The Berlin Pride attack should end one comforting illusion that has shaped much of Europe’s debate on terrorism. The principal challenge is no longer merely identifying radicalised individuals. Increasingly, it is deciding what democratic societies do with people they already know are dangerous. The alleged attacker was not an anonymous face emerging from the shadows. He had a documented history of violent crime, had allegedly attempted to join the Islamic State, had come under the scrutiny of security agencies, and had recently been convicted of preparing a serious act of violence against the state.

Yet he was living in the community under supervision when the attack occurred. That sequence will inevitably provoke difficult questions about Germany’s criminal justice system, intelligence coordination and risk assessment. Democracies have always struggled with this dilemma. They cannot imprison people indefinitely for what they might do in the future. Civil liberties, judicial oversight and proportional sentencing are defining features of constitutional states. But every successful attack by a known extremist also exposes the limitations of systems that........

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