Which is the odd one out? Barcelona, Stockholm, Paris, Berlin, Rome, Brussels, Copenhagen, London. The answer to this grim question is Italy’s capital, which has so far escaped an Islamist terror attack of the kind seen in other major European cities.

This is striking, given that one of ISIS’s repeated themes is its hatred of “Rome” – which it sees not just as the symbolic heart of Christian civilisation, but as a future military target. Last week ISIS fighters in the Philippines released a sacrilegious video in which they declared: “Remember this, you kuffar [non-Muslims] – we will be in Rome, we will be in Rome, inshallah [God willing].”

The Swiss Guards, who have defended the popes since 1506, are aware that they have a new challenge. Last week their leader, Commander Christoph Graf, spoke to the Swiss Catholic website cath.ch about the truck attack in Barcelona. “Perhaps it is only a question of time before an attack like that happens in Rome,” he said. “But we are ready also for this.”

Given that the total number of Swiss Guards is a little more than 100 men – albeit the cream of young Swiss Catholics – it is reassuring to know that they are not left to guard the Vatican alone. The Holy See’s 130-strong police force – the Corps of Gendarmerie – share the responsibility for Vatican security. (Though not quite as venerable as the Swiss Guards, they celebrated their 200th birthday last year.) The head of the Corps, Domenico Giani, oversees Vatican security; the Corps’ rapid-response unit, the Gruppo Intervento Rapido, is on standby for emergencies.

It has been a while since such an emergency took place: the last major example was on May 13, 1981, the feast of Our Lady of Fatima. The Turkish hitman Mehmet Ali Ağca fired at Pope St John Paul II, hitting him twice, then advanced with his knife. But in the words of John Paul’s secretary Cardinal Stanisław Dziwisz, Giani’s predecessor as head of the Corps, Camillo Cibin, “reacted so swiftly that the would-be assassin couldn’t finish what he had begun”.

John Paul believed that Our Lady of Fatima had redirected the bullet to save his life. The Vatican security operation, not wanting to rely on supernatural intervention, expanded its security operation in response.

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