<P> Diocletian secured the empire's borders and purged it of all threats to his power . He defeated the Sarmatians and Carpi during several campaigns between 285 and 299, the Alamanni in 288, and usurpers in Egypt between 297 and 298 . Galerius, aided by Diocletian, campaigned successfully against Sassanid Persia, the empire's traditional enemy . In 299 he sacked their capital, Ctesiphon . Diocletian led the subsequent negotiations and achieved a lasting and favorable peace . Diocletian separated and enlarged the empire's civil and military services and reorganized the empire's provincial divisions, establishing the largest and most bureaucratic government in the history of the empire . He established new administrative centers in Nicomedia, Mediolanum, Antioch, and Trier, closer to the empire's frontiers than the traditional capital at Rome had been . Building on third - century trends towards absolutism, he styled himself an autocrat, elevating himself above the empire's masses with imposing forms of court ceremonies and architecture . Bureaucratic and military growth, constant campaigning, and construction projects increased the state's expenditures and necessitated a comprehensive tax reform . From at least 297 on, imperial taxation was standardized, made more equitable, and levied at generally higher rates . </P> <P> Diocletian saw that the vast Roman Empire was ungovernable by a single emperor in the face of internal pressures and military threats on two fronts . He therefore split the Empire in half along a northwest axis just east of Italy, and created two equal Emperors to rule under the title of Augustus . Diocletian himself was the Augustus of the eastern half, and he made his long - time friend Maximian Augustus of the western half . In doing so, he effectively created what would become the Western Roman Empire and the Eastern Roman Empire . </P> <P> On March 1 293, authority was further divided . Each Augustus took a junior Emperor called a Caesar to aid him in administrative matters, and to provide a line of succession . Galerius became Caesar for Diocletian and Constantius Chlorus Caesar for Maximian . This constituted what is called the Tetrarchy (in Greek: "leadership of four") by modern scholars, as each emperor would rule over a quarter - division of the empire . After Rome had been plagued by bloody disputes about the supreme authority, this finally formalised a peaceful succession of the emperor: in each half a Caesar would rise up to replace the Augustus and select a new Caesar . On May 1, 305, Diocletian and Maximian abdicated in favour of their Caesars . Galerius named the two new Caesars: his nephew Maximinus for himself, and Flavius Valerius Severus for Constantius . The arrangement worked well under Diocletian and Maximian and shortly thereafter . The internal tensions within the Roman government were less acute than they had been . In The Decline and Fall of the Roman Empire, Edward Gibbon notes that this arrangement worked well because of the affinity the four rulers had for each other . Gibbon says that this arrangement has been compared to a "chorus of music". With the withdrawal of Diocletian and Maximian, this harmony disappeared . </P> <P> After an initial period of tolerance, Diocletian, who was a fervent pagan and was worried about the ever - increasing numbers of Christians in the Empire, persecuted them with zeal unknown since the time of Nero; this was to be one of the greatest persecutions the Christians endured in history . Not all of Diocletian's plans were successful: the Edict on Maximum Prices (301), his attempt to curb inflation via price controls, was counterproductive and quickly ignored . Although effective while he ruled, Diocletian's tetrarchic system collapsed after his abdication under the competing dynastic claims of Maxentius and Constantine, sons of Maximian and Constantius respectively . The Diocletianic Persecution (303--11), the empire's last, largest, and bloodiest official persecution of Christianity, did not destroy the empire's Christian community; indeed, after 324 Christianity became the empire's preferred religion under its first Christian emperor, Constantine . </P>

Where was rome founded and what natural defences aided its position