Complex sentence: Following the death of Theodosius I in 395 and the permanent division of the Roman Empire between his two sons, Constantinople became the capital of the Byzantine Empire, which would rule most of the territory of Turkey until the Late Middle Ages.
Split into simpler sentences: Following the death of Theodosius I in 395 and the permanent division of the Roman Empire between his two sons, the city, which would popularly come to be known as Constantinople became the capital of the Eastern Roman Empire. This, later named Byzantine Empire, ruled most of the territory of what is today Turkey until the Late Middle Ages.