A radical Muslim cleric based in London told a Portuguese magazine recently that the city would almost certainly be hit by a major terrorist attack because he knew of several Islamic militant groups there that were planning operations. In recent weeks British authorities have thwarted attacks and arrested many suspects, including 10 in Manchester and 8 in London and southeastern England, but the threat remains. London’s police chief has said that an attack is inevitable. The challenge is to prevent or mitigate it.