Ex-jihadis are best weapon against terror
Over the past 48 hours, politicians of all kinds, soldiers, counter-terrorism experts, eyewitnesses and members of the public have pitched in to discuss what happened in Paris - and why.
French President Francois Hollande has called it an "act of war", and Parisians have talked of leaving town. Many cities are awash with candles and flowers; there has been an outpouring of grief and outrage.
But even if we are beginning to come to terms with what happened, understanding why it did will prove far harder.
The Islamic State, or Isil, has claimed responsibility, and its involvement seems clear. People have asked if this represents a change of focus for the terrorist group: from state-building in Iraq and Syria to a brutal campaign of global terrorism.
Inevitably, they have speculated that France's aggressive counter-terrorism policies, not just in Syria but also in Mali and at home; its unflinching commitment to free speech; and its determined secularism have made it a particular target. Certainly, Isil propaganda has denounced France more vigorously than any other European country.
But whatever French policies towards Isil may be, they are not enough to explain why between 1,500 and 2,000 of its citizens or residents have gone to Syria to join the terrorist group - more than from any other non-Arab country.
According to official estimates, around 600 are still fighting there, many others have died, and the rest have moved on, most returning home. The challenge for the French authorities is to decide which of these returnees present the most immediate threat, and whether they present a greater threat than the other 2,000 to 3,000 individuals who have come to official attention in relation to terrorism. If Isil tries to mobilise these returnees, resources in France will be stretched even thinner.
The same situation faces most other countries in Europe. In Britain, almost half of the estimated 760 people who are believed to have gone to Syria are thought to have returned, and the authorities speak of a total of 2,000 people who could present a threat.
In Belgium, about a third of the 300-plus people who joined extremist groups in the Middle East have headed homewards. In Denmark, half of the 125 who went have now come home; in Austria about 70 of the 250-300 are back, and in Germany it is over 200.
These figures are unmanageable from a security point of view, and although many attacks have been thwarted, some plots will succeed.
Policy-makers now need to get as far upstream of the radicalisation process as possible, in order to spot risks at the earliest stages of their development. But the policy can get confused when instructions to schools and community leaders make every angst-ridden teenager a subject of terrorist concern.
I have attended two global summits and many conferences and workshops on countering violent extremism this year, at which a great variety of experts and world leaders have spoken. But I have never heard anything that suggests we are close to getting on top of this problem. It's not a surprise. How can we begin to understand, let alone dissuade, a tiny minority of society that seeks to die for a cause so unworthy of their sacrifice?
There are many theories, and much evidence, concerning why individuals join violent extremist groups, and we are becoming familiar with the arguments of alienation, powerlessness and lack of purpose that drive people towards them.
But thankfully the brutal reality of Isil membership - its squalor, inadequacy and ultimate failure - is also increasingly becoming apparent.
In the wake of Paris, it has become all the more important to determine which returnees present a risk and which do not. This is not only so that authorities can focus scarce resources where they are most needed. It also allows us to identify and seek the help of those that do not present a risk.
It is these people who can be more powerful influences than any other on those who may be inclined to join or act on behalf of Isil. Often they have tried it and seen it does not work. They have the credibility and understanding that the rest of us lack. They are invaluable.
Richard Barrett was director of global counter-terrorism operations for MI6. He is now based with the Soufan Group consultancy in New York