The family of a British teenager killed in a crash involving a spouse of a U.S. diplomat, who has since left the United Kingdom, believes she should return to that country to face justice.

British Prime Minister Boris Johnson has joined the victim's family in publicly expressing hope that Anne Sacoolas, the American diplomat's wife, would return to the U.K., and indicated he might appeal the issue to the White House.

But whether she will be subject to possible charges depends on her protection under diplomatic immunity, a centuries-old principle of international law dating back to Roman times. The Vienna Convention on Diplomatic Relations, or VCDR, universally codified this patchwork legal tradition beginning in 1961.

The VCDR has since been ratified by almost every country in the world, including the U.K. and the United States. It provides diplomats with absolute immunity from criminal prosecution, and protection from most civil and administrative actions brought in the "receiving State," or where they are stationed. A diplomat is not even required to give evidence in court as a witness under the VCDR.

There are three exceptions to a diplomat's civil immunity, none of which are likely to apply to a motor vehicle accident allegedly caused by a diplomat.

But it was not a diplomat who is alleged to have caused this fatal crash Aug. 27. It was his wife. No matter: the VCDR extends immunity to family members who form part of the diplomat's household. While every country may define "family" differently, diplomats' spouses enjoy co-extensive immunity.

If the wife is entitled to full diplomatic immunity, the U.K. plaintiffs wouldn't fare much better bringing a lawsuit in the U.S. judicial system.

Under U.S. federal law, a district court must dismiss any case against someone entitled to immunity under the Vienna Convention.

That's if diplomatic immunity still applies.

It may no longer apply to a wife of a diplomat who has left the receiving country and returned to her "sending," or home, country.

Diplomats lose much of their immunity following the termination of their diplomatic status under Article 39 of the VCDR, which provides that the "immunities shall normally cease at the moment when he leaves the country … but shall subsist until that time … However, with respect to acts performed by such a person in the exercise of his functions as a member of the mission, immunity shall continue to subsist."

Article 39 provides something less than full diplomatic immunity: "residual" immunity. Once a diplomat becomes a "former" diplomat, he or she is not immune from suit for prior acts, unless those acts were performed "in the exercise of [the former diplomat's] functions as a member of the mission."

Sometimes, a spouse of a diplomat may not even be entitled to residual immunity because it applies only to a person who was "a member of the mission." Just being a diplomat's wife is not enough; she must have conducted acts as a member of the mission.

Sacoolas' liability turns on what she did as the wife of a diplomat because residual immunity is exclusively reserved for "member[s] of the mission" only.

So now that she's left the country, either Sacoolas, 42, has residual immunity, which only protects her from liability for official diplomatic acts, or she has no immunity at all.

Either way, she is now potentially subject to the jurisdiction of the English courts — if they can get her back from the U.S. That becomes an issue of extradition, and yet another tricky issue of international law, which itself is often reduced to a tricky issue of diplomatic relations.