The Roman Emperor Michael III (r. 842-867) played a major role in stemming Bulgarian aggression against his state. He succeeded in defeating the Bulgarian Khan Boris I in 864 and having him convert to Christianity.

Michael's plan was not to simply crush the Bulgarians, but to give them a stake in Christian Roman culture- thus the Bulgarians got their own patriarch and Boris was given the title of Caesar, which had been a sort of junior imperial title, typically conferred upon the relatives of the Emperor.

Caesar over time evolved into Tsar. Another famous Tsar, Ivan III of Moscow, also claimed succession to the Roman state in Constantinople vanquished by Mehmet the Conqueror in 1453, which he legitimized by marrying one of the last Roman princesses, Sophia Palaiologina.

Warren Treadgold, A History of the Byzantine State and Society.