After Margaret Thatcher survived the Brighton Hotel bombing, some 30 years ago, the I.R.A. released a statement, which went as follows: “Today we were unlucky, but remember we only have to be lucky once—you will have to be lucky always.”

In Europe, these days, the terrorists are always lucky and it seems they do precisely what they want, whenever they want: slaughtering their fellow citizens on their way to work, or as they dine in side-street cafés, or simply as they check their baggage before flying out on an Easter break. Although interdictions are sometimes made before they embark on their murderous rampages—seven attacks were prevented in the United Kingdom during 2015—Europeans are nevertheless feeling incredibly vulnerable. And this sentiment will have a significant impact on the relations between nations and different ethnic communities.

Hence calls for unity are now a standard part of the appalling aftermath of European terror attacks. The phrase “Together we stand, divided we fall” was chalked into the main square in Brussels, on Tuesday evening, after bombers killed more than 30 people and wounded some 200 more in the twin attacks waged at the city’s airport and within its metro system. Angela Merkel, the chancellor of Germany, echoed the sentiment when she professed, “Our strength is in our unity, and that is how our free societies will prove themselves stronger than terrorism.”

This is not empty rhetoric. Unity is, after all, the essence of the European ideal. It was the chief motive behind the formation of the European Economic Community (E.E.C.) and its successor, the European Union (E.U.). It is what Europeans once prized above all on a continent that endured the worst of two world wars.

But the outrageous acts in Brussels, essentially the mass murder of men and women by their neighbors, will instead do much to drive Europeans apart. Ever since last year’s attack at the offices of Charlie Hebdo, and the subsequent massacre on the streets of Paris, the forces operating in Europe have been centrifugal rather than centripetal: this kind of bloodshed, and the shock wave that inevitably follows it, is anything but unifying. The rise of right-wing parties, such as the Front National in France, and anti-migrant parties across Eastern Europe and Germany, is one manifestation.

And then there are immediate threats to the cohesion of the E.U. “Euro skepticism” is on the rise all over the continent, especially in France and the U.K. In 10-weeks’ time, the British will vote in a referendum to decide whether the country will exit from the European Union. These latest attacks are unlikely to help the so-called Remainers who wish Britain to stay put. As Allison Pearson, a prominent British newspaper columnist, tweeted, “Brussels, de facto capital of the EU, is also the jihadist capital of Europe. And the Remainers dare to say we’re safer in the EU!”

It is obviously irrational to suggest that leaving the E.U. would make Britain safer—in fact, most believe the opposite is true—but Pearson’s isolationist passion is common. And it is difficult to argue against it when two brothers, Khalid and Ibrahim el-Bakraoui, can calmly choreograph an attack in which two bombs are detonated within an hour of one another.

It is almost beyond the comprehension of modern Europeans that young men, who came of age in Belgium’s tolerant environment—and who enjoyed many of the advantages that propel countless migrants to enter Europe’s borders—could do that to their fellow citizens. And there’s no doubt that this singularly calculated barbarity will cause much greater suspicion between Europe’s approximately 45 million Muslims and the rest of its nearly 650 million-plus citizens.