This sounds like a headline from Tzarist Russia in 1910, but in fact it was last week in Paris. A group of anti-Israel demonstrators tried to storm a synagogue, but Jews had their own undercover agents at the protests so they could raise the alarm if any of the protestors started to engage in violence. They did so, and the rioters were beaten back by a combination of “right-wing” Jewish youth groups and communal security. Unlike is Tzarist Russia, the authorities aren’t on the side of the attackers, and they eventually arrived in sufficient numbers to disperse the attackers.
Making it safe for Jews in Europe isn’t really that complicated—the police need to be proactive, using intelligence gathering, decoys wearing kippot and other Jewish garb to draw attackers and arrest them, stings, and so on. Unfortunately, though, policing in Europe is by custom almost entirely reactive, the belief is that specifically doing anything proactively stopping anti-Semitic violence would be “provocative,” and the Jewish leadership, as a well-placed European friend told me, is too ineffectual to demand anything different.