Britain's response to Irish demands for independence was devolution within the UK, or home rule. Pro-British Unionists didn't want to be governed by Dublin, so two parliaments were set up, for Northern and Southern Ireland. However, nationalists still pushed for full independence and in 1922 Southern Ireland was superseded by the Irish Free State as enshrined in the Anglo-Irish Treaty (pictured).
The Six Counties
Northern Ireland had been carved in a way that allowed Protestant loyalists to stay in control, but also ensure the region was large enough to be viable. It included four majority-Protestant counties in the ancient province of Ulster, as well as the two Catholic nationalist counties. Three of Ulster's counties — Donegal, Monaghan and Cavan — were placed on the Southern Ireland side of the border.
No laughing matter?
Involving members of the British, Irish and devolved Belfast governments, a 1924-25 boundary commission looked at the whether the border should stay where it was. It broadly remained in the same place, often cutting through communities across its 310 miles. The Spike Milligan novel "Puckoon," made into a film (above), charted the problems brought to a fictional Irish village divided by the border.
Roadside customs checks
The new border's checkpoints initially regulated the movement of certain goods, with movement of people being free. However, the Anglo-Irish Trade War of the 1930s saw tariffs imposed on foods and later coal and steel. The dispute ended in 1936, but Ireland still pursued protectionist policies into the 1950s. Customs stayed in place until the advent of the EU Single Market in 1993.
With an escalation in fighting in Northern Ireland in 1969, British troops were sent to the province, fueling nationalist resentment. The border was heavily guarded to stop weapons smuggling from the Republic. The South Armagh stretch was particularly notorious. The Irish Republican Army's South Armagh Brigade is thought to have killed about 165 British troops and police from 1970 to 1997.
South of the Border
The border was also policed by the Republic of Ireland's security forces, who intensified their anti-terror efforts in the late 1970s. They worked with the British, but the working relationship was not an easy one. To communicate with Irish counterparts, British troops at one time had to speak to the Northern Irish police, who would contact the Irish police, who would then call the Irish army.
Watchtowers and rifle sights
Despite the end of customs in 1993, the threat of terror still loomed and the border remained militarized, with watchtowers and soldiers. After the 1998 Good Friday Agreement — which brought back devolved government to Northern Ireland and sought to address issues such as policing and paramilitarism — the IRA eventually halted its campaign of violence as border security disappeared.
The border today is as invisible as it has ever been, with free movement of traffic between the Republic and the North. The picture shows two policemen, one British, one Irish, watching as a foreign leg of the 2014 Giro d'Italia crosses the border in Armagh.
Anything to declare?
There are fears that Brexit would make a hard border necessary, given that Britain appears set to leave the EU Customs Union and Single Market. The border issue is one of three conditions laid out by the EU for trade talks to begin. Brussels says there must be no hard border. Campaigners, like those pictured above, have sought to remind the public of what such a frontier would look like.
These days, the Northern Ireland peace process and free trade mean you'd hardly notice that there was a border separating two parts of the Emerald Isle. Brexit could make things complicated once again.
Based on their past meetings, Monday's Brexit talks between EU Commission President Jean-Claude Juncker and UK Prime Minister Theresa May could provide further fireworks. Among the many contentious issues, the question of how to resolve the Irish border dispute could be a major stumbling block. But what makes the issue of the Irish frontier so sensitive?
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.
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.
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 - the ed.] 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.
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 issue of customs checks looks likely to be revisited once again.
June 2016: 'The will of the British people'
After a shrill referendum campaign, nearly 52 percent of British voters opted to leave the EU on June 24. Polls had shown a close race before the vote with a slight lead for those favoring remaining in the EU. Conservative British Prime Minister David Cameron, who had campaigned for Britain to stay, acknowledged the 'will of the British people' and resigned the following morning.
July 2016: 'Brexit means Brexit'
The former Home Secretary Theresa May replaced David Cameron as prime minister on July 11 and promised the country that "Brexit means Brexit." May had quietly supported the remain campaign before the referendum. She did not initially say when her government would trigger Article 50 of the EU treaty to start the two-year talks leading to Britain's formal exit.
March 2017: 'We already miss you'
May eventually signed a diplomatic letter over six months later on March 29, 2017 to trigger Article 50. Hours later, Britain's ambassador to the EU, Tim Barrow, handed the note to European Council President Donald Tusk. Britain's exit was officially set for March 29, 2019. Tusk ended his brief statement on the decision with: "We already miss you. Thank you and goodbye."
June 2017: And they're off!
British Brexit Secretary David Davis and the EU's chief negotiator, Michel Barnier, kicked off talks in Brussels on June 19. The first round ended with Britain reluctantly agreeing to follow the EU's timeline for the rest of the negotiations. The timeline splits talks into two phases. The first settles the terms of Britain's exit and the second the terms of the EU-UK relationship post-Brexit.
July-October 2017: Money, rights, and Ireland
The second round of talks in mid-July began with an unflattering photo of a seemingly unprepared British team. It and subsequent rounds ended with little progress on three phase one issues: How much Britain still needed to pay into the EU budget after it leaves, the post-Brexit rights of EU and British citizens, and whether Britain could keep an open border between Ireland and Northern Ireland.
November 2017: May pays out?
Progress appeared to have been made after round six in early November with Britain reportedly agreeing to pay up to £50 billion (€57 billion/$68 billion) for the "divorce bill." May had earlier said she was only willing to pay €20 billion, while the EU had calculated some €60 billion euros. Reports of Britain's concession sparked outrage among pro-Brexit politicians and media outlets.
December 2017: Green light for phase 2
Leaders of the remaining 27 EU members formally agreed that "sufficient progress" had been made to move on to phase 2. Talks will now focus on a transition period and the future trading relationship between the two sides. While the Britain's Theresa May expressed her delight, European Council President Donald Tusk ominously warned that the second stage of talks will be "dramatically difficult."