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 However, nationalists still pushed for full independence and in 1922 Southern Ireland was superseded by the Irish Free State as enshrined in the Anglo-Irish Treaty (pictured).

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.

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.

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.

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.

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 remains 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 Giro d'Italia crosses the border in Armagh.

There were fears that Brexit would make a hard border necessary, given that Britain has left both the EU Customs Union and Single Market. The border issue was one of three conditions laid out by the EU for talks on future trade after the separation. Campaigners, like those pictured above, had sought to remind the public of what a hard border would look like.

Customs officials check freight trucks as they disembark from a ferry at the Northern Irish port of Larne. The inspections effectively created a customs border in the Irish Sea, avoiding the need for checks on land. The arrangement has led to supply problems for some businesses. However, it has been touted as good for Northern Ireland, giving firms there free access to both the UK and EU markets.

May became prime minister after David Cameron resigned from the post in the wake of the Brexit referendum vote in June 2016. Despite her position, she has struggled to define what kind of Brexit her government wants. Hardliners within her Conservative party want her to push for a clean break. Others want Britain to stay close to the bloc. The EU itself has rejected many of May's Brexit demands.

The leader of the British Labour Party has no formal role in the Brexit talks, but he is influential as the head of the main opposition party. Labour has tried to pressure the Conservative government, which has a thin majority in Parliament, to seek a "softer" Brexit. But Corbyn's own advocacy has been lukewarm. The long-time leftist voted for the UK to leave the European Community (EC) in 1975.

Boris Johnson's turbulent two years as UK foreign secretary came to an abrupt end with his resignation on July 9. The conservative had been a key face for the Leave campaign during the 2016 referendum campaign. Johnson disapproves of the "soft Brexit" sought by PM May, arguing that a complete break from the EU might be preferable. He became the second Cabinet member within 24 hours to quit...

David Davis headed Britain's Department for Exiting the EU and was the country's chief negotiator in the talks before he quit on July 8, less than 24 hours before Downing Street announced Boris Johnson's departure. Davis had long opposed Britain's EU membership and was picked for the role for this reason. Davis was involved in several negotiating rounds with his EU counterpart, Michel Barnier.

Raab replaced Davis in early July 2018. But he only lasted four months, resigning a day after Theresa May presented a draft withdrawal plan to her cabinet. Raab previously worked for a Palestinian negotiator in the Oslo peace process and as an international lawyer in Brussels advising on European Union and World Trade Organization law.

Jeremy Hunt was Britain's Health Secretary until he replaced Boris Johnson as foreign secretary in early July 2018. The 51-year-old supported Britain remaining in the European Union during the 2016 referendum, but said in late 2017 that he had changed his mind in response to the "the arrogance of the EU Commission" during Brexit talks. He has vowed to help get Britain a "great Brexit deal."

Nigel Farage was the leader of the United Kingdom Independence Party (UKIP) until July 2016. Under his stewardship, the party helped pressure former Prime Minister David Cameron into calling the EU referendum. He was also a prominent activist in the Leave campaign in the lead-up to the vote. Farage still has some influence over Brexit talks due to his popularity with pro-Leave voters.

Businessman Arron Banks is a friend of Nigel Farage, and donated a significant sum to the former UKIP leader's Leave.EU campaign – making him the group's biggest financial backer. He had several meetings with Russian officials ahead of the referendum, but has denied allegations of collusion with Moscow in the Brexit vote, branding the claims a "political witch hunt."

EU Commission President Juncker (left) and EU Council President Tusk (right) share two of the bloc's highest posts. Juncker heads the EU's executive. Tusk represents the governments of the 27 EU countries — the "EU 27." Both help formulate the EU's position in Brexit negotiations. What Tusk says is particularly noteworthy: His EU 27 masters — not the EU commission — must agree to any Brexit deal.

The former French foreign minister and European commissioner has become a household name across the EU since his appointment as the bloc's chief Brexit negotiator in October 2016. Despite his prominence, Barnier has limited room to maneuver. He is tasked with following the EU 27's strict guidelines and must regularly report back to them during the negotiations.

The Irish PM has been one of the most important EU 27 leaders in Brexit talks. Britain has said it will leave the EU's customs union and single market. That could force the Republic of Ireland, an EU member, to put up customs checks along the border with Northern Ireland, a British province. But Varadkar's government has repeatedly said the return of a "hard" border is unacceptable.

The leaders of the EU 27 governments have primarily set the EU's negotiating position. They have agreed to the negotiating guidelines for chief negotiator Barnier and have helped craft the common EU position for Tusk and Juncker to stick to. The individual EU 27 governments can also influence the shape of any Brexit outcome because they must unanimously agree to a final deal.