Reports that France had been warned by the US and Iraq in advance of Friday's Paris massacre that an ISIL attack was imminent, have prompted many to ask how security services could have missed the plot hatched.
Such incredulity was exacerbated by the fact that a number of those involved had been on the radar of French authorities as "radicals" and potential threats - with at least one having been charged in a terror-related case.
Neither France nor its European Union partners have the forces, assets, funding or legal provisions to take wide-sweeping preventive measures - often based on sketchy intelligence - that the US can.
And fail-safe operational monitoring of the sheer number of potential threats on European soil, in the form of sympathisers with groups like ISIL, many of whom have actually travelled to Syria and spent time with the group, is beyond the capacity of any security service.
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That much was effectively acknowledged on Monday by Manuel Valls, the French prime minister, who told an RTL interviewer that France had been aware that attacks were being plotted ahead of Friday's strike, and warned others are likely being prepared.
"Truth, lucidity obliges us to say it: terrorism has struck, and may strike again in coming days - in the coming weeks," Valls said, as police sweeps across France and Belgium detaining hundreds of people for questioning. "I don't say that to scare people, [but] everyone must be fully conscious of this."
Though virtually all European states have tightened laws in recent years to aid security services battling "terrorism" - usually provoking varying degree of protest by civil liberties opponents as they did - none have the sweeping powers of surveillance accorded to US officials under the post-9/11 Patriot Act.
And the scale of the problem confronting EU member states is far larger, with roughly 6,000 Europeans having travelled to Syria to join armed groups - nearly 2,000 of those from France alone.
In the meantime, Europe is both geographically closer and more readily accessible to flows of ISIL operatives in and out of areas controlled by the group.
And the ease with which these operatives can move between European nation-states - and the jurisdiction of different security services - exacerbates the problem.
Some of those blamed for Friday's attack, for example, appeared to have moved frequently between France and Belgium.
"There's no reason for a French or European young person to go to Waziristan for training with al-Qaeda; 100 percent of those people are joining Islamic State [ISIL] in Syria or Iraq," a senior French counterterrorism official said in an interview in September.
The likelihood of France coming under attack by homegrown fighters was probably just a matter of time, he warned. "[ISIL] is the only game in town any more. For all intents and purposes, al-Qaeda as a military and terror entity has disappeared. It still has its leaders, its followers, even backers, but it's essentially moribund."
The official complained that France's security services were struggling to meet the challenge before ISIL ranks began swelling - just as EU governments began embracing austerity.
But even subsequent efforts to boost the financing of intelligence and police forces have hardly matched the spiking numbers of ISIL-allied extremists and the threat they pose.
Last Tuesday, Britain announced it would increase intelligence staffing by 15 percent - or 1,900 officers - in response to the rising "terror" risk from its considerable contingent of citizens in Syria.
But similar steps previously taken elsewhere in Europe have made, at best, a modest difference in the current security environment.
"The task we've been given, measured simply by the number of people we now need to keep tabs on, is so enormous in certain ways it's just become impossible," the French official said. "You can't watch everyone. We're swamped."
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Suggestions in the media implying incompetence or error - rather than the overwhelming of human capacities - are particularly cruel for France's counterterrorism forces, whose track record for the past two decades has been nearly spotless.
Following the string of Paris bombings by members of Algeria's Armed Fighting Group (GIA) in 1995-96, France's security services successfully halted dozens of planned attacks on French soil until the 2012 shooting spree of al-Qaeda-supporter Mohammed Merah in and around Toulouse.
But until the end of the 1990s, French officials often got little or no help from European and US peers in battling the threat.
Having not yet suffered a strike on their own turf - or identified ISIL sympathisers in their midst - those allies did not understand the gravity of the threat, French officials said.
They resisted French urgings to adopt laws that would allow the arrest of members at all levels of violent networks, from the petty thieves who raise finance; to document forgers allowing operatives to travel undetected; logistics helpers who obtain arms and explosives; and planners of attacks.
Having been hard at work for two decades countering the threat of attacks like last Friday's one, French security officials are resentful of any impugning of their competence in the wake of the Paris massacre.
On the contrary, they note, Friday was an excruciating exception to France's success, until now, in thwarting most attacks planned on its soil.
Friday seemed to confirm the maxim expressed in a 1984 statement from the Irish Republican Army directed at the British government following a failed attack on Margaret Thatcher, the then British prime minister.
"Today we were unlucky," the IRA said, "but remember we only have to be lucky once. You will have to be lucky always."
That the French security system's good fortune would fail, sooner or later, may have been inevitable in the face of the expanding threat.
They'll be hoping that Friday was a case of ISIL being lucky just once.
Source: Al Jazeera