This has the potential to get very awkward for both leaders who are depending on each other a little more than usual recently. The US whisked a State Department official’s spouse out of the United Kingdom after an accident that killed a motorcyclist, claiming she had diplomatic immunity. Responding to the outrage at home, Boris Johnson is now demanding her return to face charges, and plans to challenge his key ally Donald Trump — who has problems of his own on international relations at the moment:

“I do not think that it can be right to use the process of diplomatic immunity for this type of purpose,” Johnson told the BBC.

“I hope that Anne Sacoolas will come back and will engage properly with the processes of law as they are carried out in this country. That’s a point that we’ve raised — or are raising today — with the American ambassador here in the UK, and I hope it will be resolved very shortly.”

Dunn died when his motorcycle collided with a car near RAF Croughton in Northamptonshire on Aug. 27.

Mother of three Sacoolas — whose husband, Jonathan Sacoolas, was based there — initially “engaged fully” with police and said “she had no plans to leave the country in the near future,” a police spokeswoman told The Guardian.

The Northamptonshire police chief constable, Nick Adderley, said they had appealed in “the strongest terms” to apply a waiver to her immunity and “allow the justice process to take place.”

How exactly does Sacoolas have diplomatic immunity in the first place? That’s another awkward point. According to The Telegraph, she had only been in the UK for three weeks before the accident. Sacoolas had stopped after the collision and admitted fault, having driven on the wrong side of the road. The husband is reportedly assigned to the military base, raising questions about what kind of “diplomacy” might be involved. Normally, diplomatic immunity only applies to dependents of officials assigned to London, although there are (cough cough) exceptions, and Sacoolas seems to be one of those:

The diplomat, his wife and their three children, had only arrived in the UK at the beginning of August, the Telegraph understands. …

Dependants of diplomats based outside London do not usually hold inviolability or immunity.

However, there are some individuals outside of London who have diplomatic immunity under the Geneva convention.

That exception for RAF Croughton personnel was arranged by a 1994 agreement, for some (cough cough) reason.  It’s the exception that appears to be rankling the Brits. With Brexit at the breaking point, this is a political headache Johnson doesn’t need, but on the other hand he can’t afford to alienate Trump over it either. If Johnson succeeds in getting a no-deal Brexit, he will need an immediate trade pact with the US to stave off as much economic damage as possible. That will require a lot of help from Trump, who up to now has promised to make it as painless for Johnson as possible.

What can Trump do about it, though? Having made the claim for diplomatic immunity, he can hardly withdraw it now. His ambassador and political ally Woody Johnson already refused the UK’s request to rescind immunity. If Trump reversed that, it would look as though he’s sacrificing a diplomat for his own political purposes — even if it might serve the interests of justice in this particular situation. It might also raise questions about how she ended up with immunity in the first place, although a refusal could actually make that question more acute than less so.

Plus, this comes as the State Department is already on edge over Ukraine-Gate. Employees at other embassies (and other facilities, cough cough) will be watching this closely to see if Trump has their backs. It’s not a propitious moment to break diplomatic traditions to throw the wife of a diplomat to the wolves for the sake of a trade deal, even if the wolves in this case are the civilized Brits and the sacrifice is someone who killed a young man by driving badly.

Now that Johnson has promised to “be raising it myself personally with the White House,” though, the temperature has escalated on the issue. If Johnson comes back empty-handed, it won’t help him project strength in the American partnership on which he’s basing his no-deal Brexit strategy. And Trump almost certainly can’t afford to help Johnson prevent it.