LONDON — When President Biden met Prime Minister Boris Johnson at the White House last month, one of Mr. Biden’s advisers passed him a note reminding him to raise the issue of trade tensions over Northern Ireland.
“We’ve already talked about that,” the president said, according to people in the room, referring to a one-on-one session he had held with Mr. Johnson before the group meeting. It is not clear what Mr. Biden told his British guest, but the president’s interest in Northern Ireland has not diminished since then.
In recent days, unprompted, Mr. Biden asked his staff for an update on the negotiations between Britain and the European Union over trade arrangements in Northern Ireland. He urged them to relay a message to the Johnson government that it should not do anything that would jeopardize the peace accord in the North, according to officials familiar with the discussions.
Mr. Biden’s deep political and familial ties to Ireland set him apart from his predecessor, Donald J. Trump, who was an avid supporter of Brexit and encouraged Mr. Johnson’s tangles with Brussels. It also makes the United States an important offstage player in the latest chapter of the long-running Brexit saga.
As Britain and the European Union kick off negotiations over revamping the complex trade regulations in the North — a legal construct known as the Northern Ireland protocol — pressure from the American president may cause Mr. Johnson to think twice about provoking another destabilizing clash with Brussels.
Mr. Biden is staunchly opposed to any move that would jeopardize the Good Friday Agreement, the 1998 accord, negotiated under the auspices of the Clinton administration, which ended decades of sectarian violence in the North.
“I feel very strongly about those,” Mr. Biden said to reporters before meeting with the prime minister. “We spent an enormous amount of time and effort in the United States. It was a major bipartisan effort.”
Defenders of the peace agreement worry that Britain’s latest demands could jeopardize it by prompting the resurrection of a hard border across the island of Ireland rather than the current arrangement in which there are no border checkpoints and the change in road signs is the only notice travelers get when they cross from North to South.
Jake Sullivan, Mr. Biden’s national security adviser, recently issued an unusually direct warning to Britain not to overturn the protocol — something it has threatened to do if the European Union does not agree to rewrite the rules.
“Without something like the Northern Ireland Protocol, and with the possibility of the return of a hard border between Northern Ireland and the Republic of Ireland, we will have a serious risk to stability and to the sanctity of the Good Friday Agreement, and that is of significant concern to the United States,” Mr. Sullivan told the BBC during a visit to Brussels on Oct. 7.
British officials tend to play down the importance of Northern Ireland to the trans-Atlantic relationship. Some suggest that Mr. Biden and other American politicians invoke it as much to mollify Irish American voters as to try to influence British policy. Others complain that the Americans conflate the protocol, a highly technical — and in their view, deeply flawed — trade arrangement, with the Good Friday Agreement, a landmark peace treaty signed by the British and Irish governments.
Besides, they point out, Britain and the United States just entered a strategically sensitive submarine alliance, along with Australia, that underscores the hand-in-glove security alignment between London and Washington.
But analysts on both sides of the Atlantic say these arguments are wishful thinking. Mr. Johnson, they said, will antagonize Mr. Biden if he does not find a compromise with the European Union. It could set back a relationship the prime minister has worked hard to build after being seen by some Democrats as an ideological twin of Mr. Trump.
“Biden doesn’t bear Boris Johnson any grudge, and he’s willing to work with the U.K.,” said Thomas Wright, the director of the Center on the United States and Europe at the Brookings Institution. “But he also cares deeply about the Good Friday Agreement, and unilaterally pulling the plug on the protocol on an issue of principal could materially affect the relationship in a negative way.”
Mujtaba Rahman, a London-based analyst at the political risk consultancy Eurasia Group, said Mr. Johnson was “underpricing” the chances of a sharp American reaction if it invoked Article 16, a provision that allows either side to scrap the protocol if it causes serious disruptions. Alienating the United States, he said, could hinder Mr. Johnson’s ambitions for a globally minded foreign policy.
“What we know is that Biden absolutely cares about the peace process,” said Mr. Rahman, who once worked at the European Commission. “Anything that potentially jeopardizes that blows back on the narrative around Global Britain.”
Further complicating matters, the European Union last week offered an unexpectedly broad package of concessions to Britain to smooth trade with Northern Ireland. Checks on food and animal products going from mainland Britain to the North would be reduced by 80 percent and customs paperwork for shipments of many goods would be slashed. The flow of medicines would also be guaranteed.
The proposal seems a genuine effort to tackle a thorny legacy of Brexit: how to treat Northern Ireland, which is part of the United Kingdom but shares an open border with a member of the European Union, the Irish Republic. Britain, however, has tacked on an additional demand — that the European Court of Justice have no role in arbitrating disputes — which Brussels has so far ruled out.
It’s a point of principle for Mr. Johnson because it strikes at the heart of Brexit’s primary rationale: That Britain liberate itself from the vexing, oppressive oversight of Brussels. If the two sides cannot work out a deal, analysts said, Mr. Johnson will likely overturn the protocol, though he is not likely to do so before a United Nations climate summit in Glasgow that Mr. Biden is scheduled to attend.
That, in turn, could snowball into an all-out trade war.
“There is a fragility to this relationship because of a provision that allow the parties to pull out of the trade agreement,” said Anand Menon, a professor of European politics at King’s College London. He noted that Mr. Johnson did not raise the European court in the forward to his government’s paper last summer outlining problems with the protocol. “They’ve been cackhanded in how they’ve handled this,” Mr. Menon said.
Relations with Mr. Biden are only one of several factors Mr. Johnson must weigh as he decides how to respond to the European Union. There are the unionists in Northern Ireland, who have become hostile toward the protocol because they view it as driving a wedge between them and the United Kingdom. There are hard-line Brexit supporters in the Conservative Party, who view any compromise as a sign of weakness.
Mr. Johnson must also consider Britain’s relations with its neighbors, chiefly France, which is already in a low-grade dispute with Britain about fishing rights and is angry about being elbowed out of the submarine alliance by Australia and the United States. Such tensions, some said, are an inevitable vestige of Brexit and are likely to linger regardless of how the current dispute is resolved.
“We are a medium-size country next to a continental economic hegemon,” Mr. Menon said. “It’s going to take forever to sort out, because like it or not, whatever they do, it’s going to have an impact on us.”