Analysts said the British government had concluded that Northern Ireland was crucial enough to risk jeopardizing other relationships, even with the United States.

“Johnson is willing not only to contemplate a trade war with Europe but also to ignore the very strong advice he got from Biden in private,” said Mujtaba Rahman, an analyst with the political risk consultancy Eurasia Group. “He was more than willing to allow the row to overshadow Britain’s coming-out party.”

? This is something about Brexit and Northern Ireland.

Mr. Johnson and Mr. Macron had a tense exchange over post-Brexit trade arrangements for the region. British officials are demanding that the European Union change the current system — designed to avoid new barriers between Ireland, an E.U. member state, and Northern Ireland — because it says that checks on some goods flowing from mainland Britain to Northern Ireland are driving a wedge between two parts of the United Kingdom.

...The French president said later he had never questioned “the integrity of the British territory” but insisted that Britain needed to abide by the terms of the deal it signed with the European Union in 2019.

Mr. Johnson, for his part, asked Mr. Macron how he would feel if sausages could not be shipped from Toulouse to Paris.
...
The subject also came up with Mr. Biden. The national security adviser, Jake Sullivan, told reporters that “they had a candid discussion of it in private” and that the president urged the prime minister to protect the Good Friday Agreement, the 1998 accord that ended decades of sectarian violence in Northern Ireland. Tensions over trade have ignited street clashes in Belfast and other cities.
...
...an ugly clash with the European Union over Northern Ireland...dramatizes the long shadow Brexit is casting on Mr. Johnson’s effort to rebrand Britain as a vital player on the global stage. Not only did Northern Ireland poison Mr. Johnson’s talks with President Emmanuel Macron of France, but it also threatens to undermine his relationship with President Biden.

I still don't understand it. I don't understand what the 2019 Brexit deal has to do with the 1998 Good Friday Agreement.

...

The “sausage wars” between Britain and the European Union escalated on Sunday when a senior British official accused the bloc’s leaders of holding “offensive” views about the status of Northern Ireland.

The rift stems from something called the Northern Ireland protocol, which was designed to avoid a hard border after Brexit between Northern Ireland and Ireland, an E.U. member country. But the protocol left Northern Ireland straddling the British and E.U. trading systems, and that could lead to shortages of some products in Northern Ireland, particularly chilled meats like sausages...

The two sides have been trying for months to find a more workable arrangement, but as the deadline approaches things have grown increasingly heated...

Mr. Johnson argues that the European Union is being overzealous in its enforcement demands and inflexible in seeking solutions. The bloc’s officials have accused Britain of failing to implement the terms of a treaty Mr. Johnson himself agreed to and ratified.

Okay. The "protocol" was the 2019 Brexit thing. How does that threaten the 1998 Good Friday Agreement?

...

The dispute has widened in recent months, with President Biden warning Mr. Johnson in quite blunt terms not to undermine or renounce the protocol. That, in turn, might lead to a hard land border that could threaten the landmark 1998 Good Friday agreement...

How? Just explain to me.

Northern Ireland’s unionist and loyalist parties, [i.e. the Protestants] which favor remaining in the United Kingdom, were outraged by the protocol, calling it a “betrayal” because Mr. Johnson broke a promise never to accept a border between them and the rest of Britain.

Interesting, guys! Like Israel!