The frontier between the Republic of Ireland and Northern Ireland remains the essential conundrum of Brexit. British Prime Minister Boris Johnson is determined to leave the European Union by Oct. 31, with or without a negotiated transition deal. But any departure must address the historically fraught Irish border, which is set to become the only land crossing between the EU and the U.K. after Brexit. Johnson has devised a plan that tries to please both British lawmakers and leaders in Brussels and Dublin. It’s not at all clear he’ll succeed.

1. Why is the Irish border such a problem?

Throughout the Brexit negotiations, the EU has insisted that only goods meeting its rigorous customs and regulatory standards will be allowed to enter the bloc once the U.K. exits. At the same time, both sides agree that the frontier should remain invisible, so that people and goods can easily cross back and forth. A return to checkpoints and watchtowers would bring back bad memories, more than 20 years after a peace agreement largely ended decades of violence, and could endanger the region’s fragile peace. A controversial compromise to resolve the problem sank former U.K. Prime Minister Theresa May.

2. How might Johnson approach this?

May’s failed plan postponed a reckoning over the Irish border by effectively keeping the entire U.K., including Northern Ireland, in Europe’s customs union until a replacement trade deal could be reached. Committed Brexit backers rejected May’s approach, known as the backstop, because it left Britain too closely tied to Europe and risked trapping the U.K. in EU rules indefinitely. The new prime minister wants to scrap the backstop and is willing to break somewhat with his predecessor’s determination to keep the U.K. unified. Instead, his plans envision keeping Northern Ireland in the EU’s single market for about four years. That would subject trade between Northern Ireland and the rest of the U.K. to checks unless and until a bespoke trade deal is hammered out. However, it would also mean customs checks returning to Ireland.

3. What exactly is Johnson proposing?

Details are emerging. He wants to put Northern Ireland into a temporary and partial trade regime with the EU that would be limited to four years. This so-called “two borders for four years” plan would require customs checks, Johnson has conceded, but those might be kept miles away from the frontier to keep the crossing itself unencumbered. He has spoken in the past about ways to do this, such as high-tech tracking with cameras and GPS and “trusted trader” programs that pre-certify qualified exporters. Under Johnson’s proposal, Northern Ireland would remain in regulatory alignment with the EU for agricultural products as well as manufactured goods. But crucially, the U.K. proposed that the region be allowed to exit the arrangements if it wishes. Northern Ireland Assembly and Executive – which currently aren’t sitting – would be given a vote on whether to retain EU alignment at the end of the four-year transition period, and every four years thereafter.

4. What’s the response from Europe?

Ireland and the EU have said they’d be open to a Northern Ireland-only arrangement, but their immediate response to Johnson’s proposals were that they fell far short of what’s needed to keep trade flowing while protecting the integrity of the common market. Skeptics of Johnson’s plan aren’t convinced that workable technological solutions exist yet and argue that inspections closer to the border would still be needed. The Irish government insists that promises from Johnson aren’t enough, and is demanding a legally binding guarantee that checks won’t be re-introduced. The EU also could consider adding a time limit to the backstop, but only with Dublin’s approval -- and there’s no sign that might be forthcoming.

5. How is Northern Ireland reacting?

The fiercely pro-British Democratic Unionist Party has indicated it could back Johnson’s plan, because it keeps the region in the U.K.’s customs zone. It also might like the idea that Northern Ireland’s assembly would have to agree to aligning the region with the EU rules. A third of the members in the 90-strong power-sharing assembly can effectively block a measure they don’t like. That suggests the DUP could block the rule alignment needed to avoid a return of a border in Ireland after four years.

6. What would happen in a no-deal Brexit?

Johnson has long promised that that the U.K. won’t erect border posts, effectively daring Ireland and the EU to set up customs infrastructure on the frontier if they insist on checks. No deal would mean tariffs -- trade between the U.K. and the EU would be covered by basic World Trade Organization rules, including a raft of duties. No one knows how it would work. Ireland is in talks with the EU on how tariffs would be collected and what form checks on goods would take.

7. Why is this such a headache for Ireland?

The Irish face a dilemma in the event of a no-deal Brexit: Either they agree to reinstate the border or run the risk of being booted out of the European single market for failing to protect its flank. That’s because without border checks, Northern Ireland could be used as a so-called back door into the EU. Brazilian beef, for example, could be shipped to Belfast, moved across the open border and sent seamlessly on to the rest of the EU. Irish Prime Minister Leo Varadkar has described some form of checks as the “price we have to pay” to protect continued Irish membership in the single market.

8. What is the border like now?

It meanders through countryside for some 310 miles (500 kilometers), dividing rivers, fields and even some houses; a change in road signs and accepted currency is pretty much the only indication that a person has moved from one country to another. The island was partitioned in 1921, a division cemented by a peace agreement between the British government and Irish rebels seeking independence. As part of the deal, Northern Ireland, where the population is majority Protestant, remained part of the U.K. with England, Scotland and Wales. The mostly Catholic southern part of the island became the Irish Free State, before formally becoming a republic in 1949.

9. Could a no-deal Brexit hasten a united Ireland?

It’s unlikely anytime soon. Nearly a century after partition, a majority in Northern Ireland want to remain part of the U.K. Still, the fact that the possibility is being openly discussed again is testament to the forces unleashed by Brexit. Under a provision of the 1998 Good Friday peace accord, a so-called border poll on Irish unification could only take place if the U.K. government considers such a referendum would likely be passed. The Irish Prime Minister has warned that a no-deal Brexit could eventually stir sentiment for a united Ireland -- an analysis rejected by unionist parties in the region.

10. What about a possible return of violence?

Customs and security checks would likely hurt the economy on both sides of the border, and perhaps offer a daily reminder of British rule of Northern Ireland. While the province has been at peace for almost two decades, tension has increased after the explosion of a car bomb in January and the killing of a journalist during riots in Derry, also known as Londonderry, in April.

To contact the reporter on this story: Peter Flanagan in Dublin at pflanagan23@bloomberg.net

To contact the editors responsible for this story: Edward Evans at eevans3@bloomberg.net, Dara Doyle, Andy Reinhardt

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