During the run-up to Ireland and Britain joining the European Union in 1973, my high school class in Dublin was completely divided over the merits of joining Europe. On one side were the nationalists. They said we would lose our language and our identity. On the other side were a group of passionate supporters of Europe. Here was our chance to break our economic, political and social dependence on Britain and become part of the European continent.

Little did we know at the time what impact Irish and British membership of the E.U. would have on the conflict in the British-ruled province of Northern Ireland. Back then, it was mired in sectarianism, violence and the apparently irreconcilable demands of the nationalists, who wanted unification with Dublin, and the unionists, who wanted to remain under the British Crown.

E.U. membership, however, gave Ireland a new and wider perspective that was no longer confined to Britain. That, plus enlightened leadership from London and Dublin and Washington and Belfast, led to the Good Friday Agreement of 1998. Peace, however uneasy, descended on Northern Ireland.

But Britain’s decision to leave the E.U. is already having a profound impact on the two parts of Ireland and the territorial integrity of the United Kingdom itself.

Unless British Prime Minister Theresa May grasps the implications, Brexit could precipitate the breakup of the United Kingdom. The Northern Irish might choose to find their way back to Europe by uniting with the South. If Northern Ireland opts to remain within the U.K., that could lead to a hard border between the North and the South and a whole host of corresponding new problems, above all calling into question the Good Friday Agreement. Such uncertainty could reignite tensions between nationalists and unionists.

“In terms of the Brexit debate on the U.K. mainland, nobody cares about the implications for Northern Ireland,” said Eunan O’Halpin, professor of contemporary Irish history at Trinity College, Dublin. But the citizens of the province clearly do.

The Good Friday Agreement allows the people of Northern Ireland “to identify themselves as Irish or British.” This means that the 1.8 million people in the province can hold both British and Irish citizenship.

Having both didn’t become an issue until the Brexit vote of June 2016. In Northern Ireland, 56 percent chose to stay in the E.U. Since then, there been a notable uptick in the number of people from Northern Ireland applying for Irish citizenship, from 3,973 applications in January 2016 to 7,045 a year later. An Irish passport is an E.U. passport.

The applications from Northern Ireland are also driven by fears that Brexit would lead to a “hard border” like the one that existed before the Good Friday Agreement, when a heavily fortified border divided the island. Since 1998, the island of Ireland became seamless. The E.U.’s cherished four freedoms of labor, capital, goods and services became a reality. Some 60,000 people now move each day across the invisible borders. Trade is flourishing, as are cultural and social links. Brexit could bring that to a halt.

The prospects of a hard border haunt the government in Dublin and the communities in Northern Ireland. For one thing, unless special arrangements for Northern Ireland are agreed upon by Brussels, E.U. funds — which jumped since the Good Friday Agreement and have totaled nearly $7.9 billion between 2007 and 2020 — would dry up. Britain itself, of course, would no longer be eligible for E.U. funds aimed at supporting poorer regions and agriculture.

Second, nationalists in Northern Ireland might campaign for unification with the Republic in order to remain in the E.U. — and thus realize their goal. But unionists who oppose unification would feel threatened. “The Brexit process is helping nationalists consolidate their vote,” said David Phinnemore, professor of European politics at Queen’s University, Belfast. “The unionists are not ready for unification. Not at all. For many unionists, the Crown is far more important than the E.U.,” Phinnemore added. As if London cares.

Enda Kenny, the Irish prime minister, knows his British counterpart May has little interest in Northern Ireland. He has embarked on a big diplomatic offensive in Brussels and with the E.U. member states to lobby for a soft border — and put the idea of unification on the minds of E.U. leaders.

On April 29, during a summit of E.U. leaders in Brussels, 27 of them endorsed the “Kenny Text” anchored in the Good Friday Agreement. It states that the Good Friday Agreement “provides for an agreed mechanism whereby a united Ireland may be brought about through peaceful and democratic means. In this regard, the European Council acknowledges that, in accordance with international law, the entire territory of such a united Ireland would thus be part of the European Union.”

The possibility of Irish unification isn’t the only potential challenge to the continued existence of the United Kingdom, of course. The Scots, who also wish to remain in the E.U., are already planning another referendum on independence. One wonders if May is truly aware of the gravity of the threat that faces her country.

This article was originally published by the Washington Post.