PARIS • The attack in Nice hammered home a point made recently by the French domestic intelligence chief Patrick Calvar: France faces "the greatest threat" of any nation in Europe.

After three devastating attacks in 18 months - and many other attempts, organised and opportunistic - the country has emerged as a focus for Islamist terror, The Financial Times said yesterday.

The question is why.

One reason is what French President Francois Hollande describes as the state of war between his country and the Islamic State in Iraq and Syria (ISIS).

France is the European nation that has most vigorously backed military action against ISIS. And as its self-proclaimed caliphate is pushed back in Syria and Iraq, striking back against its enemies has become ever more central to ISIS strategy, said the FT.

France first targeted ISIS on Sept 19, 2014. Four days later, the group's chief propagandist Abu Mohammed al-Adnani called for reprisals. Three months later, the first ISIS-linked attacks in France began: three policemen were stabbed in Tours and two vehicles rammed into crowds of pedestrians in Dijon and Nantes.

Beyond revenge for French bombings of ISIS, other deeper factors have increased France's vulnerability, the FT reported.

The country is a softer target than either the United States or Britain - the other two big Western militaries fighting the Islamic militants. The US is too far away for ISIS to strike easily and Britain has a more naturally controllable border, said the newspaper.

France is also easiest for the militants to infiltrate. At least 1,700 French nationals have gone to fight for ISIS, against around 800 Britons and 200 Americans.

"The problem is not just those that go. It's their families. Their friends. It's all the people that have been left behind by them, who might be influenced by them, or who might be angered when they die or don't come back," the newspaper quoted a British security official as saying.

"It's a hinterland of potential radicals who suddenly have emotional skin in the game," he said.

ISIS has been able to galvanise radical networks with greater success in France than anywhere else, said the report.

Its aggressive secularism makes it "a particularly appropriate symbolic target" where the ISIS message of a civilisational conflict can have the most resonance.

ISIS-aligned networks outside France also play a big role. The biggest recruiting ground for foreign fighters is the francophone Maghreb. At least 6,000 Tunisians and 1,200 Moroccans have joined the black banners, according to security consultancy Soufan group.