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 However, nationalists still pushed for full independence and in 1922 Southern Ireland was superseded by the Irish Free State as enshrined in the Anglo-Irish Treaty (pictured).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

Then-Irish Prime Minister Bertie Ahern, former US Senator George Mitchell and ex-British Prime Minister Tony Blair celebrate after finalizing the Good Friday Agreement on April 10, 1998. The pact followed lengthy negotiations and largely put an end to decades of sectarian violence, known as "the troubles," in which more than 3,000 people died.

The agreement between Britain, Ireland and the main Northern Ireland political parties, backed by the Irish Republican Army (IRA), led to a Protestant-Catholic power-sharing government in the north. In the years that followed the Good Friday Agreement, the IRA destroyed its stockpile of weapons and the British military dismantled its bases in Northern Ireland.

Two Belfast locals read a copy of the agreement the day after it was announced. A mural in the background reads: "Prepared for peace, ready for war." Two decades on, some divisions have resurfaced.

Northern Ireland's power-sharing government collapsed at the start of 2017 over disagreements between the nationalist Sinn Fein and the loyalist Democratic Unionist Party. Former US President Bill Clinton, who was due to attend at a commemorative event in Belfast on Tuesday, urged politicians to end the "paralysis." Britain's decision to leave the EU has also complicated the situation.

Former US Senator George Mitchell, who served as the chairman of the peace negotiations, is also taking part in anniversary events with Clinton, Blair, Ahern and others.

The leaders of the Good Friday peace talks — Ahern, Blair, Clinton and Mitchell — commemorated the 20th anniversary of the peace accord in Belfast on Tuesday. "Northern Ireland today is unrecognizable to the Northern Ireland of two decades ago," Mitchell said, but stressed that it will take "courageous political leadership" to meet new challenges, a likely reference to Brexit.