As the scale of the devastation wrought on Thursday night by a French-Tunisian who drove a lorry into crowds celebrating Bastille day in Nice becomes clearer, the reasons why this act was carried out remain as disputed as ever.

Many are also asking how, given France’s heightened state of security, such an attack could still be carried out. The answer to this question is much more straightforward.

The Islamic State of Iraq and the Levant (Isil), and al-Qaeda before it, has long called for supporters in the West to carry out attacks against their own populations using any means at their disposal.

Indeed, in 2010 the English-Language al-Qaeda online propaganda magazine Inspire suggested that readers used their cars to "mow down" crowds of people.