In the 280s Diocletian established Nicomedia (in north-westernTurkey) as the imperial seat in the east and Milan (in northernItaly) as the imperial seat in the west. Constantine the Greatdesignated Byzantium (in Greece, some 70 miles west of Nicomedia)as his imperial seat in 330 and renamed it Constantinople. He ruledfrom there as sole emperor. Twenty-seven year after the death ofConstantine, two brothers, Valentinian and Valens established aco-emperorship, with the former in charge of the west and thelatter in charge of the latter.
Historians argue that after the death of Theodosius the Great in390 the empire became split. This was not due to a politicaldecision. It was down to circumstances. Theodosius I was succeededby his two sons (Arcadius in the east and Honorius in the west) whowere young, inexperienced and incompetent. This resulted inpoliticians in the east and west conspiring against each other anda loss of unity. Moreover, soon after this the western part of theempire was invaded by Germanic peoples. Under the strain of this,the western part lost political cohesion. There was infighting andusurpations, resulting in the Romans being unable to deal with theinvaders effectively. Eventually this part of the empire fell. Attimes the eastern part of the empire meddled in the affairs of thewestern part.
The eastern part of the Roman Empire was not affected by thementioned invasions and continued to exit for nearly 1000 years.Historians have coined the term Byzantine Empire to indicate theeastern part of the empire after the fall of the western part.