Turtledove, Harry. "Islands in the Sea"
Divergence: 718 CE
What if: Constantinople and the Byzantine Empire fell to the Muslims in the early 700s.
Summary: Fifty years after the fall of Constantinople, the king of the Bulgars invites Muslims and Christians to decide which faith he should adopt.
Published: In Alternatives (eds. Robert Adams and Pamela Crippen Adams), q.v.; Departures, q.v.; The Best Alternate History Stories of the 20th Century (eds. Harry Turtledove and Martin H. Greenberg), q.v.; and The Mammoth Book of Alternate Histories (eds. Ian Watson and Ian Whates), q.v.