The 310-mile-long Irish border between Northern Ireland and the Republic was once marked by barbed wire, with watch towers manned by machine gun-wielding soldiers. Today, it’s been replaced by a wide motorway with cars whizzing between north and south. There are 208 public road crossings on the border and it’s almost impossible to tell where one country ends and the other begins.

The motorway is the symbol of a hard-won peace. Although the border is now all but invisible, it once marked decades of hostility and the bitter division between the traditionally-Catholic Republicans aligned with the Republic of Ireland, and the traditionally-Protestant unionists who want Northern Ireland to remain tied to the United Kingdom.

Since the Irish War of Independence against Britain ended in 1921, the island of Ireland has been divided into North and South. But reunification has always remained an aspiration for Irish nationalists in Northern Ireland and political parties in the Republic —without any clear path to fruition. Now, nearly a century after the island was divided, reunification is back on the agenda as a realistic possibility and not just a long term goal. It’s one of the main points on the election manifesto of Sinn Féin, the political party that’s topping the polls ahead of the Republic of Ireland’s general election on Saturday. The party is calling for a vote on Irish unity by 2025 and said that it is a precondition of it entering into coalition with any other political party.

That’s partly been spurred on by Britain’s departure from the E.U., which officially happened on Jan. 31. “It has always been rumbling in the background and mainly pursued by Sinn Féin because it is the number one aim of the party,” explains Muiris MacCarthaigh, senior lecturer in politics and public administration at Queen’s University in Belfast. “But also Brexit has really pushed it into the foreground.”

The Irish nationalist party, Sinn Féin, has been polling higher than expected and according to one of the most recent opinion polls, has the highest support it has ever seen in the Republic. However, Sinn Féin has only fielded 42 candidates, about half the number fielded by the two main parties, perhaps in part due to a previous loss of seats in a local election. It’s therefore unable to fully capitalize on the unexpected surge in support. The left-wing party was once considered the political arm of the Irish Republican Army (IRA), a paramilitary organization with the goal of ending British rule in Northern Ireland. Today, Sinn Féin fields candidates on both sides of the border. In Northern Ireland, the party represents a large portion of the nationalist, traditionally Catholic community and is part of the devolved government in Belfast. In the Republic of Ireland, where Sinn Féin is currently the third largest party in the Irish parliament, the party has developed broadly center-left policies such as anti-austerity measures and wealth taxes.

In the current election, Sinn Féin has mainly campaigned on a platform of anti-austerity and freezing rents to help with Ireland’s homelessness crisis. But Mary Lou McDonald, party leader in the Republic, has said that holding a border poll—a referendum to decide whether Northern Ireland should join the Republic of Ireland—by 2025 is a condition for the party entering into government and that preparations for constitutional change on the island of Ireland need to start. While the Republic’s two main political parties have both ruled out entering into coalition with Sinn Féin after the election, Sinn Féin has already managed to make the issue of Irish unity part of the conversation again.

But could Irish reunification actually become a reality? Here’s how the island’s history could shape its future.

The Partition of Ireland and the Troubles

The partition of the island of Ireland took place in 1921, after Ireland won its war of independence Britain, at that time, the majority in Northern Ireland were British settlers who wished to remain part of the U.K. A year later, the South became the Irish Free State comprised of 26 counties, an entity independent from the United Kingdom.

The partition left bitter divisions and led to a civil war (from June 1922 – May 1923) that pitted communities and families within the Republic against each other. It ultimately set the stage for decades of tensions and conflict in Northern Ireland, known as the Troubles. Beginning in the late 1960s, the 30 year sectarian conflict in Northern Ireland left more than 3,600 people dead and thousands injured. The Troubles came to an end after a peace process, chaired by U.S. senator George Mitchell, led to the Good Friday Agreement in 1998. It established a relative peace in the region.

The Good Friday Agreement also led to the formation of the Northern Ireland Assembly, a devolved government based in Belfast that makes political decisions affecting the region. The Assembly is required to be made up of members from both the Unionist and Republican communities, both sides of the divide. As a way of dealing with the thorny issue of identity within the two communities in Northern Ireland, the agreement includes the provision that anyone born in Northern Ireland can choose either Irish or British citizenship or both. This was to take into account the fact that a large portion of the population considered themselves Irish, while another considered themselves British.

