Twenty years ago tomorrow, the world changed. The threat of radical Islamic terror had been all too real for many years before. But the scope and sheer horror of 9/11 demonstrated its potency in a way that no previous incident had come close to doing.

Two decades on, we are certainly wiser to its threat, and the War on Terror — much derided by some — has certainly impacted the ability of groups such as al-Qaeda and ISIS to mount attacks. Yet there could be no more symbolic demonstration of our wider failure than the pull-out from Afghanistan. It is a fundamental mistake to think that the West somehow ‘provokes’ Islamist terror.

They seek to kill us and destroy us not because of what we do but because of who we are. Retreating inwards will not stop terror. If anything it will have the opposite effect, as we may well soon see with the Taliban back in control of Afghanistan.

And even after twenty years, many still do not take the Islamist threat seriously in our own society. Organisations which operate as, in effect, fronts for those who ally with terrorists are treated as if they have some sort of legitimate voice. And naïve fools in our own community make common cause with organisations and individuals that are the enemy of liberalism and democracy.

There are many lessons from 9/11, but one of the most important is the need for constant vigilance.