Croatia, a member of the European Union, hosted a now notorious, pyrotechnic-studded rock concert in its capital city Zagreb in July that drew a crowd of an estimated half million people. Punctuated by a signature World War II fascist-era call-and-response between the performer Marko Perkovic, known as “Thompson,” and his audience, it essentially became the largest far-right gathering in Europe since the Second World War. Croatian Prime Minister Andrej Plenkovi? was even photographed attending a rehearsal beforehand with his children. This in a country that still tries to position itself as a respected member of the EU and a key...