Five years back, around the annus questionabilis of 2016, Populism with a capital “P” was all the rage. Bookshops real and online were full of handbooks on the phenomenon. The outsize egos of Trump, Bolsonaro, Modi, Erdogan and Orban bestrode the headlines. One of the best guides was a slim volume, What is Populism? by the Princeton politics professor Jan-Werner Müller.