Fall of Roman Empire
The Roman Emperor Diocletian observed several problems in which the Roman Empire was immersed; according to those problems he reorganizes the Empire and creates the tetrarchy, the government of four. This government manages to give a stability to Rome that it had not had for years and allows the survival of the Empire.
Emperor Theodosius (379-395) divided the empire between his two sons, Arcadius and Honorius. So they would start the Eastern Roman Empire -with headquarters in Constantinople- and the Western Roman Empire.
The first division (Eastern Roman Empire) was very rich and culturally advanced, and survived for about a thousand years more.
Internal disputes and barbarian invasions progressively weakened Rome, until the last emperor of the West Roman Empire, Romulus Augustulus, was deposed in 476 by Odacro, a Goth (a people originally from the south of Sweden who advanced southwards).