Map of the Byzantium northern Balkans in the 6th century ADTrajan conquered the Dacians, under King Decibalus, and made Dacia, across the Danube in the soil of barbary, a province which in circumference had ten times 100,000 paces; but it was lost under Imperator Gallienus, and, after Romans had been transferred from there by Aurelian, two Dacias were made in the regions of Moesia and Dardania.Intr-un glas se roaga taranii romani sa-i lase a trai cu barbarii... si apoi sa ne miram ca nu pot fii invinsi getii (goti) cand satenii sant mai bucurosi sa fie cu ei decat cu noi -Salvianus: De gubenatione, V, 8 (439 AD)Jeremiah F. O'Sullivan, The writings of Salvian, the presbyter, in the Fathers of the Church series (vol. 3), and published by the Catholic University of America Press in 1947, reprint 1978 (ISBN 0813200032), and is available from Amazon.com.
The decade of the 220s was the last peaceful period in Dacia's history. The finding of a huge hoard of 8,000 coins at Romula, ranging in date from Commodus to Elagabalus (217-222) shows that insecurity antedated the mid-3rd century.
The accession of Maximinus Thrax (235-238) marked the start of a 50-year period of disorder in the Roman Empire during which the militarization of the government inaugurated by Septimius Severus continued apace and the debasement of the currency brought the empire to bankruptcy. As the middle of the 3rd century approached, a mighty swarm of Germanic tribes, migratory Goths, appeared on the horizon of the Danube. In or around 235, the Goths and Carpi occupied Histria and sacked the Danube Delta’s prosperous commercial centers, which had also served as important economic links for Dacia. The evidence of excavation is that the Limes Transalutanus did not last much beyond the reign of Gordian III (238-244): it was probably abandoned in the face of the invasion of the Carpi.
At this point of history, Rome’s only real option was to pay off the Dacian Carpi, Goths, and other barbarians in exchange for peace in Moesia. When Philip the Arab (244-249) refused to pay the annual tribute in 245, the next year the Carpi entered Dacia and destroyed the city of Romula; and the Carpi probably also burned the camp at Răcari between 243 and 247. The governor of Lower Moesia was not able to check the invaders; accordingly the emperor left Rome to take charge of the situation. The mother of a later Emperor, Galerius, fled during this period from Dacia Malvensis to Lower Moesia.But the other Maximian /Galerius/, chosen by Diocletian for his son-in-law, was worse, not only than those two princes whom our own times have experienced, but worse than all the bad princes of former days. In this wild beast there dwelt a native barbarity and a savageness foreign to Roman blood; and no wonder, for his mother was born beyond the Danube, and it was an inroad of the Carpi that obliged her to cross over and take refuge in New Dacia.—Lactantius: Of the Manner in which the Persecutors Died – Chapter IX
At the end of 247 the Carpi were decisively beaten in open battle and sued for peace; Philip the Arab took the title of Carpicus Maximus. In Sucidava, probably after a raid (245-247) by the barbarians, the townsfolk hastily threw up a trapezoidal stone wall and ditch; the building of the third phase of Romula’s town wall, in 248, was also a defensive measure against the invading Carpi. An epigraph in Apulum hails Emperor Decius (249-251) as restitutor Daciarum (the 'restorer of Dacia'). On July 1, 251 Decius was slaughtered by the Goths after his defeat in the Battle of Abrittus (Razgard, Bulgaria). The Goths now dominated the territories of the lower Danube and the western shore of the Black Sea; their influence in Free Dacia and their massive presence around Roman Dacia was overwhelming.Decius appeared in the world, an accursed wild beast, to afflict the Church, – and who but a bad man would persecute religion? It seems as if he had been raised to sovereign eminence, at once to rage against God, and at once to fall; for, having undertaken an expedition against the Carpi, who had then possessed themselves of Dacia and Moesia, he was suddenly surrounded by the barbarians, and slain, together with great part of his army; nor could he be honored with the rites of sepulture, but, stripped and naked, he lay to be devoured by wild beasts and birds, – a fit end for the enemy of God.—Lactantius: Of the Manner in which the Persecutors Died – Chapter IV
Emperor Gallienus’s many victorious fights against the Carpi and other Free Dacians brought him the title of Dacicus Maximus. However, literary sources from antiquity (Eutropius, Aurelius Victor and Festus) inform us that Dacia was lost under his reign. He moved many cohorts of legions V Macedonica and XIII Gemina from Dacia to Pannonia. The latest coins at Ulpia Traiana Sarmizegetusa and Porolissum bear his effigy. The erection of inscribed monuments in Dacia was essentially halted in 260, during Gallienus’s reign.Even the territories across the Danube, which Trajan had secured, were lost.—Aurelius Victor: De Caesaribus
In an attempt to boost the confidence of the Roman colonists, Aurelian minted coins which bear the inscription DACIA FELIX (‘Fruitful or Happy Dacia’) around 270. But finally, he was forced to accept the vulnerability of Dacia Traiana to repeated Gothic invasions. Reluctantly he decided to abandon the province and save the ruinous cost of garrisoning it. Dacia was officially abandoned in 271; other view is that Aurelian evacuated his troops and civilian administration only by 274 or 275.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.—Eutropius: Abridgement of Roman History
Moving the Romanized population to the river’s southern shore went a long way towards repopulating the provinces of Moesia, Pannonia and Thracia that had been ravaged by barbarian invasions and the recurrent plague. So the emperor created a new province of Dacia, “Dacia Aureliana” with its capital at Serdica in Lower Moesia.
