History of the East–West Schism
Disunion in the Roman Empire further contributed to disunion in the Church. The Emperor Diocletian famously divided the administration of the eastern and western portions of the Empire in the early 4th century, though subsequent leaders (including Constantine) aspired to and sometimes gained control of both regions. Theodosius the Great, who established Nicene Christianity as the official religion of the Roman Empire (see Edict of Thessalonica), was the last Emperor to rule over a united Roman Empire. Following his death in 395, the division into western and eastern halves, each for a few decades still under its own Emperor, was never reunited.
Theodosian dynasty
Its founding father was Flavius Theodosius (often referred to as Count Theodosius), a great general who had saved Britannia from the Great Conspiracy. His son, Flavius Theodosius was made emperor of the Eastern Roman Empire in 379, and briefly reunited the Roman Empire 394–395 by defeating the usurper Eugenius. Theodosius I was succeeded by his sons Honorius in the West and Arcadius in the East. The House of Theodosius was related to the Valentinian Dynasty by marriage, since Theodosius I had married Galla, a daughter of Valentinian I. Their daughter was Galla Placidia. The last emperor in the West belonging to the dynasty was Galla Placidia's son Valentinian III. The last emperor of the dynasty in the East was Theodosius II, the son of Arcadius. Later, both in the East and in the West, the dynasty briefly continued, but only through marriages: Marcian became emperor by marrying Pulcheria, the older sister of Theodosius II, after the death of the latter, Petronius Maximus was married to Licinia Eudoxia, the daughter of Theodosius II, and Olybrius was married to Placidia, the daughter of Valentinian III. Anthemius is also sometimes counted to the dynasty as he became a son-in-law of Marcian. Descendants of the dynasty continued to be part of the East Roman nobility at Constantinople until the end of the 6th century.In "italics" the Augusti and the Augustae.