The empire that was divided into East and West in 285 AD is the Roman Empire. This division was instituted by Emperor Diocletian, who recognized the challenges in managing such a vast territory. He established a Tetrarchy, with co-emperors ruling different regions—one in the West, with Rome as its capital, and one in the East, with Byzantium (later Constantinople) as its capital. The division aimed to improve administrative efficiency and governance but ultimately led to the gradual decline of Western Roman authority, while the Eastern Roman Empire continued to thrive for several more centuries.