The singular focus on the Atlantic slave trade is misplaced. Slavery was utterly evil. That said, it is not the only past injustice. Can we confidently say that slavery was worse than, say, the Cambodian killing fields, the Holodomor, the Mongol conquests or the sack of Carthage? Even if we do make that argument, why should transatlantic slavery be treated as radically different from all the other versions? Some 17 million Africans were sold as slaves in the Islamic world, for example, many of them being castrated to serve as eunuchs. Britain’s only involvement with that trade was to halt...