It is every family's worst nightmare: a traffic accident that takes the life of a loved one, often through no fault of their own. Such incidents are usually an agonizing, private tragedy for those involved. The allegations in the case of 19-year-old Harry Dunn, however, are a matter of trans-Atlantic diplomacy and threaten to become an embarrassment to British Prime Minister Boris Johnson.

They are also a reminder that diplomatic immunity is often used as a shield in ways that were never intended. Johnson, who once criticized the absurdity of the protections offered, can't let his voice be muffled this time by his need to keep the Americans onside after Brexit.

On Aug. 27, Dunn's motorcycle collided head-on with a Volvo driving outside a U.S. intelligence base about 112 km northwest of London; he suffered multiple injuries and was later pronounced dead. Dunn's devastated family say they were told by police that they believe the Volvo driver was traveling on the wrong side of the road.