… well, as Isaiah Berlin saw it anyway (as summarised by Mark Lilla in his essay in The Legacy of Isaiah Berlin):

“The puzzle is this: how did the optimistic and progressive spirit of 18th-century Europe give way to the dark and terrifying world of the 19th and 20th centuries? How did the Europe that produced Goethe and Kant, Voltaire and Rousseau, Tolstoy and Chekhov, also produce the Lager and the Gulag?”