The Irish border has become a major stumbling block ahead of Britain's exit from the European Union. What makes the issue of the Irish frontier so sensitive?

Ireland splits into two

The British response to the Irish political and armed struggle for independence around the time of World War I was a succession of Home Rule Acts that sought to allow devolution, rather than independence, on the island of Ireland. Initially, one institution was envisaged in Dublin, but unionists in the north would have been unhappy at the prospect of being ruled from such a perceived hotbed of Irish nationalism. Instead, two home rule parliaments were set up, Northern Ireland and the short-lived entity known as Southern Ireland.

Read more: Past haunts Northern Ireland 20 years after the 'troubles'

After a three-year Irish War of Independence, Britain admitted defeat. Southern Ireland was superseded in 1922 by the Irish Free State, as enshrined in the Anglo-Irish Treaty (pictured above). This state would no longer be part of the United Kingdom.

Carved for convenience

Unionists were in the minority in most of Ireland, but not in the mainly Protestant Antrim, Down, Armagh and Derry/Londonderry — all part of the northern ancient province of Ulster.

These four were not considered enough to form a viable area, and so Tyrone and Fermanagh were also incorporated into Northern Ireland. Three of Ulster's other counties — Donegal, Monaghan and Cavan — became part of Southern Ireland.

All of these considerations meant that physical practicality was never at the forefront of the minds of those who drew the borders. The border often cut through communities, a subject that was the theme of comic writer Spike Milligan's novel Puckoon, which chronicled the troubles of a fictional village cut in half by the border.

Barely noticeable boundary

The Common Travel Area, established in 1923, has long ensured that there was no need to show passports at the border — a sort of early Schengen arrangement [a European agreement signed in 1985 that largely does away with internal border checks] that was briefly suspended during World War II.

It's a very busy border. Because of the erratic nature of the boundary, regular journeys in border areas often cross the frontier several times. In addition, the northern county of Donegal is separated from the rest of the Irish Republic by a thin territorial isthmus of land. This means it's often far quicker to reach other parts of the Republic from there by crossing into Northern Ireland.

Read more: Ireland to seek emergency help in case of no-deal Brexit

There are some 300 major and minor crossings along the 499-kilometer (310-mile) border which, unlike most other borders in the EU, is not officially marked by either government although there may be "Welcome to" signs. This makes identifying the border difficult for strangers who are unacquainted with landmarks known to locals as the crossing point. 

Time of troubles

The border wasn't always so understated. With an escalation in violence in Northern Ireland in 1969, British troops were sent to the province. The border was heavily securitized to prevent the smuggling of weapons from arms dumps in the Irish Republic.

In order to control traffic entering or leaving the province at main checkpoints, the British Army blocked off smaller access points. Roads were cratered and bridges were blown up. For some communities, the inconvenience of this was crippling, helping to fuel nationalist resentment. A study by Belfast's Queen's University, "Bordering on Brexit," asked people what they felt at the time.

"I grew up a stone's throw from the border," one woman, from the Fermanagh and Omagh district on the northern side, told the survey. "I remember 22-mile detours to go 4 miles up the road. I remember the militarization of border crossings and the closure of roads. I remember how few services we had and how difficult it was for people to survive. We were completely terrorized by the British military."

Anything to declare?

Despite the Common Travel Area, movement of goods across the border was not unfettered and customs points were established. Initially, the checkpoints were only intended to regulate the movement of certain goods. However, the Anglo-Irish Trade War of the 1930s saw the introduction of tariffs on agricultural products and eventually coal and steel. 

Both governments enacted policies that were damaging to their border communities. Smuggling and black market trading picked up, exacerbated by World War II, in which Ireland remained neutral.

The trade war ended in 1936 but there were still customs checks even after both nations joined the European Economic Community (at the same time in 1973). They only ended with the opening of the European Single Market in January 1993.

Given that Britain appears set to leave the EU Customs Union and Single Market at some future point, the reintroduction of customs checks has become a possibility, albeit one with few supporters.