We know that a great deal of what Slahi describes does go on – thanks to the release of the report on the CIA’s use of advanced interrogation techniques. We also know that senior American politicians have justified these things and it is possible they will continue. Of course, Guantánamo contains men who are guilty and the desire to get information out of them is understandable. The murderous hand of Islamism stretches from Africa to Europe to the Middle East, enslaving and killing. But when one takes up arms in defence of civilisation and descends into savagery, one commits a spiritual surrender – and the war is pointless. Yes, this is a necessary book. It reminds us that the evil we’re fighting can be found in ourselves as well as in our enemies.