Fall of the Western Roman Empire
The idea of the two halves, the east and the west, re-emerged and eventually resulted in the permanent de facto division into two separate Roman empires after the death of Theodosius I, though it is important to remember that the Empire was never formally divided, the emperors of the eastern and western halves legally ruling as one imperial college until the Fall of the Western Roman Empire left Byzantium, the "second Rome", sole direct heir.