The Ifs of History is an introductory essay to Harry Turtledove's "Agent of Byzantium", written by Issac Asimov. Before going on to refer to that book's main premise - i.e. Muhammad converting to Christianity and as a result the Byzantine Empire surviving with its full power and territorial extent - Asimov discusses alternate history in general and sets out in particular a specific historic event which could be used as the basis of an alternate history story or book.