Dacia Aureliana was a province of the Roman Empire that occupied most of what is today Bulgaria (271/275-285). Its capital was in Serdica (Sofia. The Emperor Diocletian replaced Dacia Aureliana with two provinces – Dacia Mediterranea with its capital at Serdica and Dacia Ripensis, with its capital at Ratiaria. Later these two “Dacias” along with Dardania, Lower Moesia, and Prevalitana constituted the Diocese of Dacia.
Dacia Aureliana was a Roman province situated on the territory of present-day Bulgaria, Romania and Serbia. In the year 271, Emperor Aurelian retreated the Roman army and administration from Dacia Traiana (north of the Danube, in Romania) to the south, in Moesia, creating Dacia Aureliana.  The Thracian -Getic population of Dacia Aureliana had already been Romanized. 
Almost from the beginning, Dacia Aureliana was spilt into two administrative units called Dacia Ripensis and Dacia Mediterranea, as shown by an inscription from the year 283. Dacia Ripensis had its capital at Ratiaria (2 km west of the present-day village Archar - between Vidin and Lom, Bulgaria), and Dacia Mediterranea at Serdica (Sofia). 
Dacia Aureliana comprised "the eastern part of Moesia Superior" and "the western part of Moesia Inferior"; its northern area stretched from opposite the area of the bank opposite Orşova (somewhere between the Great Morava and the Timok) to the mouth of the Oescus River (present-day Iskar). Southwards, it reached the Axius River (present day Vardar) and Strymon River (present day Struma), and included the cities of Scupi (present day Skopje) and Pautalia (present day Kyustendil).
Dacia ripensis (Greek: Δακία Ρειπήσιος, English translation: "from the banks of the Danube") was the name of a Roman province (part of Dacia Aureliana) first established by Aurelian (circa 283 AD when the boundary stones were set by him and one of them was restored by Gaianus) after he withdrew from Dacia north of the Danube River. Ratiaria was established as the capital of Dacia ripensis (it was previously a colony founded by Trajan located within Moesia Superior). The capital served both as the seat of the military governor (or dux) and as the military base for Roman legion XIII Gemina. According to Priscus, Dacia ripensis was a flourishing province during the 4th and 5th centuries AD. During the early 440s, however, the Huns captured the province (prior to this, there were conflicts between the Romans and the Huns whereby the latter group captured Castra Martis through treacherous means). Even though the province recovered briefly from Hunnic rule, it was eventually decimated by the Avars in 586. On a more specific note, Aurelian developed Dacia ripensis on a stretch of the Danube specifically between Moesia Superior and Moesia Inferior. During the reign of Emperor Trajan, many fortresses and fortifications were constructed within the province. These fortresses/fortifications entailed Singidunum, Viminacium, Tanata (Τανάτα), Zernis (Ζέρνης), Doukepratou (Δουκεπράτου), Caputboes (Καπούτβοες), Zanes (Ζάνες), and Pontes (Πόντες).
The Diocese of Dacia (Latin: Dioecesis Daciae) was a diocese of the later Eastern Roman Empire, in the area of modern Serbia and western Bulgaria. It was subordinate to the Praetorian prefecture of Illyricum. Its capital was at Serdica (modern Sofia).
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 organised south of the Danube our of central Moesia, with its capital at Serdica.
The abandonment of Dacia Traiana 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.||”|
Later, during the administrative reforms of Diocletian and Constantine I, the Diocese of Moesia was created, encompassing most of the central Balkans and the Greek peninsula. After a few years, however, the diocese was split in two, forming the Diocese of Macedonia and the Diocese of Dacia, encompassing the provinces of Dacia Mediterranea, Moesia Inferior, Dardania, Praevalitana and Dacia Ripensis.
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. However, upon his death in 395, it reverted to the Eastern Empire, forming, together with the Diocese of Macedonia, the Praetorian prefecture of Illyricum.