In the Roman Empire
The Romans were different from other major conquerors of the Balkans in that they first arrived in the west. Later attacks were launched from the southeast as well, so that by the 1st century ce the entire peninsula was under Roman control. At the height of Roman power, the Balkan peoples were the most united of any time in their history, with a common legal system, a single ultimate arbiter of political power, and absolute military security. In addition, a vibrant commerce was conducted along the Via Egnatia, a great east-west land route that led from Dyrrhachium (modern Durrës, Albania) through Macedonia to Thessalonica (modern Thessaloníki, Greece) and on to Thrace. The northwestern part of the peninsula, including Dalmatia along the Adriatic coast as well as Pannonia around the Danube and Sava rivers, became the province of Illyricum. What is now eastern Serbia was incorporated into Moesia, which reached farther eastward between the Balkan Mountains and the Danube all the way to the Black Sea. The southeastern part of the peninsula was ruled as Thrace, and the southern part was brought into Macedonia.
The Romans largely regarded the Danube River as their northern frontier, but in the 2nd and 3rd centuries their authority was extended northward into Dacia, in what is now western Romania. Dacia had been the home of a people closely related to the Thracians. The Dacians had suffered invasion by a number of peoples, including the Scythians, a mysterious people probably of Iranian origin who were absorbed into the resident population. In the 3rd century bce they managed to contain Macedonian pressure from the south, but in later years they were much less able to fend off Celtic invaders from the northwest. By the 1st century ce a substantial Dacian state extended as far west as Moravia and threatened Roman command of the Danube in the Balkans. The extension of the Dacian state and Dacian raids across the river into Moesia prompted the emperor Trajan in the first decade of the 2nd century to march into Dacia, obliterate the Dacian state and Dacian society, and establish a Roman colony that lasted until barbarian incursions forced a withdrawal back across the Danube beginning in 271.
Christendom
The abandonment of Dacia in the second half of the 3rd century was a symptom of Rome’s decline, leading to major changes in the 4th century. In 330 the imperial capital was moved to Byzantium, so that any tribe intent on attacking the seat of Roman power and opulence would thenceforth move through the Balkans rather than into Italy. In 391 Christianity became the official religion, and in 395 the empire was divided in two. The dividing line ran through the Balkans: Illyricum went to the western sector under Rome; the remainder went to the eastern half and was ruled from Byzantium (by this time named Constantinople). This deep and long-lasting division did little to alleviate the barbarian incursions of the times. The 5th century saw devastation by, among others, the Alani, the Goths, and the Huns. Most of these invaders soon left or were assimilated, but such was not to be the case with the Slavs, who first arrived in the 6th century.
The Slavs were settlers and cultivators rather than plunderers and within 100 years had become a powerful factor in the region. They separated into four main groups: Slovenes, Croats, Serbs, and Bulgarians (the last being a Turkic tribe, the Bulgars, that was eventually absorbed by Slavs who had already settled in the eastern Balkans). Although in 681 the Bulgars established their own state, the Slavs acknowledged the suzerainty of the emperor in Constantinople.
In the second half of the 9th century, Christianity was adopted by the Bulgarians and the Serbs, both of whom chose the Byzantine rather than Roman variant of the new religion. To the north of the Danube, the Romanians, though not Slav, made the same choice, while the Croats, together with most of the rest of what had been Rome’s section of the divided empire, became part of the western Christian community. The Albanians, isolated behind their mountain chains, were not much affected by either branch of Christianity. The divisions and competition between Rome and Constantinople intensified, with the two communities separating irrevocably in 1054. The dividing line of 395 was thus reinforced: the Croats and Slovenes became an integral part of Roman Catholic Europe, with its Latin script and culture, and the Serbs, Bulgarians, and Romanians joined the Greeks in their allegiance to Eastern Orthodoxy.