It’s not often that Brits can say that the US is behind the UK. But in understanding the dynamic between the successors to the old socialist left and radical Islam, US thinkers have years of catching up to do. It is not as if American commentators are wrong or uninteresting, it is just that, unlike their counterparts in Europe, they have not begun to come to terms with the Islamisation of the worst strains of left-wing politics, and the wider consequences for the progressive cause.