THE FIRST fatality identified Tuesday in the horrific bombing of an arena in Manchester, England, was an 18-year-old woman who had been studying health and social care in college. The second was an 8-year-old girl, whose teacher said she was “simply a beautiful little girl in every aspect of the word.” A dozen other children were among the 59 people wounded in the attack, while others were likely among the 22 killed. That these innocent young people, fans of the pop singer Ariana Grande, would have been targeted by a suicide bomber allegedly affiliated with the Islamic State is as incomprehensible as it is sickening. It should redouble the determination of civilized nations to combat the evil that imbues the Islamic State and its followers.

It’s not yet known precisely what role the shrinking terrorist state in Syria and Iraq may have played in the attack, though it claimed responsibility. On Tuesday police identified the attacker as 22-year-old Salman Abedi, who the BBC reported was born in Manchester to a family of Libyan origin. Police were conducting searches in the city and said it was possible accomplices would be identified; one 23-year-old man was reported detained . But defenders of vulnerable immigrants and asylum seekers, who in Britain as elsewhere in the West remain the targets of populist demagogues, could take some comfort from the fact that the assault apparently did not originate with those communities.

Britain had not suffered such a serious terrorist act since July 2005, when bombs exploded in the London subway and on a bus . Effective policing and intelligence operations have stopped or deterred other plots. But the Manchester strike bore signs of a sophisticated operation, including the apparent use of a suicide belt packed with bolts and other improvised shrapnel. Such weapons are frequently used by Islamic State attackers in the Middle East. A key question for investigators will be how the alleged attacker obtained such a device, and whether he had help in constructing it.

The tactics of the bombing were coldly calculated. Ms. Grande can be counted on to attract a crowd of younger fans; some were said to be attending their first concert. The bomber positioned himself near a box office just outside the arena hall and detonated explosives as people were streaming out following the performance.

As nations across the West have learned, it is not possible to prevent all such terrorist attacks, especially when they are staged by homegrown militants. What is possible is a response that focuses on uniting rather than dividing a diverse society. That’s what was happening in Manchester on Tuesday, as thousands of people of all races and faiths gathered for a vigil in the city’s Albert Square. “I’m not here as a person with brown skin or someone born Muslim,” a man named Amir Shah told a Guardian reporter. “I’m here as a Mancunian.” If that spirit prevails, the terrorists will have failed.