Last week’s appalling atrocity in Christchurch, New Zealand, in which a solitary gunman killed 50 worshippers at two local mosques, is a tragic reminder of the one-sided nature of the battle against terrorism. In 1984, after the IRA unsuccessfully attempted to kill Margaret Thatcher, it issued a statement saying, “You have to be lucky all the time. We only have to be lucky once.” The security services are well aware of that. Their counter-terror tactics seek to disrupt an attack before it is imminent by collecting and acting on good intelligence. In the case of the New Zealand attacker, however, there did not appear to be any visible signs of what he was planning. But he followed an ideology, that of the European far-right, which is only too visible.

As with terrorism inspired by extreme Islamist ideology, the principal way it spreads is via social media on the internet. Those seeking to stir up violence do not need to organise an actual conspiracy when they can run a website or open a Facebook account. Closing them down is one way of responding, but that may well happen only after the attack.

It is possible to move even further back in the chain of circumstances, to challenge the ideology itself for instance, or even to alter the cultural assumptions on which it implicitly relies. In the case of jihadism, that might mean undermining the assumption that Islam and Western values are mutually incompatible or that they are locked in an undeclared “war of civilisations”.