As Theresa May announced that she will resign the Conservative Party leadership, she spoke warmly about the Union that makes up the United Kingdom. “Because this country is a union,” the

prime minister said, “not just a family of four nations, but a union of people, all of us – whatever our background, the colour of our skin, or who we love, we stand together. And together

we have a great future.” These were fine words, no doubt sincerely and passionately felt, but it was the damage that her Brexit withdrawal agreement would have inflicted on the Union –

particularly the Union between Great Britain and Northern Ireland – that eventually consigned Mrs May’s premiership to failure. Without the so-called ‘backstop’ that threatened to create an

resignation statement, she spoke about reaching consensus only, “if those on all sides of the debate are willing to compromise”. That seems like a reasonable position, but every negotiator

has to have her limits, and Mrs May repeatedly assured MPs that “no British prime minister” would countenance an Irish Sea border between Great Britain and Northern Ireland. Early in

negotiations, the EU advocated a ‘special status’ for the province, that would keep it within the single market and customs union, even while the rest of the country left. When this language

alarmed unionists, it restyled its land grab as a ‘backstop’ and sold it as an ‘insurance policy’ against new checks and infrastructure between Northern Ireland and the Republic. In

unguarded moments, with the diplomacy stripped away, it became apparent that this policy was conceived as a punishment for the UK’s decision to leave the EU. The former Brexit secretary,

Dominic Raab, revealed that the team around Martin Selmayr, the European Commission’s secretary general, openly talked about Northern Ireland as the ‘price’ Britain had to pay for Brexit.

Mrs. May’s resistance to this extraordinary attack on the Union was largely rhetorical. Her government conceded the principle that a backstop was needed in December 2017, in a joint report

on the progress of negotiations, issued by both sides. The Politico journalist, Tom McTague, quoted a senior EU27 official who said, “we just could not believe the British had accepted the

text. We knew it would not be acceptable to the unionists.” The December document promised that, if there was no agreement on a trade deal or specific solutions for Ireland, “the United

Kingdom will maintain full alignment with those rules of the Internal Market and the Customs Union which, now or in the future, support North-South cooperation, the all-island economy and

the protection of the 1998 agreement”. The next paragraph expressly ruled out “new regulatory barriers … between Northern Ireland and the rest of the United Kingdom,” unless the Stormont

Assembly legislated for “distinct arrangements”. The government might yet have salvaged something from deliberately oblique provisions. Instead, it waited until June 2018 to set out its

understanding of the backstop, by which time the EU Commission had translated the joint report into a ‘legal text’ demanding a ‘common regulatory area’ on the island of Ireland, with

Northern Ireland required to stay in the single market and customs union, while the rest of the UK left. This was the arrangement to which Mrs. May told the House of Commons no prime

minister could ever agree. Yet, eventually, she accepted these demands with only a few cosmetic changes, in the Draft Withdrawal Agreement. She claimed that she’d won a significant

concession from Brussels, because the whole UK was to be included in a ‘customs territory’ with the EU. Perhaps she thought that few people would care about the distinction between a customs

territory and the customs union, of which Northern Ireland was to remain a full member. As the prime minister tried three times to win the House of Commons’ support for her plans, the

attorney general, Geoffrey Cox, explained that firms in Great Britain would have to fill in customs declarations in order to sell goods in Ulster, if the backstop were ever enacted. In

addition, while there was talk of maintaining alignment between Northern Ireland and the rest of the UK, the province would have to adhere to swathes of single market rules that were not

applicable on the mainland. There were discussions about what exactly they might entail, but it was acknowledged that regulatory checks would be needed on shipments crossing the Irish Sea.

Mrs. May tried to spin and repackage it several times, but the whole purpose of the backstop was to ensure that any new political and economic dividing line fell between Northern Ireland and

the rest of the UK, rather than along the existing land border in Ireland. She acquired several letters from EU officials and drafted various joint statements, designed to reassure critics

that these provisions would not be used, but none could remove the threat completely. Again, the attorney general’s contribution was decisive, as he concluded that, despite these apparent

safeguards, the “legal risk remains unchanged”, of the UK becoming stuck in an arrangement that carved up its territory. This week, her ‘bold offer’ to MPs included legal commitments to work

toward replacing the backstop, but she admitted in her speech that it could not be removed from her deal with the EU. They changed nothing. The cabinet forced the prime minister to announce

her resignation after her eye-catching pledges to hold votes on a second referendum and a full customs union, but her adherence to the backstop had already turned her premiership into a

hollow husk.