Let’s assume he is indeed who he sounds like – another deluded British-born jihadi. I am afraid I have listened, on the Telegraph website, to the voice of the man who claims to be the killer of James Foley, and there seems little doubt that he grew up in this country. He was probably born in our wonderful NHS. He was schooled in our broadly excellent education system. He and his family have very likely spent their lives, like the rest of us, cushioned by our welfare state. And this is how he chooses to pay back the gift of nurture – by engaging in terrorism, declaring his allegiance to a novel barbarian state, and publicly beheading an entirely innocent journalist.

We are going to have to make up our minds very quickly about this “caliphate”: how we will respond to the irruption of a new and hellish country on the map, and how we deal with these Brits who go off and fight in its name. These Isil wackos now control an area the size of Great Britain, considerable oil reserves, a population of about six million, some industry, and a military capability said to be second in the region only to Israel. To take them on will not be easy, and I can see all the arguments for doing little or nothing – letting “history” take its course.

All these considerations are being actively weighed now, in London and Washington – and above all the imperative of not committing to “boots on the ground”. No option looks very appealing, to put it mildly; and yet doing nothing is surely the worst of all. If we let Isil get their way, then we will be acquiescing, first, in a gigantic and violent change in international borders. Next, we will be allowing a new and hideous regime to be born: a country where black-flag waving jihadis compete to show they have the most bigoted and reactionary understanding of their religion by persecuting women, Jews, Christians, gays, Yazidis and Shi’ites. The place would be a giant training ground for terrorists and wannabe jihadis. We need to try to close it down now, before it gets worse.

We also need to be far more effective in preventing British and other foreigners from getting out there (I am interested to see how many Belgians are there); and the Turks need to shut that border. We need to make it crystal clear that you will be arrested if you go out to Syria or Iraq without a good reason. At present the police are finding it very difficult to stop people from simply flying out via Germany, crossing the border, doing their ghastly jihadi tourism, and coming back. The police can and do interview the returnees, but it is hard to press charges without evidence. The law needs a swift and minor change so that there is a “rebuttable presumption” that all those visiting war areas without notifying the authorities have done so for a terrorist purpose.

There are perhaps five or six hundred Britons currently out there – overwhelmingly, though not exclusively, young men. If and when there is a real attempt to take on Isil, they may come back in a hurry and in a group. Some of them will present more of a risk than others, but the evidence seems to be that most of the jokers – such as those portrayed in the satirical film Four Lions – have already come home. It is the harder nuts that are staying longer. When they come back, they will need surveillance at the very least, and we must look again at our system of monitoring these people.

The Lib Dems will oppose the return of control orders; but even Nick Clegg would surely accept that times have changed. If we have to bring back control orders for some of the more serious risks, we should do so immediately. And unless they come back – and if they continue to give allegiance to a terrorist state – then absolutely we should take away their citizenship.

All this will be difficult, but the problem has got worse fast, and it could get worse still. What is the point of having a defence budget if we don’t at least try to prevent the establishment of a terrorist “caliphate” that is profoundly hostile to civilised values? Do nothing now, and the tide of terror will eventually lap at our own front door.