The Third Rome
Ivan the Great, who was able to unite all the Russian Slavs under his rule, now controlled a vast empire whose size and territory was as great as any earlier empire. Ivan married Sophia, the niece of the last Byzantine emperor. This marriage allowed him to claim the title of Czar, or ‘Caesar,’ which was the title the emperors of both Byzantium and Rome took.
Ivan also incorporated many symbols of the Byzantine and Roman empires into his own empire, including a double headed eagle, which was the symbol of rulers in the Byzantine Empire.
All Czars from Ivan the Great down until the twentieth century would lay their claim to rule through their ancestry leading back to the Roman Empire.