The failed attack against the Amsterdam-Paris Thalys train on Friday is a clear reminder for all of us: jihadist terrorism is the biggest security threat Europe has faced for decades. But it is also a sign that something is wrong with counter-terrorism. One of the main characteristics of attacks since the Mohamed Merah case in March 2012 is that the perpetrators were all “well known” to the intelligence services. And in France, some were the subject of a special file kept when a person is considered to be a threat to the security of the state. Obviously, the fact that people known to be potentially dangerous were able to commit terrorist attacks is worrying. Especially when you realise that about half the Europeans who went to Syria or Iraq were not known to these same services before being “spotted” in the field – if police and intelligence cannot stop a known terrorist what happens when an unknown one decides not to go to Syria but to attack in France, Belgium, the UK or Germany? But there’s a bit of confusion going on here. The public probably think that if someone is known to the authorities and the subject of a special file, they are under surveillance. However this is not the case.Radical extremists: Some 6,000 Europeans are or were involved in the fighting in Syria (they went there, they were killed in action, they are still in IS camps, they are on their way there or their way back.) If you have 6,000 “active” jihadists, that probably means that if you try to count those who were not identified, the logistics people who help them join up, their sympathisers and the most radical extremists who are not yet involved in violence but are on the verge of it, you have something between 10,000 and 20,000 “dangerous” people in Europe. To carry out “normal” surveillance on a suspect on a permanent basis, you need 20 to 30 agents and a dozen vehicles. And these are just the requirements for a “quiet” target. If the suspect travels abroad, for instance, the figure could go up to 50 or 80 agents and necessitate co-operation between the services of various countries. Work it out: to keep watch on all the potential suspects, you’d need between 120,000 and 500,000 agents throughout Europe. Mission impossible! So counter-terrorism must adapt if it is not to be overwhelmed. One way this can be done is to use new legislation. An intelligence law passed in France this spring which allows the intelligence services to conduct massive digital surveillance of suspects (heavily criticised by human rights organisations) is probably a good step.Innocent blood: It enables the authorities to keep an eye on the communications and virtual contacts of would-be jihadists and to detect any sign or radicalisation that could precede commitment to action. But it is not enough. Our societies must accept the very fact that we are at war with an enemy which counts dozens of thousands of “soldiers”, has an international presence (including inside our own borders), could at any moment attack any target in any country and is not afraid to shed innocent blood. Actually, this enemy wants to shed innocent blood because he thinks (or perhaps knows) that repeated atrocities will make us sick of Middle Eastern affairs and give up trying to fix the region’s problems. Which is exactly what they want: to be left alone in a confrontation with their own rulers and governments so they can overthrow them and achieve power.This situation requires a new approach. We should probably decide to strengthen the laws and stiffen the penalties for terrorism-related offences (including financing, spreading propaganda, travel to “jihad territories” etc.). And we should probably devise additional penalties: for instance why not make a convicted terrorist wear an electronic surveillance device or tag for some years after his release? Strengthening surveillance: Of course, as with phone-tapping, electronic surveillance would be under the strict control of judges or an independent commission and subject to regular re-assessment. The whole debate about strengthening surveillance on trains is going nowhere – unless it is decided to post armed guards or police officers on those trains, like in the United States, which could be a sort of deterrent.Trains go from point A to point B with some (or lots) of stations in between. Can you imagine each station having scanners and checkpoints? Can you imagine turning up at train stations one or two hours before departure, like at airports? Of course not. And even if it did happen, the threat could switch to other targets: buses, tramways, underground, stores, theatres, restaurants, bars, churches and sporting events. Could we equip all these places with scanners and checkpoints? The answer is obvious. The only way to protect society against the terrorist threat lies in strengthening intelligence and laws.