“For people in Northern Ireland, issues of identity had been largely settled under the Good Friday Agreement,” says MacCarthaigh. But two decades later, Brexit scratched the deep wound of identity, reviving the question of reunification.

How Brexit — and the Irish border — complicated the issue

For many in Northern Ireland, the U.K.’s 2016 vote to leave the E.U. raised the question of whether or not to stay in the E.U. or the U.K. Many in Northern Ireland consider themselves more closely aligned to the E.U. and even “embracing or recognizing that element of Irish identity,” MacCarthaigh says.

Northern Ireland voted to remain in the E.U. by a majority of 56% to 44%. Since then, there has been a large increase in the number of people from Britain and Northern Ireland applying for an Irish passport. “I’m not saying they are turning from British into Irish but they are sort of manifesting their right to Irish citizenship by getting an Irish passport,” adds MacCarthaigh.

Feelings in Northern Ireland are complicated because membership of the E.U. has played an invaluable role in providing financial assistance to the peace process, helping to deliver much needed infrastructure projects and European trade provided an economic boost to the region. The E.U. also helped fund bodies aimed at fostering cross-community relations, helping to ease tensions.

During the Brexit negotiations, there was a feeling among many in Northern Ireland that the government in London was not taking account of the impact that Brexit would have on the region, particularly the border areas. This was something particularly highlighted by Sinn Féin representatives in Northern Ireland. “Irish interests have never been served in Westminster,” Michelle O’Neill, vice president of Sinn Fein and deputy First Minister of Northern Ireland told TIME in September. “There’s no attempt to even understand what it [Brexit] means here for the people and for the peace process. Minister after minister demonstrated and MP after MP have demonstrated their reckless approach to the Good Friday agreement.”

And Brexit means that an E.U. member state (the Republic of Ireland) now borders a non-E.U. territory, Northern Ireland (which is part of the United Kingdom.) The Irish border was one of the main sticking points during Brexit negotiations. Both Ireland and the E.U. maintained that there should be no return to a hard border of any kind between North and South.

A return to any form of a physical border would have reopened scars from the conflict. A physical border also had the potential to be economically devastating given the interconnectedness of the two sides of the border with trade and people flowing freely. The Republic accounts for one third of Northern Ireland’s exports.

After protracted negotiations, British Prime Minister Boris Johnson agreed in principle to a customs border being located in the Irish sea as opposed to along the land border of the north and south of Ireland. This essentially unifies the island of Ireland for customs purposes. But politically, the region remains complicated.

Demonstrators from 'Border Communities Against Brexit' attend an anti-No Deal Brexit protest at the Carrickcarnon border crossing on the road between Dundalk, Ireland on October 16, 2019, and Newry in Northern Ireland. Paul Faith—AFP via Getty Images

Where do the Republic of Ireland’s political parties stand?

Since the Good Friday Agreement, both of the main parties — Fine Gael and Fianna Fáil — have said that they ultimately want to see a united Ireland. But it’s remained a talking point without a clear pathway. In a previous interview with TIME, Taoiseach Leo Varadkar said he believed a border poll in the wake of Brexit “would not achieve a united Ireland but what it would do is give rise to further nationalism, further sectarianism and further polarization.”

He added that he believed in working with all sides of the divide in Northern Ireland. “What I believe in is the politics of John Hume [who was the recipient of a Nobel Peace Prize for his role in the peace process] who who spoke less about united Ireland and more about an agreed Ireland, that we need to agree the arrangements and the relationships, north and south, east and west”

While Sinn Féin has long held a goal of Irish reunification, the party’s surge in the polls is unprecedented. A recent opinion poll by the Irish Times newspaper showed Sinn Féin with 25%, with Fianna Fáil on 23% and Fine Gael at 20%. “I think it’s remarkable that Sinn Fein are ahead in the polls,” says Professor Jonathan Tonge at the Department of Politics at the University of Liverpool. “Sinn Fein were until recently a pariah party that no one would touch,” he adds referring to the party’s links to the IRA. Because the party has polled so highly, Sinn Féin was on Tuesday granted a position in a televised debate on the national broadcaster RTE for the first time. It was less than 30 years ago that Sinn Féin was entirely banned from giving interviews on the national airwaves as part of efforts to prevent the IRA and Sinn Fein from getting their message across during the Troubles.

