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.

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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.