The comment’s a few days old, but as prophecy it might be gaining some traction. Boris Johnson has gone on a tour of the United Kingdom to build support for his no-retreat strategy on Brexit, only to find little unity among the other three nations of the union. As Gordon Brown quipped last week, Brexit might take on a whole new meaning, and even Johnson’s Tory colleagues wonder about it:

May’s de facto deputy prime minister, David Lidington, told the BBC this month that the union “would be under much greater strain in the event of a no-deal.”

He added, “My view comes not just from Scottish nationalism and pressure for Irish unification — it comes from indifference among English opinion to the value of the union.”

Gordon Brown, a former Labour Party prime minister, said at an event in London last week that Johnson could be remembered “not as the 55th prime minister of the U.K. but as the first prime minister of England.”

Now that Johnson has wrapped up his first four-nations tour, the returns suggest that disunity may become a very big problem if a hard Brexit takes place on Halloween. Johnson got booed in Wales and Scotland, the latter of which intends to push a new independence referendum in the event of a no-deal Brexit. Johnson stayed away from public forums on his visit to Northern Ireland, but the political parties in deadlock over forming an executive showed a lot more consensus when it came to Johnson’s Brexit plans:

The party leaders were united on one thing: They warned Johnson that his threat to take Britain out to the European Union without a deal, without a trade pact or a transition period, was folly, or worse.

“We are in a crisis, and Brexit is adding to the chaos,” said Naomi Long, a leader of the Alliance Party.

Mary Lou McDonald, the Sinn Fein president, said Johnson’s plan for a no-deal Brexit has increased the likelihood that the United Kingdom will splinter — by boosting the case for Irish reunification.

“Traditionally, the argument and the discourse has been between green and orange, between Irishness and Britishness. But Brexit changed that and added a new dimension, a critical dimension, which is European or not? Inside the European Union or not?” McDonald told BBC Radio on Wednesday morning.

The hard-left Sinn Féin warned Johnson that a hard Brexit will trigger a plebescite on Irish reunification. The Northern Ireland vote on Brexit had been solidly negative in 2016, and the worries over the impact on the Good Friday agreement has only made it less popular:

It would be “unthinkable” if a no-deal Brexit was not followed by a poll on Irish reunification, the leader of Sinn Féin has warned Boris Johnson, also telling the prime minister that no one believed he was impartial on Northern Ireland.

“In the longer term, we have advised him that constitutional change is in the air. He can’t say that he hasn’t been told,” Mary Lou McDonald said after meeting Johnson at Stormont on Wednesday morning.

Any Brexit, but particularly no deal, “represents in anybody’s language a dramatic change of circumstances on this island, and … it would be unthinkable in those circumstances that people would not to be given the opportunity to decide on our future together”, McDonald said.

A generation ago, the success of such a move would have been nearly unthinkable. It’s becoming a lot more thinkable now, not just because of Brexit but also because of demographic changes in Northern Ireland and a sense of dislocation over its present political stalemate.

It’s not helping matters that Johnson is demanding that Irish PM Leo Varadkar drop the backstop and back reopening negotiations that already took place in large part on Ireland’s behalf. Johnson’s allies in the UK have taken to accusing Varadkar and his government of being “bought by Brussels” for insisting on solid guarantees of a soft border after Brexit. Varadkar in return shot back today that Ireland was not about to be “bullied” by the UK into submission:

New British Prime Minister Boris Johnson has called for Ireland to scrap the border “backstop” clause in the deal negotiated under his predecessor Theresa May.

“Ireland isn’t going to be bullied on this issue and as a government and as a country, I think we are going to stick by our position,” Varadkar said in an interview with the Irish Daily Mirror newspaper.

Varadkar said Ireland had “total support” from other EU countries on the backstop, designed as an insurance policy to prevent border controls between EU-member Ireland and British-ruled Northern Ireland but which Johnson says will keep Britain tied to EU customs rules.

Johnson essentially told Varadkar, trust us:

Johnson also told Varadkar he wants the controversial backstop plan to protect the Northern Irish border scrapped.

“He [Johnson] said that in all scenarios, the government will be steadfast in its commitment to the Belfast Agreement and will never put physical checks or physical infrastructure on the border,” the statement from Downing Street says, adding: “The prime minister made clear that the government will approach any negotiations which take place with determination and energy and in a spirit of friendship, and that his clear preference is to leave the EU with a deal, but it must be one that abolishes the backstop.”

Ahem. Don’t expect the Irish to exhibit a sudden swell of trust in the Brits when it comes to borders and unilateral actions regarding sovereignty. Besides, as Varadkar told Johnson, Brexit is the UK’s idea, not theirs. It’s up to the UK to meet all of its international obligations while pursuing it:

Brexiters have had three years to come up with a plan to avoid import/export controls over the border without having an actual border and checkpoints, and so far they haven’t come up with a workable plan. Why would Ireland simply trust that Johnson will magically work it out in the next few weeks? Especially without a concrete legal commitment to the consequences of failure?

Johnson’s demand may well end up backfiring where it counts. The backstop issue isn’t just important for the Republic of Ireland; it’s also important in Northern Ireland, where the current open border allows for cultural connections and most importantly provides no catalyst for violence. A hard Brexit will require border checks on goods and services exchanged between two customs jurisdictions, and that means a return of a policed border that will become a magnet for protests and worse. If anything, it may well accelerate momentum toward a reunification plebescite, and it might accelerate support for it as well.

Johnson didn’t even get a good reception in Wales, where its first minister warned that the UK might not be so U if Johnson persists:

The next day in Wales, Johnson met a similarly dubious Welsh first minister. Mark Drakeford, a member of the opposition Labour party, warned in an interview with the Guardian that a no-deal Brexit would endanger Wales’s agricultural and manufacturing sectors and “a whole way of life that has existed for centuries.” He stressed that Johnson’s characteristic “bluff and bluster” was testing the unity of the United Kingdom itself.

“I think the union that is the United Kingdom is more at risk today than at any time in my political lifetime,” he said, pointing to how both Scotland and Northern Ireland overwhelmingly voted to remain in the European Union.

Johnson has painted himself into a corner on a very large bet. He and his fellow Brexiters torpedoed the Theresa May-negotiated Withdrawal Agreement after more than two years of negotiations on the premise that the EU would blink within weeks of a hard Brexit. And who knows? They still might; there’s no doubt that it would do damage to the other 27 nations within the EU. However, it’s going to do a lot more damage to the UK, especially in the short run, plus the EU cannot afford to set a discount for disunification based on bullying and brinksmanship from a disaffected government.

Johnson may well regret setting those discounts at home too now that the incentives are all set for a hard Brexit on Halloween. It makes the incentives clearer and clearer for to disunification among  the UK’s constituent nations, especially Scotland and Northern Ireland. The sun may be about to set on the last vestiges of the British Empire.