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A series of short stories by Harry Turtledove. Set in the year 1305 and later, it depicts a universe based on the POD that Mohammed, instead of founding Islam, became a Christian monk and later St Mouamet, the Patron Saint of Change and one of the most influential figures in Christianity.
The absence of Islam leads to a powerful, resurgent Byzantine Empire that builds on Justinian's reconquests. By the time the stories take place, Constantinople has reclaimed all of Italy, Narbonese Gaul (southern France), Ispania (Spain) and North Africa for itself. Mesopotamia, Arabia and the Caucasus (the latter two via local chieftains) are contested between Byzantium and its eastern rival, a surviving Zoroastrian Persia. To the north, something roughly equivalent to the Carolingian empire exists, referred to as 'the Franco-Saxons'. The Angles still came to Britain, which is referred to as Angleland. The Anglelanders and Franco-Saxons follow a form of Arian-influenced Catholicism which is regarded as heresy by the Orthodox Byzantines. As Rome and Jerusalem are firmly held by the Byzantines, Orthodoxy is definitely the strongest Christian creed, though persecuted Nestorian and Monophysite minorities still exist within Byzantine territory (and are sometimes assisted by the Persians to stir up trouble).
The stories follow Basil Argyros, a Byzantine government agent, through a remarkable career which involves the earlier discovery of a number of inventions, including gunpowder and the printing press.