Britain and the European Union are heading for another bust-up over Brexit, with the long-running dispute over the Northern Ireland Protocol coming to the boil again. The UK threatened on Thursday to rewrite a key part of the Brexit accord. Here’s how such a move could trigger a trade war between the two allies. What’s the dispute about? The future of Northern Ireland was the most controversial and difficult part of the original Brexit negotiations. Both sides wanted to avoid putting customs checkpoints on the 499km (310-mile) border between the Republic and the North – once a major flashpoint for the sectarian violence that plagued the region for a generation. The solution UK Prime Minister Boris Johnson voluntarily signed up to was to keep Northern Ireland in the EU’s single market for goods – at the cost of putting the customs border in the Irish Sea. That means goods travelling into the province from the rest of the UK are subject to checks. Unionists argue that has driven a wedge between Northern Ireland and the rest of the UK and diverted trade flows to the south. Johnson’s government has long threated to trigger an emergency clause in the protocol known as Article 16, allowing it to suspend parts of the agreement if it causes “serious economic, societal or environmental difficulties” or a “diversion of trade”. Why now? The elections to Northern Ireland’s assembly on May 5 marked a major shift in the region’s political balance as Sinn Fein secured a victory – a historic first for the party, which supports Northern Ireland leaving the UK and reunifying with the Republic of Ireland. Sinn Fein, along with the Alliance Party, which also gained seats in the election, both support the protocol – while the Democratic Unionist Party has pushed hard for the accord to be scrapped. Sinn Fein hails ‘new era’ for Northern Ireland after historic polls The DUP has so far refused to join Northern Ireland’s power-sharing assembly, a key legacy of the Good Friday Agreement, until the UK takes action to change the protocol. Without DUP cooperation, key decisions at Stormont, Northern Ireland’s parliament, have to be put on hold. The parties now have six months to form a government or another election could be called by the UK Secretary of State. That has created an opportunity for Johnson’s government – after months of grappling with Covid-19 and partygate – to try and renegotiate the protocol and end the need for checks on most goods. What’s different this time? While the UK has long threatened to trigger Article 16, Foreign Secretary Liz Truss’ position appears to have shifted in recent days. According to The Times newspaper, she is considering new legislation to override whole sections of the protocol. In a call with European Commission Vice-President Maros Sefcovic on Thursday, she warned that the UK would have no choice but to act if the EU does not show “requisite flexibility”. In response, Sefcovic said unilateral action by the UK “is simply not acceptable” and would “undermine the conditions which are essential for Northern Ireland to continue to have access to the EU single market for goods.” Unlike Article 16, which allows the UK to take limited and proportionate action to address specific disruptions caused by the protocol after a one-month cooling-off period for talks with the EU, legislation would let the government take immediate unilateral action. But lawyers warn that such a move would be in breach of international law. UK Foreign Secretary Liz Truss takes on the Brexit job How would it work? Truss could introduce a new parliamentary bill – which would likely face strong opposition in the House of Lords – or use so-called secondary legislation, which lets a minister make a direct change to the law without needing the active approval of parliament. The legislation may dis-apply parts of the protocol relating to trade and tax – or give government ministers the powers to override it. Alternatively, Truss may simply threaten to introduce legislation in the hope of encouraging the EU to negotiate. How would the EU react? The EU is not afraid of retaliating. The bloc is likely to launch infringement procedures against the UK and could suspend the trade agreement the pair so carefully crafted years ago. That would halt tariff-free trade in goods between the two sides – leaving Britain in largely the same position as it would have been had it failed to secure a Brexit deal in the first place. While the European Commission, the EU’s executive arm, is responsible for both negotiations and retaliatory measures, a final decision on details and timing requires approval by member states, a process that could take months. What does the US say? President Joe Biden, who is of Irish descent, has repeatedly warned Johnson that peace in Northern Ireland cannot be jeopardised by tensions over the protocol. In a letter to Truss on May 10, US congressmen William Keating, chairman of the foreign affairs subcommittee on Europe, and Brendan Boyle, co-chair of the congressional EU caucus, warned that a unilateral move to rip up the protocol would undermine the Good Friday Agreement. Northern Ireland’s first minister resigns in row over post-Brexit trade What’s at stake? A trade war risks adding enormous economic pressure at a time when Europe is already grappling with a cost-of-living crisis fuelled by rising gas and oil prices. There is also Russia’s war on Ukraine to consider, which over recent months has bound the UK and EU together after years of tensions over Brexit. Acting unilaterally also risks sending a message to the rest of the world about the UK’s willingness to abide by its international agreements, former prime minister Theresa May warned this week.