AMSTERDAM, Nov. 18 - His telephone was tapped, his apartment was watched and many of his friends were already behind bars, so the Dutch authorities were not surprised by evidence that it was Mohamed Bouyeri, a Dutchman of Moroccan descent, who murdered the filmmaker Theo van Gogh in broad daylight one morning this month. Yet they had been powerless to stop the crime. It is a problem faced by most European governments as radical Islam spreads across the Continent: how to arrest suspected militants before they act, without trampling on individual rights or risking charges of discrimination. The government of...