Theodosius I
383–384

According to the Chronicon Paschale, Theodosius celebrated his quinquennalia on 19 January 383 at Constantinople; on this occasion he raised his eldest son Arcadius to co-emperor (augustus). Sometime in 383, Gratian's wife Constantia died. Gratian remarried, wedding Laeta, whose father was a consularis of Roman Syria. Early 383 saw the acclamation of Magnus Maximus as emperor in Britain and the appointment of Themistius as praefectus urbi in Constantinople. On 25 August 383, according to the Consularia Constantinopolitana, Gratian was killed at Lugdunum (Lyon) by Andragathius, the magister equitum of the rebel emperor during the rebellion of Magnus Maximus. Constantia's body arrived in Constantinople on 12 September that year and was buried in the Church of the Holy Apostles on 1 December. Gratian was deified as Latin: Divus Gratianus, lit. ' the Divine Gratian '.

Theodosius, unable to do much about Maximus due to ongoing military inadequacy, opened negotiations with the Persian emperor Shapur III (r. 383–388) of the Sasanian Empire. According to the Consularia Constantinopolitana, Theodosius received in Constantinople an embassy from them in 384.

In an attempt to curb Maximus's ambitions, Theodosius appointed Flavius Neoterius as the Praetorian Prefect of Italy. In the summer of 384, Theodosius met his co-emperor Valentinian II in northern Italy. Theodosius brokered a peace agreement between Valentinian and Magnus Maximus which endured for several years.

Theodosius I was based in Constantinople, and according to Peter Heather, wanted, "for his own dynastic reasons (for his two sons each eventually to inherit half of the empire), refused to appoint a recognized counterpart in the west. As a result he was faced with rumbling discontent there, as well as dangerous usurpers, who found plentiful support among the bureaucrats and military officers who felt they were not getting a fair share of the imperial cake."
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