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 However, nationalists still pushed for full independence and in 1922 Southern Ireland was superseded by the Irish Free State as enshrined in the Anglo-Irish Treaty (pictured).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

After a shrill referendum campaign, nearly 52% of British voters opted to leave the EU on June 23. 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.

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.

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."

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 split talks into two phases. The first would settle the terms of Britain's exit, and the second the terms of the EU-UK relationship post-Brexit.

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.

Leaders of the remaining 27 EU members formally agreed that "sufficient progress" had been made to move on to phase two issues: the post-Brexit transition period and the future UK-EU trading relationship. While Prime Minister Theresa May expressed her delight at the decision, European Council President Tusk ominously warned that the second stage of talks would be "dramatically difficult."

British ministers appeared to back a Brexit plan at May's Chequers residence on July 6. The proposal would have kept Britain in a "combined customs territory" with the EU and signed up to a "common rulebook" on all goods. That went too far for British Foreign Minister Boris Johnson and Brexit Secretary David Davis, who resigned a few days later. May replaced them with Jeremy Hunt and Dominic Raab.

May's Chequers proposal did not go down well with EU leaders, who told her at a summit in Salzburg in late September that it was unacceptable. EU Council President Tusk trolled May on Instagram, captioning a picture of himself and May looking at cakes with the line: "A piece of cake perhaps? Sorry, no cherries." The gag echoed previous EU accusations of British cherry-picking.

EU leaders endorsed a 585-page draft divorce deal and political declaration on post-Brexit ties in late November. The draft had been widely condemned by pro- and anti-Brexit lawmakers in the British Parliament only weeks earlier. Brexit Secretary Dominic Raab resigned along with several other ministers, and dozens of Conservative Party members tried to trigger a no-confidence vote in May.

In the face of unrelenting opposition, May postponed a parliamentary vote on the deal on December 10. The next day, she met with German Chancellor Angela Merkel to seek reassurances that would, she hoped, be enough to convince skeptical lawmakers to back the deal. But while she was away, hard-line Conservative lawmakers triggered a no-confidence vote. May won the vote a day later.

The UK Parliament voted 432 to 202 against May's Brexit deal on January 16. In response to the result, European Council President Donald Tusk suggested the only solution was for the UK to stay in the EU. Meanwhile, Britain's Labour Party called for a no-confidence vote in the prime minister, her second leadership challenge in as many months.

May tried to get legal changes to the deal's so-called Irish backstop in the weeks that followed. She eventually got assurances that the UK could suspend the backstop under certain circumstances. But on March 12, Parliament voted against the revised Brexit deal by 391 to 242. EU leaders warned the vote increased the likelihood of a no-deal Brexit. Two days later, MPs voted to delay Brexit.

Following the second defeat of May's divorce deal, the European Council met in Brussels on March 21 to decide what to do next. EU leaders gave May two options: delay Brexit until May 22 if MPs vote for the withdrawal deal or delay it until April 12 if they vote against the deal. If the deal were to fail again in Parliament, May could ask for a long extension.

On March 29, the day that the UK was supposed to leave the EU, British lawmakers voted for a third time against May's deal — rejecting it this time with a vote of 344 to 286. Following the latest defeat, May approached the main opposition Labour leader Jeremy Corbyn in an attempt to find a compromise, angering hardline Brexiteers in her own Conservative party.

With the April 12 deadline looming after the third defeat of May's deal, EU leaders met again in Brussels to discuss a second delay. The only question was how long should it be? In the end, the UK and EU agreed to a "flexible" extension until October 31 — which can end sooner if the Brexit deal is approved. The UK had to take part in EU elections in May because their exit wasn't secured in time.

Weeks of talks between Prime Minister Theresa May and the Labour party to reach a deal proved unsuccessful and further eroded her political capital. She triggered an angry backlash from her party after she tried to put the option of a second referendum on the table. The series of failures led May to announce her resignation, effective June 7, in an emotional address.

After Theresa May announced on June 7 that she would leave office, other members of her Conservative party began clamoring for the top job. Within a month, the leadership battle came down to Jeremy Hunt (left), an EU proponent who fears a no-deal scenario, and Boris Johnson (right), one of the main proponents of Brexit.

At the end of July 2019, Johnson was officially named Theresa May's successor as British prime minister. "We are going to energize the country, we are going to get Brexit done by October 31," he said after he was elected leader of the Conservative Party.

Conservative rebels and opposition MPs backed efforts to delay an October 31 Brexit deadline in fear of a no-deal departure. In response, Johnson called for a general election, saying his government cannot rule without a mandate after he stripped 21 rebel MPs of their Conservative status. The Labour Party said it would not back elections until legislation to block a no-deal Brexit was in place.

In late September, Britain's highest court ruled that Johnson's decision to suspend Parliament ahead of the UK's planned exit was unlawful. "This was not a normal prorogation in the run-up to a Queen's Speech," said the Supreme Court. Political rivals immediately called on Johnson to leave his post. Johnson said he would abide by the court ruling, though said he "strongly" disagreed.

British Prime Minister Boris Johnson managed to secure a deal with European negotiators that would allow the UK to leave the EU in an orderly manner. The deal received unanimous backing from the leaders of 27 other member states. But an attempt to get the UK Parliament to sign off on the deal failed. Instead, Parliament pushed for the Brexit deadline to be extended until the end of January 2020.

On December 22, UK lawmakers vote for Prime Minister Johnson's European Union withdrawal bill, which will see a leave date of January 31 2020 enshrined in law. Getting a majority to vote to pass the bill in the lower house has proven a major sticking point for the PM, but following a general election Johnson's Conservative party won control of the house and the bill passed with a 124 majority.

After months of disagreements over fishing rights and future business rules, the EU and UK clinched a post-Brexit trade deal on Christmas Eve. Prime Minister Boris Johnson hailed the deal, saying the UK has "taken back control of our laws and our destiny." The deal will allow the UK and the EU to trade without tariffs, but also impose limitations on free movement and financial services.