Just when the Western world had absorbed the shock of a truck attack in Berlin in December that claimed 12 lives, it is reminded again of the dangers of “lone-wolf” attacks inspired by Islamic State (IS) that are almost impossible to guard against.

When a sole attacker drove randomly across London’s Westminster Bridge towards the Houses of Parliament – one of the most trafficked thoroughfares in the Western world – killing and maiming innocent bystanders, it served as a reminder, if that were required, that open, global cities are vulnerable to such attacks.

These are moments that serve as a reality check for those in authority who are striving to maintain a balance between oppressive policing and surveillance and a free society. This is enormously challenging in an environment in which strains of fanaticism have been let loose.

Regrettably, the London terrorist attack leading to five deaths, including the perpetrator and a policeman, will find its way into a racially tinged political discourse – and not in a way that will be particularly edifying.

But there is also no point in pretending that mayhem in the Middle East can be separated from what takes place on the streets of London or Brussels or Berlin or Nice, or in other places that become victims of continuing upheaval in a crescent that stretches from the Mediterranean to South Asia.

Now that the weapon of choice for lone-wolf terrorists seems to have become a vehicle to mow down people innocently going about their business, a policing task becomes even more difficult.

Peter Bergen, a terrorism expert, noted in a post for CNN that as long ago as 2010, al Qaeda’s Yemen branch had encouraged its recruits in the West to use vehicles as weapons.

A headline on its webzine, Inspire, had described vehicles as “the ultimate mowing nachine” – not to “mow grass, but mow down the enemies of Allah”. He wrote:

These attacks are hard to defend against in free societies where crowds will gather, as was the case for Bastille Day in Nice, or the Christmas market in Berlin … and now throngs of tourists and visitors that typically crowd the sidewalks around the Houses of Parliament.

The utter cynicism and brutality of these random low-tech attacks pose enormous challenges for security.

This latest episode will not be the last such vehicle attack with the possibility that something much worse might eventuate, including the detonation of a truck packed with explosives and shards of shrapnel. Open Western cities will always be vulnerable to these sorts of attacks.

The threat of IS-inspired terrorism is now embedded in Western societies. It is no good pretending it is not.

Since 2014, when IS proclaimed its caliphate, there have been more than 70 terrorist attacks “conducted or inspired” by its followers in 20 countries (not including Syria and Iraq), according to a running total kept by CNN.

If Syria and Iraq were added, such terrorist attacks would number in the hundreds.

In 2014, CNN lists seven terrorist incidents, including the stabbing of two Australian police officers in New South Wales. Six died and 12 were injured in 2014, in Belgium, Australia, Canada, the US and France.

That was the beginning.

By 2016, the numbers of casualties from IS-inspired terrorism had risen sharply across the Middle East and in Europe. This included the Brussels bombings at a metro station and an airport, in which 32 people died and 340 were injured.

It is not least of macabre coincidences that the London terrorist attack occurred on the first anniversary to the day of the Brussels bombings.

So far this year, there have been five major incidents. Most, if not all, are linked to IS.

London was the first such episode in continental Europe. The others occurred in Turkey, Afghanistan, Pakistan and India.

Out of all this, it is a depressing conclusion, but as IS in its strongholds in Mosul in Iraq and Raqqa in Syria is further degraded, chances are it will step up its terrorist activities elsewhere.

In other words, risks to countries involved in the war against IS will rise as its fortunes in its so-called caliphate slide. IS is on the ropes in its Middle Eastern strongholds. This makes it more dangerous to Western interests.

In London, and among Britain’s allies, political leaders have hastened to express solidarity, but all would be aware that such ritualistic professions of support and concern will not provide a foolproof shield against the next Islamist-inspired terrorist attack.

The question is not if, but when and where.