In February 2016, before Brexit became a reality, Boris Johnson made a flying visit to Northern Ireland as London Mayor to close a multi-million pound deal for 195 bright red London buses. From a factory floor at the family-owned Wrightbus company in Antrim, after testing safety glass with a sledgehammer at a neighboring factory, he proclaimed the benefits of voting to leave the EU later that summer.

Clad in a yellow high-vis vest, Johnson swung from the suspended bus chassis for the cameras while a grinning Arlene Foster and former Northern Ireland secretary Theresa Villiers looked on. He was certain that the border in Ireland would remain “absolutely unchanged”.

The free travel area in place for a hundred years would remain the same, he assured, perhaps forgetting the decades of militarised border checks between north and south that people worried might return after Brexit, or that exactly a hundred years before, Ireland was undivided. He was possibly unaware that the year marked the centenary of the 1916 Easter Rising, an insurrection against British rule in Dublin that was catalyst for the war of independence.

“Time for freedom folks is what I would say”, he quipped, warning people not to fall victim to “Project Fear”. But voters in Northern Ireland didn’t buy Johnson’s spiel. Nearly four months later, the majority voted to Remain.

Johnson exhibited a trait that characterises the Conservative party’s approach to Ireland: selective memory, which stems not just from ignorance, but from a legacy of opposing Irish sovereignty and treating the “Irish Question” as an awkward inconvenience.

A brief history lesson may be in order here. The Conservative Party joined together with Liberal Party dissenters to oppose Irish Home rule in 1886. The allegiance with unionists secured the Conservative party political majorities over the decades that followed. History repeated itself in 2017, when a calamitous snap election led the Conservatives into a confidence and supply agreement with the Democratic Unionist Party. That year, the Tories launched a manifesto under their full name of the Conservative and Unionist party.

Many unionists in Northern Ireland descend from Protestant families in Ulster, gifted land by the British Crown in an attempt to control the restive province. When the case for home rule began to gain traction at the end of the 19th century, the Ulster Volunteers emerged as the armed wing of unionism, threatening war if Ireland were allowed independence. The Irish Volunteers – a group that would later become the contemporary IRA – were formed in response. In 1921, the island of Ireland was partitioned.

Failure to recognise the present significance of the Irish border that created is now central to the unravelling of the Brexit process. “As historical errors go, this is pretty large. And an error it most definitely was,” former Leave campaigner Oliver Norgrove wrote in his recent mea culpa published in the Irish Times. The Leave campaign “did not sufficiently take into account the interests of Northern Ireland”, he added.

Why not? Norgrove claimed the reasons stemmed from ignorance. He recalls one evening towards the end of May 2016, when BBC Newsnight rang the campaign to request a representative debate the effects of Brexit on the border. Nobody – not even “polished media performers” – wanted to take the request, he concedes, because they didn’t know enough about the subject.

But examining the Leave campaign in detail suggests Brexiteers were not simply ignorant of Irish history. In fact, they actively encouraged voters to ignore the issue altogether, repeatedly dismissing genuine concerns about the Irish Border as a tactic of “Project Fear”, and mocking Irish sovereignty.

In February 2016, Vote Leave campaign chairman Nigel Lawson was asked directly about the effect of Brexit on the Irish border during a debate at Chatham House. He joked that it “would be great” if the Republic of Ireland were to admit it “made a mistake in getting independence” and “come back within the United Kingdom”. Asked if Brexit would herald a return to border checks, he claimed that there were already checks in place “because of the terrorist problem”.

Brexiteers like Lawson repeatedly ignored signs that Brexit could rupture Northern Ireland’s fragile peace. Yet the warnings were clear; former Northern Ireland Secretary Peter Mandelson told the British-Irish Chamber of Commerce that Brexit could “irrevocably reorder” the UK and restore a hard border.

A 28-page government report published in March 2016 stated that border controls between North and South would be re-imposed if Britain left the EU. Former Conservative leader John Major even joined with former Labour leader Tony Blair in Belfast to raise the alarm that Brexit would not only undermine the peace process but threaten the Union itself.

DUP leader Arlene Foster also dismissed concerns about the border as “scaremongering”, and later made the guileful claim that no hard border had ever existed in Ireland. Bolstered by DUP support, the Conservatives have continued to discuss the Irish border from a revisionist perspective that positions Britain as an almighty imperial power, and Ireland as her importunate child.

Yet the DUP’s support for Brexit, much like their opposition to marriage equality, does not represent the public majority in Northern Ireland. The party was formed during the Troubles by evangelical preacher and hard-line unionist Ian Paisley, who claimed his ancestors “cut a civilisation out of the bogs” while Irish people were “wearing pig skins and living in caves”. Brexit has shown the DUP is willing to ally with Conservatives against the interests of the majority of voters in Northern Ireland, in order to preserve its union.

Conservative MPs have rushed to headline DUP fundraisers. These have included MP Priti Patel, whose suggestion that the government should use food shortages to pressure Ireland during the Brexit process betrayed a wilful ignorance of the Irish famine, and Michael Gove, whose contempt for the peace process in Northern Ireland is well documented. In a pamphlet Gove wrote in 2000, he called the Good Friday Agreement a “humilitaiton of our army, police and parliament,” comparing it to condoning paedophilia or appeasing Nazis.

The Brexit motto “Take Back Control” promotes a dangerous nostalgia for a return to empire that the DUP strongly shares, with the Leave campaign suggesting Britain can reassert its former dominance. But laughable claims that Ireland should rejoin the empire expose a narrow-minded detatchment from reality, and an ignorance of Ireland’s history. The wilful amnesia of the Brexit campaign has isolated Britain while helping to reignite a movement for a united Ireland.

When resigning as Foreign Secretary, Johnson warned that Britain was “truly headed for the status of a colony”. Johnson’s mistake was to view the Irish as subjects of a dead empire. In reality, Brexit has forced the UK to finally reckon with Ireland as an equal, sovereign nation.