Passing through the border was almost unnoticeable. I only recognized a difference because of a few signs along the road pronouncing a change in currency. Euros would be no good here; in Northern Ireland, they only accepted British sterling.

I visited the Republic of Ireland in 2016, not long after the United Kingdom voted to leave the European Union (EU). A debate immediately ignited over what was, for so long, an issue of the past: the border between Ireland, part of the EU, and Northern Ireland, which belongs to the United Kingdom (U.K.).

Recent years have seen the debate about the border between the U.S. and Mexico reach a fever pitch. But now a debate which was thought to be over is bubbling hundreds of miles across the ocean. Memories of a “hard border” are still fresh in Ireland. Hardly more than 20 years ago, the military checked identification at border crossings. Unrest between Catholics and Protestants led to violence and fear.

“The Troubles,” as this time was called, led to the deaths of thousands. Irish nationalists and British loyalists radicalized. There were bombings in pubs and city centers which killed dozens and injured hundreds. The Bloody Sunday massacre led to the shooting over a dozen peaceful protesters. Some might be aware of the drink known as the Irish Car Bomb. It’s made by dropping a shot of whiskey and Irish cream into a Guinness stout. It derived its name from the car bombing which were so frequent in Ireland in the late 20th century that they became near ubiquitous.

A series of back-to-back car bombings in 1974 in Dublin killed 26 in one day. “It’s like having a drink called the ‘9/11’,” an Irish-American friend once told me, half-joking. For years, these atrocities were a not-so-distant memory. Now they threaten to become a future. Britain’s withdrawal from the EU would mean that the border between Ireland and Northern Ireland would now become a border between an EU country and a non-EU country.

This could lead to high tariffs, political unrest and, as some conservatives in the U.K. desire, a hard border.

It’s hard for Americans to understand. The conflict between Protestants and Catholics in Ireland have deep roots. But from an onlooker, one sees only an island; it almost feels silly to divide such a small land mass into different parts.

But looking at this Irish past provides a mirror for the American present. For decades, the border between Northern Ireland and Ireland was a symbol of fear. Fear for the military who guarded it and harassed those who wished to cross. Fear that extremists would slip through and hurt those on the other side. Fear that a proud nation may never again be unified. When British citizens voted to leave the EU, many liberal commentators considered it a warning sign for America’s impending presidential election.

Brexit was an unprecedented maneuver by the British nationalists. There was no play book on how to leave the EU. But there is a long, storied history on how to treat the Irish border, and there are many lessons to be learned from the pain that it caused. It’s one we should all look toward.