It always starts with words. Genocide is largely remembered for its depraved acts, but it is incubated in language. Words can cast dark spells on a population, stirring hatred in those who otherwise see themselves as moderate, humane, normal. This is why the genocide convention of 1948 criminalises “direct and public incitement to commit genocide”. Like Britain, Israel was a signatory nation and, two years later, it translated the convention into domestic law. There were four acts, it decreed, that leave the offender “treated like a person guilty of genocide”: one is “incitement to commit genocide”. As the British lawyer...