Final answer:
Emperor Diocletian divided the Roman Empire into Eastern and Western halves, ruling the Eastern part and appointing a co-emperor for the Western part. This division was done through a tetrarchy system.
Step-by-step explanation:
Emperor Diocletian was responsible for fundamentally restructuring the Roman government, initiating the division of the Empire into two parts: the Eastern Roman Empire and the Western Roman Empire. This division came under the framework of the tetrarchy, where four rulers would share power in an attempt to stabilize the vast territory of Rome. Diocletian ruled the Eastern part of the Empire while appointing a co-emperor to rule the Western part.
Even though this structure of government held firm during Diocletian's life, it began to crumble following his death, eventually leading to a civil war from which Constantine emerged victorious. Constantine then established himself as the single ruler and moved the capital to Byzantium, which he renamed 'New Rome' and is now known as Istanbul.
The long-term ramifications of Diocletian's efforts were profound, as they shaped the governance and territorial divisions that would continue well into the Middle Ages, especially with the permanence of Constantinople as a significant center of power in the Eastern Roman Empire, also called the Byzantine Empire.