Boris Johnson declared last month during his speech announcing his resignation as Conservative Party leader, and therefore UK prime minister, that he “got Brexit done.” Yet he leaves his successor with a significant number of EU-related challenges still to be faced in the fall and beyond. The recent long queues of trucks and holidaymakers at UK ports reminds one that many of the ramifications of leaving the EU, long suppressed by the effects of the COVID-19 pandemic, are beginning to be more strongly felt by voters and businesses. They include new trade barriers that have gone up as a result of leaving the bloc, which have hurt importers and exporters — and despite a sharp fall in the value of the pound since the 2016 vote, there is little to suggest that businesses have benefited from increased competitiveness. Indeed, the UK economy lagged behind the trade performance of other big nations before the pandemic and has failed to fully share in the global trade rebound as the health crisis recedes. Britain’s new prime minister will have to grapple with all of these Brexit practicalities, plus a relationship with the bloc that has been frayed by Johnson’s repeated threats to rewrite the deal he signed, and an electorate that is now showing some greater signs of remorse about the result of the 2016 vote to leave the EU. UK-EU relations are at a post-Brexit low as London pushes ahead with plans for a unilateral rewriting of the Northern Ireland Protocol, which is a lynchpin of the 2020 trade agreement. It governs the movement of goods crossing the border between Ireland, which is an EU member, and Northern Ireland, which — following Brexit — is not. The EU has said the plans to alter the protocol — spearheaded by Foreign Secretary Liz Truss, who is the clear favorite to win the Conservative leadership contest and become the new UK prime minister next month — breach international law. The bloc has threatened to tear up the 2020 agreement and/or introduce sanctions. While few in Brussels are sad to see Johnson leave office, EU officials are under no illusions that there will be a big change under Truss or even if her rival, former Finance Minister Rishi Sunak, manages to pull off a surprise victory in the leadership election. With much goodwill squandered since 2016, the EU’s willingness to compromise might be significantly reduced. Despite the solidarity between the EU and UK over their support for Ukraine, this raises the prospect of a major bust-up between Brussels and London between now and 2024. While the most likely prognosis is for a continuation of poor relations, the one window of opportunity for the new prime minister will come in the second half of the current financial year when the contentious Northern Ireland Protocol bill goes to the Lords, where it could be debated for months. If it becomes law, the political fireworks with the EU could begin soon after. Brexit is by no means in the rear-view mirror as a UK policy issue. Indeed, the worst confrontations with the EU may be yet to come. Andrew Hammond The Conservative leadership debate over Brexit underlines what a strange, Alice in Wonderland-style contest it has been. Although Sunak was proponent of Brexit in 2016 and Truss a “Remainer,” the latter has won more support from hard-line Brexiteer MPs than the former. Meanwhile, the only group of Conservative Party members among which Sunak has a lead over Truss are those who voted to remain in the EU. Although Truss supported the Remain campaign in 2016, she subsequently transformed herself into one of the party’s most steadfast Brexiteers. This has earned her strong support from Brexiteer Conservative MPs, so she will be under pressure to get tough with the EU. If she scores a big victory in the leadership election when the result is announced next month, she might feel especially emboldened to pick fights with Brussels, channeling the spirit of her political hero, Margaret Thatcher, who was the UK’s first female prime minister. Truss claims that one of her key achievements since 2016 has been negotiating, while trade secretary, a new generation of post-Brexit economic deals with non-EU nations, stretching from Asia-Pacific to the Americas. Yet a significant number of these were similar, or even virtually identical, to the accords the UK already had in place as part of its EU membership. As foreign secretary, Truss has driven the plans to unilaterally override the Northern Ireland Protocol, which could have a very negative effect on relations with the EU, potentially even leading to a mini trade war, despite the ongoing Ukraine conflict. In Brussels, the widespread view is that she has made a domestic calculation about her own political ascent that has subordinated the interests of Ireland to her ambition to move into Downing Street. She has also pledged to review by the end of next year all of the EU laws that were retained by the UK after Brexit, as part of a “red-tape bonfire,” and vowed to scrap or replace those deemed to be hindering UK growth. She also reportedly said she would seek to reform the European Convention on Human Rights and would be “prepared to leave” it if necessary. While Sunak has a longer Brexit pedigree than Truss, it is surely a remarkable feature of the leadership contest that he has won significantly less support from Brexiteer MPs and party members than Truss has. That said, should he pull off a surprise victory next month, he might have more political cover than she does to seek compromises with Brussels, given his status as an original Brexiteer. Taking all of this together, therefore, Brexit is by no means in the rear-view mirror as a UK policy issue. Indeed, the worst confrontations with the EU may be yet to come. If the Northern Ireland Protocol issue descends into further acrimony, that could yet result in a trade war — or even the collapse of the 2020 deal. • Andrew Hammond is an associate at LSE IDEAS at the London School of Economics.