The populist right has wasted no time waiting for facts to emerge about the identity of the attacker in Berlin or a motive to slam Chancellor Angela Merkel for her humane asylum policy and to push its xenophobic agenda. This dangerous — if predictable — reaction plays directly into the hands of the Islamic State, which would like nothing better than to start a war between Christians and Muslims in Europe.

Shortly after the attack on Monday, Marcus Pretzell, a member of the far-right Alternative for Germany party, viciously tweeted, “These are Merkel’s dead!” On Tuesday, Geert Wilders, the leader of the Netherlands’ Party for Freedom, tweeted an image of Ms. Merkel spattered with blood; Nigel Farage, of Britain’s U.K. Independence Party, tweeted that such events “will be the Merkel legacy”; and Marine Le Pen, the French nationalist, issued a statement on the “Islamist” attack in Berlin and called for reinforcing Europe’s national borders there.

More may soon be known about the person who drove a truck into a Christmas market near Berlin’s Memorial Church, killing 12 people and injuring at least 48. A Pakistani immigrant detained after the attack was freed Tuesday, and the assailant, still unidentified, remains at large.

As the police asked the public to stay vigilant, Ms. Merkel, who said “we must assume” the attack was an act of terrorism, appealed to Germans not to let terrorism steal their way of life: “We do not want to live with the fear of evil paralyzing us.” Still, Christmas markets in Berlin remained closed on Tuesday. London’s Metropolitan Police assured that it had “detailed plans for protecting public events,” and France’s interior minister, Bruno Le Roux, said that after the attack, “security for Christmas markets was immediately reinforced.” Heightened fears across Europe are understandable; the attack resembled one on Bastille Day in Nice, where a truck was used to slaughter more than 80 people.