DIOCESE OF DACIA

Emperor Aurelian (270-275), confronted with the secession of Gallia and Hispania from the empire since 260, with the advance of the Sassanids in Asia, and the devastations that the Carpians and the Goths had done into Moesia and Illyria, abandoned the province of Dacia created by Trajan and withdrew the troops altogether, fixing the Roman frontier at the Danube. A new Dacia Aureliana was reorganised south of the Danube, with its capital at Serdica (today's Sofia). Later on, Diocletian and Constantine I would reorganise the provinces Dacia Mediteranea, Moesia Inferior, Dardania, Prevalitania and Dacia Ripensis into a Diocese of Dacia, which along with the Diocese of Macedonia formed the Praetorian prefecture of Illyricum.
The abandonment of Dacia Trajana by the Romans is mentioned by Eutropius in his Breviarium historiae Romanae, book IX :
The province of Dacia, which Trajan had formed beyond the Danube, he gave up, despairing, after all Illyricum and Moesia had been depopulated, of being able to retain it. The Roman citizens, removed from the town and lands of Dacia, he settled in the interior of Moesia, calling that Dacia which now divides the two Moesiae, and which is on the right hand of the Danube as it runs to the sea, whereas Dacia was previously on the left.
The diocese was transferred to the western empire in 384 by Theodosius I, probably in partial compensation to the empress Justina for his recognition of the usurpation of Magnus Maximus in Britannia, Gaul and Hispania.
Provinces

★ Dacia mediterranea

★ Moesia I

★ Praevalitana

★ Dardania

★ Dacia ripensis