The lesson from the prosecution of crimes against humanity at the International Criminal Court and other tribunals — for Sierra Leone, Rwanda and former Yugoslavia — is that the prosecution must have a multi-skilled team in place as soon as possible, gathering intelligence on the history of the warzone and the system and pattern of atrocities as well as seeking to identify individual jihadists. This must be fully resourced and well under way, so that returning jihadists can be arrested and charged with committing war crimes, then denied bail but brought promptly to trial.

The answer might be to appoint a Special Prosecutor — an American invention, but none the worse for that — who would work with police and intelligence services and have sufficient expert staff and funding to be in a position to bring returning IS fighters to trial and to obtain their conviction on evidence that proves guilt beyond reasonable doubt. Unless a major initiative is taken by the newly appointed (and largely unknown) law officers, prosecutions will waste a lot of time and money, and, worst of all, may not succeed.

A cheaper alternative might be to hand them over for prosecution to the ICC itself. But this would be complicated — neither Iraq nor Syria has ratified the ICC treaty, and the Security Council has declined to refer the situation in either country to the ICC prosecutor. Although technically the UK, as a signatory to the treaty, could send its citizens for trial at The Hague, this is a "court of last resort" where defendants could argue that the trial would be an abuse of process because there is no impediment to trial in their own country. That argument would be correct — and could lead to their release after several years of arid hearings. On the other hand, war crimes trials in the UK might have a useful deterrent effect on young men minded to join a foreign jihad, especially on their young friends and families who might be more likely to tip off police about their projected departure. ICC proceedings are, moreover, intolerably cumbersome and lengthy and partly in French: the sooner these people are severely punished in Britain for participating in a crime against humanity, the better.

There does seem to be a reluctance to accept that the UK has an international duty to punish its own citizens if they commit crimes against humanity — we cannot simply wash our hands of them and wish them on some other state or a court in The Hague. The Prime Minister's initial response — to stop them returning — was a mistake. So was his reason, namely that their allegiance was to another country. Their make-believe caliphate is not a state and hopefully never will be. The UK has a duty to the international community to look after its own citizens, and a duty under international law to secure their prosecution if they commit crimes against humanity.

The soft-hearted suggestion recently made by former Metropolitan Police Deputy Assistant Commissioner Peter Clarke, that jihadis should be pardoned if they return to Britain and recant, is inappropriate, given the seriousness of their crime. Of course, as in all criminal proceedings, there would be scope for plea bargains in return for confessions and Queen's evidence, and release might come earlier if they were prepared genuinely to assist de-radicalisation schemes like the excellent "Channel" programme, which makes use of imams, psychologists and social workers — and why not repentant but imprisoned jihadis? But it is difficult to see how they could be offered any sort of amnesty: these crimes are unforgivable. They may be "political", in the sense that they are fighting to create an idealised international entity, but that cannot excuse the killing of civilians and children because they belong to different races and religions. No doubt their youth might be taken into account, or the cunning of their recruiters or the fact that they may have volunteered out of boredom (this is the latest theory, deduced from the fact that most European jihadis come from Belgium). But this is slight mitigation for travel taken with malice aforethought — IS recruiting websites make no bones about the executions they will be called upon to conduct.

This does not necessarily mean that the government can leave everything to beefed-up enforcement of existing laws. The police do have ample powers to investigate any incitements or encouragement to go out to Syria to fight for IS, although the recent suggestion by the Metropolitan Police that it would be a crime merely to look at or download execution pictures from IS websites was (fortunately for free speech) wide of the mark. The Home Secretary, under the Royal Prerogative, already has the power to cancel passports of dual nationals and naturalised Britons, so it is not a stretch to allow her to suspend (or refuse to issue) passports for British citizens, subject to their right to judicially review her decision and subject to Magna Carta's reminder that use of such a power should only be temporary, while the war lasts.