In response to Sinn Féin raising the issue in the election, Leo Varadkar, the Taoiseach (Ireland’s word for Prime Minister) and leader of Fine Gael, said that it would be “divisive” if a border poll was held too early. Meanwhile, Michael Martin, the leader of Fianna Fáil, said a border poll should happen “when the time is right.”

Irish reunification isn’t the main reason Sinn Féin is polling so well. Both of the main political parties seem to have lost the confidence of the Irish electorate. While Fianna Fáil’s popularity crashed during the post-2008 economic downturn, Fine Gael is broadly seen as not having made sufficient progress on bread and butter issues such as healthcare and housing, despite its success in Brexit negotiations that prioritized Ireland’s interests.

“Sinn Fein represents a left wing alternative and to some degree the others parties of the left are not that strong at the moment, whether it be the Greens, whether it be Labour,” says Tonge. “That allows Sinn Fein a fairly clear run at the center left.”

And while Irish unity is not a priority for most of the electorate in the Republic of Ireland, the fact it’s a crucial part of Sinn Fein’s agenda means more people are considering it. “People aren’t going to vote for Sinn Fein because they think a united Ireland is imminent but they are sympathetic to the idea,” Tonge says. “For many people in the Irish Republic, it is an aspiration at least.”

What would it take for Irish unity to happen?

While Brexit highlighted the complicated issue of identity in Northern Ireland, most experts say the path to Irish unity is too murky for it to happen any time soon.

While a border poll might be successful in the Republic, there is a good chance it would be voted down in the North, says Dermot Ahern, a former Fianna Fáil minister who was a key player in the peace process. “Having a border poll within five years is one-dimensional,” says Ahern. The Good Friday Agreement recognized that a substantial portion of the population of Northern Ireland wished to bring about a united Ireland and that a border poll could be called by the Northern Ireland Secretary of State if it “appeared likely” that a majority of those voting “would express a wish” for Northern Ireland to form part of a united Ireland.

If this ever occurred, “then you would have a referendum in the North and similarly in the Republic. It’s not clear exactly what the terms of the referendum would be, so on the legal constitutional side, the pathway to a united Ireland is a little bit fuzzy about how it would come about,” says MacCarthaigh.

Given the decades and even centuries of divisions between the two communities in Northern Ireland, it is not clear where the Protestant Unionist communities would fit in a united Ireland. Until January, the main political parties in Northern Ireland, the conservative, pro-British Democratic Unionist Party (DUP) and Sinn Féin had been so politically divided they had been unable to work together to form a functioning devolved government for the past three years. Before a border poll can be planned, “the DUP and Sinn Féin have to show they can work with each other,“ says Ahern.

Tensions are also rising in other parts of the U.K around Brexit, particularly with Scotland having also voted to remain in the E.U. If there was a referendum on Scottish independence, that would strengthen the case for Irish unity of some form. Because of Brexit and the vote by a majority in Northern Ireland to remain in the E.U., the region is somewhat more detached from the U.K. than it has been at any other point previously. “It is possible total detachment is likely to ensue at some point,” says Tonge. “It probably is on borrowed time but when that time expires is a difficult call to make.”

And Irish unity may not take the exact form that Sinn Féin and other parties have aspired to since the partition. Experts say it could be more about retaining membership of the E.U. than a united 32 country republic. It is more likely “that you might have some kind of federal arrangement where you have a united Ireland but the North having its own governing system within it,” explains MacCarthaigh. “Some things would remain the same and that would be possibly a more plausible scenario rather than a quick switch to Dublin controlling everything.”

Tonge agrees that the future of Irish unity is likely to be more about the European political project. Under this vision, the North would align with the South as part of a “grander European Irish project,” rather than “unfettered, undiluted Irish sovereignty.”

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