EU leaders agreed to negotiating guidelines during a summit in April 2017 that divided the divorce talks into two phases. Phase I, in which both sides aimed to settle the basic terms of Britain's departure, started in July and ended with an agreement on "sufficient progress" in December. Officials are now holding Phase II negotiations on the post-Brexit relationship between Britain and the EU.

London agreed to a formula for calculating what it owes in its "divorce bill" to the EU in early December after months of haggling by British officials. The current EU budget expires in 2022 and EU officials have said the divorce bill will cover financial obligations Britain had committed to before triggering article 50. The final bill will reportedly total around £50 billion (€67 billion).

Both sides agreed in early December that the 3 million EU citizens currently in Britain and the 1.1 million British citizens in the EU keep their residency rights after Brexit. British courts will have immediate jurisdiction over EU citizens living in Britain. But the EU's highest court, the ECJ, can hear cases until 2027 if British judges refer unclear cases to them.

Britain and the EU also agreed in December that no border checks between the Republic of Ireland and Northern Ireland would return post-Brexit. How feasible the commitment will be is unclear, as Britain's commitment to leaving the EU Single Market and Customs Union makes it difficult to avoid customs checks at the Irish border.

Theresa May envisages a two-year transition period after March 2019. Both sides still have to hash out the details of the transition period in Phase II, including the exact end-date, whether new EU laws passed during the period will apply to Britain, and whether Britain can negotiate its own free trade deals. British officials hope to agree on the terms of the transition by March 2018.

May has repeatedly said Britain will leave the European Single Market and the EU Customs Union. Leaving both could disrupt British-EU trade, but allow Britain to negotiate its own free trade deals and restrict EU migration — key demands by pro-Brexit politicians. London has said it wants to negotiate a new EU-UK trade deal during Phase II to minimize trade disruption before March 2019.

Britain has also vowed to restrict EU migration into Britain after Brexit. However, some British lawmakers are wary that a sharp drop in immigration could lead to shortfalls in key sectors, including health, social care and construction. The EU has warned that Single Market access is out of the question if London decides to restrict the ability of its citizens to live and work in Britain.

Recent terror attacks across Europe including a string in Britain underline both sides' support for continued security cooperation after March 2019. But access to EU institutions such as Europol and programs such as the European Arrest Warrant require compliance with EU laws. Whether Britain will still be compliant after it leaves is unclear.

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.