IN the version of the story presented by Gerard Butler, the emissaries of King Xerxes, who were demanding “earth and water” as a symbol of Greek fealty, got kicked into a Spartan well. Herodotus says that the Athenians, too, flung Persian envoys into a pit, supplying the earth, though he says it happened earlier, under King Darius.

Unlike Butler in the gung-ho 300, Herodotus, though Greek, took a dim view of this approach to diplomatic relations. Since earliest times, the principle that diplomats represent their own sovereign and enjoy protection has been respected; even Genghis Khan took it very seriously, and it developed into the diplomatic immunity enshrined in the Vienna Convention of 1961.

It has always been a highly ambiguous doctrine, however, dependent on the interpretation of governments, the seriousness of the offence (and whether the offence exists in both countries), the seniority of the diplomat, the degree of friendliness between the nations concerned, and so on. In at least some instances, it is the most obvious example of balancing competing priorities; the interests of high policy and the state, and the administration of domestic and natural justice.

No full judgment can be made about the merits of the case of Anne Sacoolas, the wife of an American diplomat, because immunity has been invoked by the US, and she has left the UK rather than face trial. Ms Sacoolas was involved in a traffic collision in Northamptonshire in August which resulted in the death of Harry Dunn, a 19-year-old. It is suggested she was on the wrong side of the road.

If that cannot be established in the absence of a trial, the merits of whether Ms Sacoolas should have to stand trial are a slightly different question. That comprises those considerations of international relations and the nature of Ms Sacoolas’s role, though seems at least problematic to elevate those above establishing the truth, applying justice and addressing the concerns of Mr Dunn’s family. And it looks, from the Prime Minister’s response, as though this is not the sort of case in which immunity should have been invoked, and that Ms Sacoolas should face the sort of trial any British citizen involved in such an accident would expect.

There is an issue, however, not all that different from the clash of practicalities, personal or national interests and the principle that justice be done which apply to diplomatic immunity, to be raised when it comes to collisions of the sort that led to Harry Dunn’s death. Of course, we cannot judge whether Ms Sacoolas was at fault, precisely because she is not facing the court. But in similar cases, where people have been found to have caused death through careless or inattentive driving, it is quite common for a custodial sentence to follow.

That seems to almost everyone to be a reasonable response to the gravity of having killed or seriously injured someone, even though we might not think the intention was seriously criminal. Indeed, that’s the attitude the law applies in practice, because the punishment for using a mobile phone, or much other inattentive driving, is a matter of a small fine, a couple of points, or a remedial driving course. Yet it is largely a matter of chance, or at least outwith the culprit’s intention or control, that those “minor” incidents did not lead to catastrophic results.

It is possible to support a custodial sentence – and many argue that sentences for causing death by negligent driving are not nearly strict enough – while acknowledging that imprisoning the driver responsible does very little practical good either to him or her, or to the families of the victim. It may, at best, offer the offender expiation and the victims or their relatives the satisfaction that the matter has been recognised as serious, and justice served. The chief purpose, however, must surely be to mark the gravity of the outcome of the incident and act as a deterrent, rather than to punish or rehabilitate people who had no conscious criminal intent.

If that’s the case, the logic should be to regard minor driving infractions much more gravely; to point out that using a phone, changing a CD without keeping your eye on the road or a lane without indicating, are in principle exactly the same offence as killing a pedestrian or other road user through carelessness. Only good fortune (for the driver and potential victim) differentiates the offence from one which should land you in jail.

This problem applies in ethics, when trying to formulate fixed rules in cases where the correct course of action is judged by the outcome, and is a fundamental objection to utilitarianism. The law is – or it may be safer to say, most people think it ought to be – less concerned with transcendental ideals, and geared instead for practical and common-sense resolutions. What is certain is that no judgments can be made if there is no case, and that if there are overwhelming reasons why a case is not in the public interest, even if that is because of diplomatic Realpolitik, those reasons should also be open to scrutiny.