But these are not merely international crimes, triable at the International Criminal Court (ICC). In 2001, under the International Criminal Court Act, they were made crimes in the UK as well, carrying a maximum sentence of 30 years. Any UK citizen or resident assisting IS in any capacity would be guilty — the law covers accomplices of all kinds who give assistance to groups committing crimes against humanity. The doctor, the paramedic, the IT operator, the website propagandist are all liable to conviction as ancillary offenders, if they have joined an enterprise that they know to involve the commission of international crimes.

And let us not forget the good old national crime of murder, to which IS soldiers have been confessing on their bloodstained Facebook pages. Parliament gave British courts the power to try UK citizens for murdering foreigners in foreign countries way back in 1861.

So the way to deal with returning jihadists under existing law is very clear. Anyone who has served IS in any capacity should be arrested and charged with participating in a crime against humanity (or murder, if there is evidence of involvement in killing a civilian or a hostage). Normally, bail until trial will be refused because of the heinousness of such offences, so there will be no need for Asbos or Control Orders. Then there will be a trial, probably at the top security court at Belmarsh, with the prosecution bearing the burden of proving guilt beyond reasonable doubt — the standard that should apply at every criminal trial in this country. Upon conviction, a prison sentence would in practice be long enough to keep the ex-IS fighter off the streets for many years — long enough perhaps, to deter most jihadis from returning home. Some would continue to live and die in their desert of choice whilst those who did return here would be denied martyrdom and locked up for life.

This procedure is readily available — it involves using present laws, without need for parliamentary additions to the already extensive arsenal of anti-terrorist powers. There is even a special war crimes/crimes against humanity division in the Crown Prosecution Service (CPS) Counter-Terrorism Division, and a "war crimes team", known as SO15 at the Metropolitan Police Counter-Terrorism Command.

At present, these divisions with long names have few staff and not much success. The CPS has a website devoted to "successfully concluded war crimes prosecutions since 2001" but it has only one entry, the case of an Afghani warlord, torturer and hostage taker. He was in residence in the UK, easily arrested and convicted as long ago as 2005. The Home Office admitted recently that it had managed to charge only 12 of the 69 jihadis caught on their return to Britain this year, and it is unclear whether any of these charges relate to international crimes.

So the real problem is of police and CPS resources and resolve. It must be addressed, but not by changing the law. The reality is that local law enforcement officers have little experience of prosecuting war crimes, seen as somewhat exotic and calling for investigatory skills that police do not possess and resources that the CPS on its present budget cannot afford. They are not geared up for the task of proving international crimes — beyond reasonable doubt, and not simply by reference to propaganda websites and Facebook pages which can easily be fabricated.