When the Labour government introduced anti-terrorism legislation 25 years ago, it stressed that it was targeting extreme crimes. “Terrorism involves the threat or use of serious violence for political, religious or ideological ends. It … aims to create a climate of extreme fear,” said Jack Straw, the then home secretary. Some MPs still feared that a group like Greenpeace, which had destroyed genetically modified crops and temporarily halted nuclear weapons production at Aldermaston, might be proscribed. Mr Straw reassured them that such bans would be used only when absolutely necessary; he knew of “no evidence whatever” that the actions of...