The heightened security isn’t driven by baseless panic. Last week, Mohammed Moguchkov—a Chechen-origin asylum seeker already under surveillance by French authorities for pro-jihadist sympathies, and brother to a jailed terrorist—stabbed schoolteacher Dominique Bernard to death in a killing authorities said was linked to Gaza. Another attack, authorities said, was foiled. Following the brutal Hamas attack on Israel, police forces across Europe have been on high alert: Antisemitic incidents have quadrupled in the United Kingdom, and Germany has banned the pro-Hamas organisation Samidoun. Eighty-five years old, Mireille Knoll had escaped the notorious Nazi round-up of Paris Jews in 1942 and survived...