These are Europeans attacking their own homelands. Roughly 5,000 European Union citizens have fought in Syria, and most come from four countries: France, Germany, Britain and Belgium. Their lives sound very familiar: Mr. Abdeslam was apparently caught after the police noticed a suspiciously large order for a pizza delivery. The three men who planned to bomb the Brussels Airport were briefly stymied when a Brussels taxi company sent the wrong type of car to take them there.

Even their motivations are recognizable. According to ProPublica, a Belgian militant who was training in Syria called his mother to ask what people in Molenbeek were saying about a 20-year-old friend who blew himself up outside the Stade de France in November. “Are they talking about him?” he asked. “Are they praising him? Are they saying he was a lion?”

Europeans are becoming more vigilant. A friend in Berlin said that, on a recent train to Düsseldorf, everyone started getting nervous when no one claimed a suitcase left by one of the doors. A bomb squad met the train at its next stop, and told passengers they’d be herded into a secure area. Just then, the older Turkish man who owned the suitcase woke up, and revealed its contents: a pile of clothes.

Despite the inevitable false positives, it’s hard not to be on guard. I’m constantly making a series of mundane existential calculations: Is it worth it to risk going to a movie? Should I let my kids ride the metro to their soccer practice? Daily life has a chiaroscuro quality: One minute you’re riding a bus and enjoying a view of the river; the next you’re wondering about the fellow with an unusually large backpack.