There has been outrage in certain quarters of the British media regarding the insistence of the Irish government that Westminster provide a detailed outline of how it plans to deal with the border dividing Northern Ireland from Ireland. From an Irish point of view, the Dublin government’s position is understandable. Many Irish people are aware that the last time the British government refused to make its position on the border clear, it caused considerable problems.

In 1920, with the Anglo-Irish war raging, Westminster belatedly sought to introduce Irish home rule. The Government of Ireland Act essentially provided for two home rule parliaments, one in Dublin and one in Belfast. Six of Ireland’s 32 counties became Northern Ireland, with the old county boundaries serving as the border between the two states.

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From the beginning, the line of partition was arbitrary. Most of the county lines in Ulster were drawn up in the early 1600s. Using county borders to form the basis of the boundary meant that several nationalist and unionist communities got left on the “wrong side” of the line.

This issue came to a head during the Anglo-Irish Treaty negotiations in 1921. Once they accepted that some form of partition was inevitable, Sinn Féin representatives pressed for clarification about where exactly the border between the Irish Free State and Northern Ireland would lie.

Just as supporters of Brexit today insist that the specifics of the Irish border can be addressed at some time in the future, the British negotiators refused to tackle and solve the question of the border during these talks. Instead, they proposed a boundary commission to decide the exact position of the border after the Anglo-Irish Treaty had been signed. This is the current Westminster approach to its negotiations with the EU, saying, “Let’s make a deal first, and we can figure out the details later.”

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Nor is this the only parallel in discussions about the border today and a century ago. In 2017, Brexiteers have taken a contradictory position in insisting that the UK will leave the customs union, but that a frictionless border will be in operation between Ireland and Northern Ireland.

In the early 1920s, the British government adopted an even more duplicitous stance, assuring nationalists in Dublin that the border of 1920 would be drastically revised, while promising unionists in Belfast that the frontier would remain unchanged.

The outbreak of the Irish civil war meant that the boundary commission to redraw the border did not convene until 1924. Initially, the commission was to include three members, representing the Dublin, Belfast and London governments respectively. But Northern Ireland refused to appoint a representative. Why would it? Unionists had already been assured that the border would not change. In the end, the Labour government in Westminster had to choose Northern Ireland’s commissioner.

The lesson is that failing to address issues surrounding the border question can have ugly and unforeseen consequences

In Dublin, there was considerable confidence that the boundary commission would transfer large parts of Northern Ireland that had nationalist majorities to the Free State. But in 1925, the conclusions of the boundary commission were leaked. These revealed that the proposed changes to the border would be minimal and that some territory in Donegal would transfer to Stormont rule.

The loss of a portion of Donegal was too much of a humiliation for the Free State government to take, and it struck a deal with Westminster to suppress the commission’s report. In return for the cancellation of the imperial debt that the Free State had agreed to pay in 1921, the border remained unchanged. But the feeling lingered in Ireland that “perfidious Albion” had tricked the Free State into accepting a borderline it did not want.

In the long term, this border was problematic. In making Northern Ireland, unionists had sought as big a hinterland for Belfast as possible, to create an economically viable state, while ensuring that the nationalist population trapped in its borders would not be large enough to challenge unionists in elections. This approach resulted in counties with nationalist majorities, Fermanagh and Tyrone, being included in Northern Ireland. The resentment and tension that this created was to eventually reap a terrible harvest in 1969.

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Had the British government taken direct responsibility for drawing the border in 1921 and sought to respect the wishes of local communities regarding which state they wanted to join, it is quite possible that the Troubles would never have broken out. A reduced Northern Ireland with an overwhelming unionist majority would have had nothing to fear from its small nationalist population. It may have treated this minority with the tolerance and fairness that the six-county state failed to do, thereby removing the root cause of the Northern Irish conflict.

The lesson for the British government is that failing to address issues surrounding the border question can have ugly and unforeseen long-term consequences. Let’s hope it is one they heed.

• Caoimhín de Barra is assistant professor of history at Drew University in New Jersey, and author of The Coming of the Celts, AD1860: Celtic Nationalism in Ireland and Wales (University of Notre Dame Press), to be published next year