THE series of terrorist attacks in Paris last night, which appear to have claimed the lives of up to 140 people, was the nightmare scenario that Western intelligence and security agencies have been warning against for years. Ever since a commando-style raid by a jihadist group in Mumbai eight years ago that killed 166 people and lasted for four days, there have been fairly regular instances of similar plots either being uncovered or turning out to be false alarms. But as the IRA once grimly warned: “You have to be lucky every time; we only have to be lucky once.”

What has clearly greatly increased Europe’s vulnerability to such attacks is the continuing civil war on its doorstep in Syria and the emergence of Islamic State (IS) as an even more potent magnet for would-be jihadists than al-Qaeda (AQ). While so-called “core” AQ in the badlands of Pakistan’s tribal areas has withered under assault from America’s drones, IS has gone from strength to strength. With its rampage through Iraq last summer, its pretensions to establish a caliphate and its slick exploitation of social media to publicise its successes and brutalities, IS has radicalised, recruited and trained many thousands of young European Muslims, not least in France.

The ease with which such recruits have been able to travel to IS’s stronghold in Raqqa, Syria by just getting a plane ticket to Turkey and then continuing by road is not quite matched by their ability to return to their homes. Security agencies and police do their best to monitor them and prosecute them if they have evidence. But it is estimated that at least half the jihadists who have gone to Syria in the past four years have come back hardened, brutalised and plugged into technically savvy support networks. In both France and Britain, there may be around 400-500 such people.

To them can be added unknown numbers of radicalised individuals who have never left home, but are persuaded by the ceaseless flow of jihadist propaganda on social media to carry out so-called “lone wolf” attacks. Although Ayoub El-Khazzani, the young man who attempted to massacre passengers on a train in Belgium last August was a Moroccan national, he could just as easily have been a radicalised local. Lone-wolf attackers are regarded as a particularly acute problem by security agencies because of their sheer unpredictability and the impossibility of keeping all those with the potential to carry out such attacks under surveillance.

As yet, the identity of the eight perpetrators of last night’s series of attacks is not in the public domain, but given the careful coordination and ruthless efficiency of the operation it would be surprising if they had not received training at an IS-run camp in Syria or similar establishment elsewhere, such as in Yemen where AQ remains powerful. (That is also where the terrorists responsible for the attack on Charlie Hebdo, a satirical magazine, in Paris in January were trained.) It would also be surprising if they did not have accomplices providing them with support and infrastructure.

What will be most worrying to the authorities is how such a complex plot could have been conceived, apparently without more specific details appearing on their radar screens. While concerns that such an attack might be brewing have grown in recent weeks, that is nothing new—security alerts are frequent and, more often than not, prove to be false alarms.

If it is true, as early reports suggest, that Western surveillance agencies failed to pick up any of the “chatter” that usually precedes a major operation (and which, for example, was evident before the bombing of the Russian Metrojet airliner two weeks ago) that will be a source of considerable alarm. The planning that went into last night’s attacks must have taken several weeks and involved more than the immediate perpetrators. (Theories that the operation might have been brought forward as a response to the killing of the IS executioner Mohammed Emwazi, better known as “Jihadi John”, by an American drone on November 12th are highly speculative.)

Fears have been growing that the strong encryption routinely built into smartphones and messaging apps by tech firms to protect their customers’ privacy is also a gift to terrorists who want to “go dark” when an operation is being planned. In both France and Britain, security agencies have been getting new powers to eavesdrop on suspects, but even with those it is possible that they are still falling behind in the technology arms race with the terrorists.

It is too early to say what the consequences of Europe’s most lethal terrorist attack since the Madrid train bombing in 2004 will be. But it is bound to ratchet up the sense of crisis over Europe’s porous external borders and non-existent internal ones (at least for the signatories of the embattled Schengen Agreement). It is an understandable fear that among the many refugees flocking to Europe from Syria are some experienced fighters who mean to do harm when the opportunity arises. The attack will also exacerbate the anxiety about an “enemy within” that is at war with both the West’s cultural mores and foreign policies that are perceived as “anti-Islam”. Anti-radicalisation programmes that almost every country in Europe with a significant Muslim population has a version of are well-meaning, but of dubious effectiveness. Across Europe, there is no shortage of populist politicians who are only too happy to exploit the fallout from what happened last night.

A broader question will be what, if anything, Europe can do to reduce the risk of such attacks becoming a recurring horror. A more concerted attempt to end the war in Syria and destroy the IS caliphate might help reduce the flow of refugees but it is not likely to eradicate the threat. Indeed, IS might become much more focused on taking the fight to the “far enemy”.

One major concern must be the easy availability of automatic weapons in Europe (although not, mercifully, in Britain with its strict gun laws) and the way in which they can be moved from one country to another. If large-scale terrorist attacks start to take place more or less anywhere that people gather in large numbers, from sports stadia to rail stations to entertainment venues and political events, will it mean introducing Israeli-style security screening and checks across Europe’s cities? That would seem politically inconceivable and economically disastrous, the clearest manifestation that the terrorists had won.

Much will depend on the scale and timing of the next atrocity. Life has a way of returning to normal quite quickly. But if Europe’s security agencies, which have done a pretty good job keeping its citizens safe in the past, are unable to prevent attacks of this kind becoming a more regular occurrence, then keeping calm and carrying on may not be an option.

Read more:

Je suis Charlie 2? Er, non. Why Europe’s reaction is different this time

A mood of “defiant normality” in Paris, the day after the horror

Vilifying all Islam is the wrong way to counter bloody medievalism (January 2015)

France and its Muslims (February 2015)