Ten years ago, in a piece for The Wall Street Journal, I noted that during the age of Caesar and Livy the Roman Empire still had more than a millennium of life ahead of it. This observation generated an irate letter to the editor, berating me for the suggestion that the Byzantine Empire, which fell in 1453, had anything to do with the ancient Roman Empire, which collapsed in 476. Well, now with the publication of Anthony Kaldellis’s fascinating new book, which forcefully argues that the two empires are one and the same, I am at last vindicated. So there, Mr. Letter Writer.

Of course, it is not new to claim that the Roman Empire’s successor was the Byzantine Empire—the Eastern remnant left after the West was swept away. Historians are also aware that the easterners continued to refer to themselves and their state as Roman. The term “Byzantine” is modern, developed by scholars to differentiate the medieval Greek-speaking state with its capital at Constantinople from the ancient empire of Augustus based in Rome. Although this medieval state had its origins in the Roman Empire, the dramatic changes in language, culture, religion, and ethnic makeup led historians to rename it the “Byzantine Empire”—something that no one, least of all its citizens, ever called it. Looking back on it, the retitling was an odd decision. History, after all, is the story of change, so renaming something simply because it changes invites confusion. Nevertheless, European writers found it hard