In the run-up to the UK\u2019s 2016 Brexit referendum, the Irish border barely featured in the debate. Two years on, the question of how to keep the 310-mile (500-kilometer) frontier free of checkpoints and patrols threatens to derail Britain\u2019s plans for an orderly departure from the European Union. How did we get here? And what happens next? Tell me about Ireland\u2019s border Long controlled by Britain, Ireland was split in 1922 into the Catholic-dominated south and the mainly Protestant north. The south won independence, the north remained under British rule. For decades, customs officers patrolled the new border to stop smugglers. Later, soldiers manned heavily fortified checkpoints and watchtowers on the border, as violence raged across the region. Today, the frontier is pretty much invisible. Violence largely ended with a 1998 peace deal, and the fact Ireland and the U.K. were both members of Europe\u2019s single market meant there was little need for checkpoints. Visitors often only realize they\u2019ve crossed into the north when they encounter the Union Jack flag flying or curbstones painted red, white and blue. So, what\u2019s Brexit got to do with all this? Now the north and south of Ireland are a little bit akin to two adjoining U.S. states. Think of Ohio and Pennsylvania. When the U.K. leaves the EU, that relationship will becomes something closer to that between the U.S. and Canada. So, notionally, checkpoints, patrols and cameras need to return after Brexit. What\u2019s the problem with that? The absence of border controls are a powerful symbol of the region\u2019s peace process, which ended violence between nationalists fighting for a united Ireland and loyalists defending Northern Ireland\u2019s place in the U.K. A return to checkpoints and cameras risks stirring up old hatreds, as well as presenting obvious targets for new attacks. Business will suffer too The island of Ireland in many respects works as one integrated economy, with supply chains criss-crossing the border. Take Guinness, the most iconic of Irish exports. Diageo brews the stout in Dublin, loads it into \u2018silver bullet\u2019 tanker trucks and ferries the beer 90 miles north to Belfast. There, it\u2019s bottled and canned before being ferried back south. While the whole operation today is seamless, a hard border could cause delays of between 30 minutes and an hour for each truck crossing. So, what\u2019s the solution? Both sides have agreed that no new border must emerge on the island after Brexit. But the UK also wants to avoid a border between Northern Ireland and mainland Britain. Some kind of border has got to go somewhere as the U.K. is leaving the single market that makes the current setup permissible. The EU has a plan - and it\u2019s called the "backstop." What that means is that if all else fails, Northern Ireland would keep most of the bloc\u2019s rules after Brexit so that no border needs to go up. So, for example, food standards in the region would match those in Ireland, even if the British government opted to slash standards elsewhere in the U.K. Is that it? No. The EU doesn\u2019t want Northern Ireland used as a back door for sub-standard goods to slip into the single market. After Brexit, U.K. markets may be opened up to food like chlorine-treated chicken and genetically modified crops, which are banned in the EU. So checks will be needed at British ports to ensure such products aren\u2019t shipped to Northern Ireland, and then transported south and into the single market. The British have reacted furiously to that. Why are the British so angry? The U.K. says the EU effectively wants to annex Northern Ireland, with some even muttering that the Irish government is seeking to unify the island by stealth. And even if U.K. Prime Minister Theresa May liked the plan, her political weakness complicates matters. She\u2019s propped up in power by the DUP, a Northern Irish party which views Belfast as quintessentially British as London. It\u2019s threatening to veto any deal which sets up barriers with the mainland. So are we heading for a no-deal Brexit? The EU won\u2019t start sign off on the divorce deal unless the border riddle is solved. So, there is a possibility the U.K. could crash out of the bloc without a deal. In practice, that could mean tariffs on goods traded between the EU and the U.K., vast lines at ports, grounded airplanes and maybe even food and medicine shortages in Britain. Sounds fairly apocalyptic Yes, and that\u2019s one of the reasons it probably won\u2019t happen. Both sides are edging towards a compromise, and most expect a deal between the EU and U.K. government. It\u2019s worth remembering that more than possibly anybody else, the Irish want to avoid the U.K. crashing out. Without a deal, border controls would likely return between the north and south, while the vast trade that runs east-west between Ireland and Britain would be devastated by tariffs and delays. Ah, no need to worry so? Hold on. While May wants an agreement, she faces a massive challenge getting virtually any deal through Parliament. She doesn\u2019t have a majority at Westminster, and wherever she turns, there\u2019s opposition. Her hope is that enough lawmakers peer into the abyss, contemplate the chaos no deal could bring and back her plan. Final question; is this all leading to a united Ireland? Hours after the Brexit referendum, Martin McGuinness, the former Irish Republican Army leader turned politician, said it had bolstered the case for a plebiscite on a united Ireland. But for that to happen, Britain must judge it likely that Northern Ireland would opt for reunification. Despite polls showing support inching up, demand is not yet overwhelming. There\u2019s also little appetite in London or Dublin for a vote both sides fear could jeopardize the region\u2019s fragile peace.