A truck crosses from the Republic of Ireland into Northern Ireland.Paul Faith / AFP / Getty

It was music that first brought me to Belfast.

In the 1990s, I would travel from Dublin to see bands play at Queen’s University, though I never ventured much beyond the school. There was a justifiable tension in the city—British army patrols were commonplace, and all vehicles crossing the border that separated the Republic of Ireland from Northern Ireland were stopped and checked by security forces who were young, armed, and nervous. Getting through was always a relief, a palpable lifting of the tension, tempered only by the fact that I knew I would have to cross again, hours later, to go home.

Music still brings me to Belfast, though now it’s for vinyl. Secondhand music stores have sprouted up across the city, and spending an afternoon browsing for bargains is a joy. I now live about a 30 minutes’ drive from the border, and were it not for the change in speed-limit signs—the republic uses kilometers, while Northern Ireland uses miles—I would not notice the border at all. With Britain’s impending withdrawal from the European Union, that might soon change.

Ostensibly about Britain’s relationship with the Continent, in reality Brexit is the playing out of a fractious, decades-long struggle within the Conservative Party, one that spilled over into the public sphere through a referendum that became a lightning rod for much of the deep-rooted social and economic anger across British society. The decision to leave the EU will affect all aspects of life in the United Kingdom, from the testing of drugs to the negotiation of international trade deals.

But perhaps none will be as important as that of the now-intangible Irish border. The question of the frontier between the U.K. and the Republic of Ireland played a relatively minor role in the 2016 Brexit referendum. In recent months, however, it has become the singular issue on which Britain’s withdrawal from the EU depends. Lawmakers in British Prime Minister Theresa May’s Conservative Party have threatened to torpedo any deal over the so-called backstop—the mechanism by which Britain would have to abide by the bloc’s customs rules until the two sides reach an agreement on their future relationship—arguing that it tethers the U.K. to the EU for an indeterminate period of time. Failing to reach a deal by March 29, when Britain is legally set to withdraw, could result in a “hard Brexit,” which might, among other things, reimpose a tangible border between the republic and Northern Ireland.

In effect, the transformation of the border, an achievement that was critical to the development of peace and stability on the island of Ireland, is now at risk, and many are justifiably worried.

About 300 roads and pathways intersect the meandering line between the Republic of Ireland and Northern Ireland, some of which are so hidden that they were mapped only last year. About 45 million cross-border vehicle journeys take place each year, and the republic is Northern Ireland’s single biggest export market. (As with all trade between members of the EU’s single market, customs and regulatory checks are nonexistent.)

It was not always this way. The island was partitioned in May 1920, although the first customs posts were not opened until April 1, 1923—Easter Sunday. Initially, they were a source of bemusement, as there had never before been a border on the island, but it was not long before they became a source of frustration and resentment, and, eventually, the target of attack.

Partition brought disruption to centuries-old trade and supply routes, particularly in agriculture, the dominant form of economic activity on the island at the time. It resulted in a marked deterioration in the economic well-being of the towns and communities that straddled the border, a shift that was exacerbated by a dispute that erupted in the 1930s between the two countries over land annuities. The border was tightened and trade plummeted. It was only after the end of World War II that things slowly began to improve.

The border, however, remained a contentious political issue. In the 1950s, the Irish Republican Army (IRA), which opposed British rule of Northern Ireland, began to bomb police stations and border posts in the region. Then the Troubles began.

The Troubles remains an understated term for the political and sectarian violence that engulfed Northern Ireland for decades. The border was quickly militarized, with significant knock-on economic effects. Many “unapproved” roads and bridges—crossings with no police or customs checkpoints—were blocked or blown up by the British army in the 1970s, causing huge disruption to local communities but having little effect on IRA activity. By the 1980s, the border area was bandit country, with significant parts of it inaccessible to security forces, except by helicopter.

Things finally began to improve in the 1990s. All tariffs and sales-tax restrictions between the Republic of Ireland and the U.K. ended in 1993 with the creation of the European Single Market, though border controls remained in place. The security measures were intense, often resulting in delays for hours at a time and requiring an infrastructure that was subject to near-constant attack. It was only after the signing of the Good Friday Agreement in 1998 and the demilitarization that followed that the benefits of EU rules, which allowed for the free travel of goods and services, were felt. Peace brought stability to Northern Ireland as well as a strengthening of the all-island economy, particularly in agriculture, which remains to this day a core industry along the border. The production of beef, milk, and poultry products in Ireland is a seamless affair. One dairy farmer told The Detail, a Belfast-based news site, how his produce crosses the border twice before it is even sold in grocery stores.

It was both the single market and the Good Friday Agreement that made the border invisible and ensured that peace became a viable, ongoing reality. The two are needed to sustain that peace. The people of Northern Ireland, who voted to remain in the EU, are fully aware of the dangers of the U.K. leaving the EU without a deal or a solution for the border issue.

Brexit is not just about tariffs or trade: It is about history, and making sure it is not repeated. This is a real concern. Last month, a car bomb exploded outside a courthouse in Derry , Northern Ireland. It was planted by a dissident republican group, a reminder that there are those who are all too willing to see the peace process tossed aside.

Belfast emerged from under the shadow of the Troubles to shine as a city of venues, culture, books, and bargains. The elimination of border and security controls was crucial to that process. Peace came slowly to Northern Ireland. It needs to be nurtured and protected so that the music and the vinyl—not the violence—define the region.

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