Boris Johnson Might Break Up the U.K. That’s a Good Thing.

The United Kingdom may be finally coming to an end.

On Thursday, Parliament passed the withdrawal agreement on which Prime Minister Boris Johnson successfully campaigned in last month’s general election. By the end of the month, it will be signed into law. The United Kingdom will leave the European Union on Jan. 31.

For decades, membership in the European Union helped glue together a fragmenting United Kingdom; now Brexit is tearing it apart. The short-lived fantasy of the “British nation,” too, may finally meet its end.

Mr. Johnson’s plan is likely to lead to a border between Northern Ireland and Britain for the first time in modern history. The policy — designed to allow Britain to radically break with Europe while Northern Ireland remains aligned with the rest of the bloc, including the Republic of Ireland — is an astonishing betrayal of the Ulster unionists, whose politics is predicated on the sanctity of the United Kingdom. And drawing Northern Ireland into the same regulatory system as its southern neighbor poses a remarkable opportunity for the nationalists. A once-more united Ireland is firmly in view.

That certainly hasn’t gone unnoticed in Scotland. The pro-independence Scottish National Party, which took 48 of Scotland’s 59 seats in December’s election, reads the writing on the wall. A large majority of Scots voted to remain in the European Union during the 2016 referendum; to allow Northern Ireland but not Scotland to remain aligned with the European Union’s market will only add to the sting. Nicola Sturgeon, the S.N.P.’s leader, has already formally requested that the Scottish parliament be given powers to hold an independence referendum. Mr. Johnson, for his part, has made clear that he intends to stand in the way of such a vote, but he may not be able to block it forever.