This month marks the 100th anniversary of the United Kingdom partitioning Ireland into two domains, north and south, under the terms of the Government of Ireland Act. Ever since May 3, 1921, that fateful split has bedeviled British politics and today is an obstacle to the nation’s withdrawal from the European Union.

Could this be the moment for Britain to embrace a Unified Ireland? These columns have raised that question twice. My own column, issued two years ago, suggested Brexit could bring about Irish unification. The New York Sun issued an editorial the following month, saying the question was beckoned by Brexit.

Now, since March and incited by Protestant youths, Northern Ireland has experienced its worst street violence in years. While it’s rioters might, or might not, have much awareness of the nuances of the province’s status under Brexit, there’s no question leaders of the province’s Democratic Unionist Party will leverage the disorder in their campaign to undo the Northern Ireland Protocol.

When the UK left the EU in January, the Protocol was deemed necessary to preserve the open border in products and people between Northern Ireland and the Irish Republic. It resembles under another name Theresa May’s ill-fated “backstop” proposal that cost her the premiership. Customs checks are now required at Northern Irish ports of entry on food and other products arriving from Great Britain.

That is something Boris Johnson’s Tory government had pledged would never happen. So fearing any steps that diminish the province’s union with the rest of the United Kingdom, both the DUP and its Protestant supporters are crying betrayal — and with some justification.

Northern Ireland’s Agriculture Minister, Edwin Poots, is threatening to take Her Majesty’s Government to court. His department has dragged its feet on enforcing the border checks on food products. Playing for time, the UK made a unilateral decision to impose a six month grace period on some border controls, risking a legal response from EU authorities.

The British minister responsible for Northern Ireland admitted this week that “it is hard to see that the way the Protocol is currently operating can be sustainable for long.”

Meantime, the Biden Administration, staffed with Brexit opponents from the Obama foreign policy team, has done little but warn the Johnson government to do nothing to compromise the north-south border. All parties to the problem fear a violent response to the Unionist riots from Catholic militants, including a resurgent Irish Republican Army.

It’s worth remembering this is not the first time that Northern Irish Unionists have clashed with the government at Westminster over the future of the island. Before World War I, the empire’s last Liberal cabinet was negotiating Home Rule for all thirty-two counties of Ireland. The 1912 legislation would have given the province modest control over its domestic affairs, and expressly prohibited a devolved Irish Parliament from establishing Catholicism as an official religion.

Nonetheless, Protestant Unionists were outraged to the point where their leaders organized paramilitary volunteers and imported guns from Germany. In their pre-emptive rebellion, they had willing allies in the Conservative Party.

Out of office for six years, the party was looking for an issue with which to bludgeon the Liberals and their progressive agenda. Tory leader Andrew Bonar Law urged British troops to refuse any orders to enforce Home Rule, and by 1914, regiments near Dublin threatened to mutiny if asked to take arms against the Unionist volunteers.

Only the August outbreak of World War I avoided the greatest constitutional crisis in Britain since the Cavaliers fought the Roundheads in the 17th century. Home Rule was put on hold. But the War put the initiative in the hands of Irish independence movement and by 1919 the Irish Republican Army was waging an armed revolt.

The Ireland Act was Westminster’s last ditch attempt to revive Home Rule by permitting six northern counties to opt out. A scant six months after its effective date, Home Rule was rendered moot when Westminster conceded de-facto independence to the Catholic south in the Anglo-Irish Treaty of December, 1921.

For the next 50 years, both Dublin and London paid lip service to possible reunification, followed by a three decade IRA campaign of guerrilla warfare and terrorism. “The Troubles” ended in the Belfast Agreement, signed on Good Friday1998. The pact established a basis for sectarian power sharing.

That agreement also required the UK to repeal the Government of Ireland Act, which had mandated the North remain a UK province, and acknowledge that the Irish Republic had a legitimate claim to a united Ireland. It was this end-of-century about face that has agitated hardline Unionists ever since.

Sectarian differences would make reunification by referendum a challenge. In the 2016 Brexit results, 56% of Northern Irish voters cast ballots to Remain in the EU. Yet while 85% of Catholics voted to Remain, thereby sustaining ties with the Republic, only 40% of Protestants chose to do so. With 48%of the Northern Ireland’s opulation of 1.8 million identifying as Protestant, and 46% Catholic, it’s unlikely a majority vote for unification would include a majority of Protestants.

Dublin is committed to unification, but Prime Minister Martin has made clear that a referendum in the near term would be “very explosive and divisive” if forced upon Belfast. Looking ahead, though, northern Protestants have little to fear from a Republic that has become an economic powerhouse and largely secular 21st century European state. The White House could play a constructive role in echoing that sentiment, and this week 25 members of Congress urged the President to appoint a special envoy to Northern Ireland.

I’s up to the Johnson government, though, to commit to a timetable for one Ireland. This would involve the UK offering guarantees of security assistance to the Republic should any violence accompany the unification process.

Plus, London would need to mount a major sales campaign to reassure 900,000 Northern Protestants they have nothing to fear from 5.9 million Catholic, Republican countrymen, and everything to gain from joining with the South.

The Johnson government would be wise to avoid replicating the cynicism of Bonar Law’s Tories in those waning days of Empire and take the steps necessary to the economic and political future of Great Britain. The final consummation of Brexit demands the reversal of an ill-fated Irish compromise made a century ago.

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Mr. Atkinson covers the 20th century for The New York Sun.