The Theodosian dynasty was a Roman imperial family that produced five Roman emperors during Late Antiquity, reigning over the Roman Empire from 379 to 457. The dynasty's patriarch was Theodosius the Elder, whose son Theodosius the Great was made Roman emperor in 379. Theodosius's two sons both became emperors, while his daughter married Constantius III, producing a daughter that became an empress and a son also became emperor. The dynasty of Theodosius married into, and reigned concurrently with, the ruling Valentinianic dynasty (), and was succeeded by the Leonid dynasty () with the accession of Leo the Great.
History
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 Valentinianic 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.
According to Polemius Silvius, Theodosius the Great was born on 11 January 347 or 346. The epitome de Caesaribus places his birthplace at Cauca (Coca, Segovia) in Hispania. Theodosius had a brother named Honorius, a sister referred to in Aurelius Victor's De caesaribus but whose name is unknown, and a niece, Serena.
In 366, Theodosius the Elder attacked and defeated the Alamanni in Gaul; the defeated prisoners were resettled in the Po Valley. In 367 Roman Britain was threatened by the Great Conspiracy, defeated 368–369 by the magister equitum Theodosius the Elder, accompanied by his son Theodosius. At this time was the unsuccessful usurpation in Britain by Valentinus. Theodosius the Elder was made magister equitum in 369, and retained the post until 375. The magister equitum and his son Theodosius campaigned against the Alamanni 370. The two Theodosi campaigned against Sarmatians in 372/373. Valentinian's rule in Roman Africa was disrupted by the revolt of Firmus in 373. Theodosius the Elder defeated the usurpation.
In 373/374, Theodosius the magister equitum's son, was made dux of the province of Moesia Prima. At the fall of his father, Theodosius the dux of Moesia Prima retired to his estates in the Iberian Peninsula, where he married Aelia Flaccilla in 376. Their first child, Arcadius, was born around 377. Pulcheria, their daughter, was born in 377 or 378. Theodosius had returned to the Danube frontier by 378, when he was appointed magister equitum.
First generation emperor: Theodosius the Great
After the death of his uncle Valens (), Gratian, now the senior augustus, sought a candidate to nominate as Valens's successor. On 19 January 379, Theodosius I was made augustus over the eastern provinces at Sirmium. His wife, Aelia Flaccilla, was accordingly raised to augusta. The new augustus's territory spanned the Roman praetorian prefecture of the East, including the Roman diocese of Thrace, and the additional dioceses of Dacia and of Macedonia. Theodosius the Elder, who had died in 375, was then deified . In October 379 the Council of Antioch was convened. On 27 February 380 Theodosius issued the Edict of Thessalonica, making Nicene Christianity the state church of the Roman Empire. In 380, Theodosius was made Roman consul for the first time and Gratian for the fifth; in September the augusti Gratian and Theodosius met, returning the Roman diocese of Dacia to Gratian's control and that of Macedonia to Valentinian II. In autumn Theodosius fell ill, and was baptized. According to the Consularia Constantinopolitana, Theodosius arrived at Constantinople and staged an adventus, a ritual entry to the capital, on 24 November 380.