Due to the death of Theodosius I, the emperor of the East (with his young second son Honorius being the emperor of the West as figurehead) in 395, his first son Arccadius became the emperor of the East, and the two capitals were placed, one in Constantinople in the East and the other in Mediolanum (later to Ravenna) in the West, which promoted the Roman Empire to be divided into the West and the East.
395年には、東帝テオドシウス1世（西帝は形式的に幼い次男ホノリウス）の死により、長男アルカディウスが東帝とされ、都が東のコンスタンティノポリスと西のメディオラヌム（後にラヴェンナへ遷都）に置かれ、東西二分が進むことになった。