So now we understand how terrorism works. The clue is in the name. Its purpose is to spread terror - and the grim truth is, it's working.
Right now, very many Jews are terrified. I know it from my own conversations. After Paris and Copenhagen - after Brussels and Toulouse - parents of children at Jewish schools are seized by anxiety. There's nervousness about the most mundane Jewish activity: after Paris, buying candles and challah on a Friday afternoon feels like a grave risk.
In Denmark, a man is dead for doing no more than trying to protect a batmitzvah party. This week, one of the smartest, sanest men I know called to ask if he should attend an upcoming Jewish talk: no hysteric, he was scared.
Of course, this is not true of all Jews. One friend admits, guiltily, that he has no fear because he does not live a visibly Jewish life.
But that very reasoning only highlights why this situation is one most of us thought we would never know in our lifetimes. The notion that doing basic Jewish things - wearing a kippa, going to shul - might now entail a mortal risk: well, that was something we assumed belonged to the history books, in their darkest chapters.
In this climate, there are many things we need. One of them is security. No one likes to see guards or police outside a house of worship or learning. But for now, if that's what it takes, then that's what we must do. The alternative is worse.
We also need understanding. Thanks to Twitter, I probably see more of the outpourings of the hardcore anti-Zionist left than most. Perhaps naively, I assumed the torrent might subside - if only for a while - after the murders in Copenhagen. Not a bit of it. The haters carried on hating, adamant that antisemitism is exaggerated, a mere tool deployed by "the Zionists" to silence their critics. They tweeted that the killing of a Jew in Denmark was either a random coincidence or, as several hinted, a cunning "false flag" operation by Israel.
No compassion, no sympathy. Just the shouted insistence that all this fuss over Jews was designed to deflect attention from Islamophobia. I thought of pointing to the string of columns I've written over many years denouncing anti-Muslim hate or noting that the Community Security Trust, which monitors antisemitism, gives valued help and advice to its Muslim counterpart, Tell Mama. But the haters weren't listening.
Above all, we need somehow to remain calm. Benjamin Netanyahu's call for Europe's Jews to pack their bags and leave did not help. He made the same call after Paris and it was wrong then, too. It came too soon, before the dead had even been buried. It smacked of exploitation, turning a tragedy into a talking point for his own re-election campaign.
It also chafed against his earlier positions. Netanyahu now projects Israel as a place of safety, yet last summer he was telling the world that Israelis live in daily fear of a terror attack. Now he tells Jews the best response to a murderous threat is to run, yet a matter of weeks ago he was taunting his electoral opponents as weaklings who would "capitulate" to the terrorist enemy.
Little wonder that most Jewish leaders, including the Danish chief rabbi, rejected his call - saying that if Denmark's Jews moved to Israel it would be out of love, not fear.
This is a moment of grave anxiety. We need protection, empathy and above all calm, from leaders who will cool our fears - not fuel them.