“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.”