On the basis of a random sample of English-language internet websites about empires, we can now formulate the first law of comparative imperialisms as follows: as an online discussion of empire grows longer, the probability of a comparison involving the Roman Empire approaches 1. (This is a variant of the general law that states that ‘as an online discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches 1’.) The comparative study of empires is thriving, and the recent intensity of interest is connected, at least in part, to the international military interventions of the United States. But comparisons between empires are nothing new, and, in the 1960s, Peter Brunt wrote an insightful article on British and Roman imperialism. That analysis was the product of the age of decolonization, an age which also acted as a spur to comparative approaches within classical scholarship: witness Nicole Loraux's suggestion that it was anti-colonial movements associated with the Algerian and Vietnam wars that led Jean-Pierre Vernant to embark on his series of comparative investigations into Greek thought and religion. Brunt's article was written in a retrospective key at a time when it was possible to look back to the completion, or the near completion, of a major period of European colonialism and arrive at a sort of reckoning. Some two generations prior to Brunt, in the early twentieth century and at the apogee of the British Empire, Lord Cromer delivered an address to the Classical Association on ‘Ancient and Modern Imperialism’ in which he found it unimaginable to think of independence for Britain's overseas colonies. Francis Haverfield responded sympathetically to Cromer and in his own writings associated the British and the Roman empires. Any discussion of comparative imperialisms, therefore, will need to consider not just the recent concentration of debates over empire but also a lengthy trajectory that extends back to Cromer and Haverfield and indeed further beyond into the eighteenth century. None of the books under review reflects in detail on the intellectual history in which they may be situated, but this is a subject that at least needs to be acknowledged and that we shall have occasion to return to later.