What Is the Northern Ireland Protocol That Britain and the E.U. Are Squabbling Over?

LONDON — For months, a battle over the status of Northern Ireland has been the thorniest legacy of Brexit, even sparking a conflict known as the “sausage wars.” Now, Britain has upped the ante by demanding that post-Brexit trade rules for Northern Ireland that it agreed to two years ago be scrapped and replaced.

The European Union responded to that call Wednesday with a far-reaching plan to resolve practical problems raised by that Brexit treaty — the Northern Ireland protocol — that has provoked a full-scale confrontation between Britain and the bloc. It is a spat that could upset the United States.

The protocol aims to resolve one of the most complex issues created by Brexit: what to do about the border between Northern Ireland, which is part of the United Kingdom, and the Republic of Ireland, which remains part of the European Union.

According to the new proposal from Brussels, checks on food and animal products going from mainland Britain to Northern Ireland would be reduced by 80 percent, customs paperwork for shipments of many goods would be slashed and the flow of medicines would be ensured.

“Today’s package has the potential to make real, tangible difference on the ground,” said Maros Sefcovic, vice president of the European Commission, the executive body of the 27-nation bloc, adding that this amounted to an “alternative model for implementation of the protocol.”

But he offered no concession over a demand made on Tuesday by Britain for a completely new agreement, one that would remove any role for the European Court of Justice, the bloc’s top court, as an arbiter in disputes. That idea had already been rejected by Brussels.

For critics of Mr. Johnson, the rift over the protocol is evidence of his lack of trustworthiness, his willingness to break international commitments and his denial of responsibility for the consequences of the withdrawal from Europe he championed. Mr. Johnson’s allies accuse the European Union of inflexibility in applying rules, a pettifogging lack of sensitivity to feelings in parts of Northern Ireland and vengeful hostility toward Britain for exiting the bloc.

Behind all the bluster lie fears about the fragility of the Northern Ireland peace that raise the stakes beyond those of typical trade disputes. President Biden, who talks often about his Irish heritage, has already warned Mr. Johnson not to do anything to undermine the Good Friday Agreement that helped to end the violence.

What is the Northern Ireland protocol?

It’s fair to say that while the accord sounds like the title of a spy thriller, it’s actually a dry legal text that won’t be found on most people’s vacation reading lists.

The frontier between Northern Ireland, which remains in the United Kingdom, and Ireland, which is in the European Union, is contested, and parts of it were fortified during the decades of violence known as “The Troubles.” But after the Good Friday peace deal in 1998, those visible signs of division have melted away along the open border. No one wants checkpoints back, but as part of his Brexit plan, Mr. Johnson insisted on leaving Europe’s customs union and its single market, which allows goods to flow freely across European borders without checks.

The protocol sets out a plan to deal with this unique situation. It does so by effectively leaving Northern Ireland half inside the European system (and its giant market), and half inside the British one. It sounds neat — logical, even — until you try to make it work.

Why doesn’t Britain like it?

The plan means more checks on goods entering Northern Ireland from mainland Britain, effectively creating a border down the Irish Sea and dividing the United Kingdom. Faced with all the new bureaucracy, some British companies have stopped supplying stores in Northern Ireland, saying they simply can’t handle the added paperwork now needed.

This has enraged some Conservative lawmakers and inflamed sentiment among those in Northern Ireland who want the region to remain part of the United Kingdom. The unionists, mostly Protestants, identify as British and believe the changes could threaten their future in the United Kingdom.

So while not being able to get the right kind of sausages might seem like a small inconvenience, to many unionists, it feels as if their British identity is what’s in the fryer.

Why is the E.U. insisting on it?

The bloc has dug in its heels, partly because Mr. Johnson signed on to the protocol, but also because he negotiated it himself and pushed it through the British Parliament.

British critics accuse the Europeans of being overly rigorous and legalistic in their interpretation of the protocol, and of being overzealous about the checks required.

But E.U. leaders believe that the bloc’s existential interests are being put at risk. For Brussels, the single market is one of its cornerstones and it says it needs to control what enters it. If that is undermined, it could threaten the building blocks of European integration.

What about those sausages?

Under the protocol, foods with animal origins — yes, like sausages — coming from mainland Britain into Northern Ireland need health certification to ensure they meet European standards should they end up in Ireland, which is still part of the European Union’s single market.

The British want a light-touch system — that is, one in which there are minimal checks — on goods that companies promise will stay in Northern Ireland.

But the European Union wants Britain to sign up to Europe’s health certification rules to minimize the need for controls. So far many of the regulations have been waived during a “grace period,” and, if enacted, the latest proposals from Brussels should take the sizzle out of the “sausage wars.”

What would happen if Britain withdrew from the protocol?

Britain says it has grounds already to deploy an emergency clause known as Article 16 that permits it to act unilaterally, effectively allowing it to suspend parts of the protocol. It doesn’t plan to do so for the moment, but the option remains on the table.

If Britain does this, the European side would most likely accuse Mr. Johnson of breaking a treaty. This could lead to retaliation and a possible trade war between Britain and the European Union.

Is this all just a negotiation tactic?

That’s likely.

During the endless Brexit talks, Mr. Johnson often played hardball with the Europeans, sometimes relying on a so-called madman strategy and threatening to quit the bloc without any deal at all.

So this may just be another roll of the negotiating dice, and most analysts believe that, for the British, the best outcome would be winning concessions on the protocol from Brussels.

The European Commission’s response has been to talk to business and other groups in Northern Ireland and to focus on resolving their practical problems. It hopes the concessions offered on Wednesday will satisfy business groups in Northern Ireland, if not all the demands of the government in London. Brussels has limited room to maneuver, however, if Britain really presses its demand for a change to the role of the European Court of Justice in arbitrating disputes.

So isn’t it risky?

Yes, because ultimately, Mr. Johnson has no real alternative to the protocol short of ripping it up and daring the Republic of Ireland to resurrect the Irish border. That could inflame sectarian tensions in Northern Ireland, provoke a trade war with Brussels and heighten tension with the Biden administration.

Monika Pronczuk contributed reporting from Brussels