In the days when the self-styled "Islamic State" was advancing across Syria and Iraq, its official spokesman singled out France in a chilling threat.
Abu Mohammed al-Adnani urged Muslims to "kill a disbelieving American or European - especially the spiteful and filthy French".
No one can be sure why Mohamed Lahouaiej Bouhlel, a 31-year-old Frenchman of Tunisian origin, crushed 84 people to death beneath the wheels of his lorry in Nice. But he may have acted on the incitement to murder issued by the Islamic State of Iraq and the Levant (Isil).
If so, this tragedy would provide further proof of France's acute vulnerability to Islamist terrorism. That stems from several factors.
The truth is that France has the biggest Muslim minority in Europe, approaching 10 per cent of the population - a larger pool within which a small minority can be radicalised.
Moreover, the social divide between some Muslim communities and the national mainstream appears to be wider than elsewhere in Europe.
In this polarised situation, radical Islamists find their echo on the far Right - and the two strands of extremism feed off one another.
Patrick Calvar, head of France's domestic intelligence service, spelt out the danger in May. The "confrontation between the extreme Right and the Muslim world" risked placing France on the "verge of a civil war", he said.
In blunt remarks to a closed parliamentary inquiry - later leaked to the press - Mr Calvar asked what would happen if vigilantes retaliated after a terrorist attack, or a French version of the sexual violence in Cologne, by assaulting Muslim immigrants in general? Suppose there were "punitive expeditions in the suburbs", triggering a cycle of retaliation and counter-retaliation.
"Where is the spark going to come from that will light the powder, transforming France into an uncontrollable country where groups take up arms and hand out their own justice?" asked Mr Calvar. "Nothing is excluded in a country already as eruptive as France today."
Another telling indicator is the fact that France has provided more foreign fighters for Isil in Syria than any other European nation.
In addition, France is part of the Schengen area, giving it open borders with the European continent. Unlike Britain - which has always retained its frontier controls and has the natural advantage of being an island - France can do little to prevent the flow of suspected terrorists or weapons into its territory.
Some of the terrorists who planned and executed the Paris attacks last November lived in Brussels. They were able to take advantage of free movement between the two capitals.
Finally, there is evidence that France's intelligence agencies and security services are overwhelmed by the scale of the threat. Monitoring thousands of suspects is exceptionally labour-intensive.
The government had planned to lift the state of emergency, in place since the Paris massacre, on July 26.
But that aspiration has gone by the board. The tragedy in Nice shows that any country that holds free celebrations and public gatherings is inherently vulnerable to murderous fanatics.
