The most immediate murderous threat comes predominantly from Islamist extremism. Only last week, marking the 25th anniversary of 9/11, Chief Rabbi Sir Ephraim Mirvis warned that “extreme Islamism has continued to stain our world”, while MI5 chief Sir Ken McCallum warned that “mass-casualty terrorist ambition endures”, citing the recent conviction of an Islamist extremist who sought automatic weapons for an attack on Manchester’s Jewish community. Yet violent extremism does not exist in a vacuum. It sits within a wider climate in which antisemitism has become more socially permissible and, in some places, almost routine, visible in professional bodies, unions, universities,...