Brexit was meant to be about the restoration of British sovereignty. But there are few starker examples of loss of sovereignty than being unable to trade within your own country without filling in a customs declaration.

That is what Britain agreed to when Boris Johnson’s government negotiated the Northern Ireland protocol as part of the Brexit deal. Ironically, the imposition of an internal border within the UK is a far more intrusive loss of sovereignty than anything Britain ever had to accept while it was actually an EU member.

Theresa May, Johnson’s predecessor, said no British prime minister could ever agree to a border down the Irish Sea. But Johnson made that concession to “get Brexit done”, to force an election, to avoid creating a border between Northern Ireland and the Irish Republic and to keep Britain out of a customs union with the EU.

The inevitable tensions caused by the protocol are now threatening to create a new crisis between the UK and the EU — one which has the potential to lead, quite quickly, to a trade war and the imposition of tariffs. That prospect has also created strains between Washington and London, with the acting US ambassador in London delivering a firm message to Britain to dial down the rhetoric and respect the protocol.

America’s intervention points to the threat of a broader crisis within the western alliance. There is a very real prospect that a G7 summit intended to showcase western unity could be swiftly followed by a trade war within the western bloc.

Avoiding this sorry outcome will require movement in both London and Brussels — and, quite possibly, mediation from Washington.

If tensions are to be dialled down, the British have to kill the idea that their real intention is to junk the protocol altogether. That was the implication of Johnson’s recent statement that the people of Northern Ireland should enjoy “free and uninterrupted access to goods and services from the whole of the UK”. That would indeed be highly desirable. But it is impossible under the terms of the agreement that he negotiated. The only legitimate and achievable goal right now is to get the protocol applied with maximum flexibility.

As for the Europeans, if they are sincere in their insistence on the central importance of peace in Northern Ireland, they should recognise the dangers of further inflaming unionist opinion. They need to find ways of respecting the protocol and protecting the EU’s internal market, while minimising symbolic flashpoints.

The danger is that the EU will instead yield to the temptation to use this row to demonstrate (once again) the folly of Brexit, Johnson’s mendacity and the superior power of the bloc. But we know all that already. Nonetheless, Brexit has happened and Johnson is firmly in place, backed by a strong electoral mandate.

However annoying it finds the Johnson government, the EU has a strong geopolitical, economic and human interest in establishing a decent relationship with the UK.

Before triggering a trade war, Brussels should be able to point to actual, rather than theoretical, harm to their internal market. Take the great British sausage, which is currently (and ludicrously) at the centre of the dispute. From July, this iconic item can be exported from Britain to Northern Ireland only in frozen form. Failure to enforce the rules from now on, according to the EU, will endanger the European single market. But we have just witnessed unfrozen sausages traversing the Irish Sea for six months without any evident damage to the EU.

The EU can point to the danger of setting precedents and of regulatory divergence over time. But those problems could be managed, with good will on both sides.

Unfortunately good will is in short supply — in both Brussels and London. So there may be a role for the US to play. For that to happen, the Americans would need to allay fears in London that they will simply act as an enforcer for Brussels and Dublin. Any American special envoy to Northern Ireland should follow the stellar example of George Mitchell in the 1990s peace process. He took the concerns of all sides equally seriously.

The Biden administration is certainly capable of playing that role. Although the US has repeatedly emphasised the need to avoid a border between Ireland and Northern Ireland, US officials have also said privately that they do not intend to take sides between Brussels and London over the implementation of the protocol. Their overriding concern is that the dispute should be settled, not the terms of the settlement.

The hope must be that the current war of words between Brussels and London is the kind of posturing that typically precedes a deal. The fear is that both sides may see an upside in confrontation. The Johnson government may see a benefit in blaming the dastardly Europeans for the loss of sovereignty that is actually built into the protocol. Emmanuel Macron, the most hardline of the European leaders, is facing a difficult French presidential election, and getting tough with Britain can only help.

But while these stances may make domestic political sense, they are international nonsense. The unity of the western alliance is worth more than a frozen sausage.

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