What emperor divided the roman empre into eastern and western half to try and stop its decline?

Answer 1

No emperor divided the empire into two halves. The empire was never divided. Eastern Roman Empire and Western Roman Empire are terms which have been coined by historians. The Romans had only one term: Roman Empire.

The emperor Diocletian created the tetrarchy (rule by four) and created fourmain administrative units (not two) which were called praetorian prefectures. Diocletian appointed fellow general Maximian as co-emperor. Maximian took overall control of the western part of the empire, while Diocletian took overall control of the eastern part. These two men became senior emperors (Augusti) when Diocletian appointed the creation of two junior emperors (Caesars). The Caesars, Galerius and Constantius, were subordinate to the Augusti and took charge of defending the troubled frontiers along the rivers Danube (in the east) and Rhine (in the west) and respectively, which were under constant attack from outside. Each co-emperor was in charge of one of the four praetorian prefectures. Diocletian took charge of Oriens: eastern Libya (Cyrenaica) Egypt. the territories in Asia and Thrace (the south-eastern corner of the Balkan Peninsula). Maximilian took charge of Italia: Italy Africa (north-western Africa). Constantius I took charge of of Galliae: Britannia (England and Wales) and Gaul and Hispania (spain and Portugal). The Galerius took charge of Illyricum: most of the Balkan Peninsula in south-eastern Europe. These were units of a single empire which, Diocletian stressed, was indivisible. Each emperor had an imperial seat. They were Nicomedia(modern Izmit in north-western Turkey) for Oriens Sirmium(modern Sremska Mitrovica, near Belgrade, in Serbia) for Illyricum, Mediolanum(modern Milan in Italy) for Italia et Africa and AugustaTrevorum(modern Triers in Germany) for Galliae. Rome ceased to be the capital of the Empire.

This reform had several aims: 1) The bad experience of recent sole emperors suggested to Diocletian that sole emperors were vulnerable and that co-emperorship made power more secure. 2) Diocletian had no male issue. Therefore, he needed a trusted co-ruler from outside the family. 3) There was a need to improve imperial control over the empire. There had been conflict between powerful men in every province of the empire. Diocletian shared controlling the provinces with Maximian. 4) There was also a need to improve the efficiency of the defence of the vast frontiers of the empire from constant attacks from outside. The creation of these four units was not a division of the empire which, as Diocletian stressed, was indivisible. It was an administrative arrangement.

Co-emperorship was not new. Previously emperors had made their sons co-emperors with one in charge of the east and the other in charge of the east. The purpose was the same, improving the defence of the frontiers. After the tetrarchy, there was an alternation of periods of co-emperorship and periods with a sole emperor. This clearly shows that the empire was not divided.

The creation of rule by four men was accompanied by a doubling in the number of provinces and a tightening of the grip of the imperial bureaucracy over them and local affairs. This enormously increased the size of the imperial bureaucracy and the cost of imperial administration.

Answer 2

There was no Roman emperor who divided the empire into eastern and western halves. The east/west division was made by later historians.