their mistakes in the way that normally keeps a reckless politician in check. Jacob Rees-Mogg is happy because the fish are happy. There's the depth of his commitment.
There are some unionist hopes of reversing the protocol: claims that it can be revoked, hopes that a vote in the Assembly in four years time might scrap it.
In the meantime, Ian Paisley seeks to be his father's son, even making appeals to the consciences of Tories and asking them to search their hearts, expecting them to comprehend the hurt suffered by unionists. Ian had thought that he was riding some kind of wave of unionist enthusiasm. Plucky little Britain was standing up to the Eurocrats again and he was in the frontline.
Sammy Wilson seems to have become a libertarian. I suspect both he and Ian have spent too much time in the company of Nigel Farage. Basically, the Brexiteers were happy to have the DUP onboard, but they were never going to let them steer the ship and, if they didn't like where it was going, they were free to jump off.
Boris Johnson had got what he wanted and won an election with a majority even he would have difficulty squandering in a single parliament, though he might well do.
It's a long way down from Arlene Foster being able to pick up a phone and demand that the Prime Minister think again.
The hope remains that a future big deal between Britain and the EU will make no borders necessary, but until then the fall-back position, the backstop, is that Northern Ireland will do things the EU's way as Great Britain sails ahead into its glorious sovereign future, or wherever.
But having seen how poorly the DUP thought through the challenges that Brexit posed for them, one can hardly hope that they can come to some settled understanding among themselves on how to respond when they are further eclipsed, or when they have another big constitutional question in front of them - like a border poll.