During the early years of the Roman Republic, the lands between the Danube and the upper Balkan peninsula were inhabited by various Illyro-Dacian tribes. The Romans frequently came into conflict with Illyrian raiders and pirates active in the Adriatic Sea. In 229 B.C. the Romans fought a war against Queen Teuta of Illyricum, in an effort to put an end to Illyrian piracy. But it was not until the conclusion of the Third Macedonian War that Rome finally conquered the Illyrian coast, and annexed it as a province.
Thrace had been a client kingdom of Rome since the First Macedonian War. It was finally incorporated into the Roman Empire as a province under the Emperor Claudius, in 46 A.D.
Augustus Caesar launched a series of campaigns to push the northern frontier of Rome to the Danube River. In 6 A.D. there was a campaign on the lower Danube and Moesia was annexed. Four years later, a campaign on the upper Danube led to the annexation of Pannonia.
The region was divided between the Eastern and Western halves of the Roman Empire in the fourth century, with Pannonia and Illyricum largely remaining under the control of the Western Empire, and Moesia and Thrace under the control of the Eastern Empire. The capital of the Eastern Empire, Constantinople, was founded in the fourth century by the emperor Constantine on the southern shores of Thrace. Between the fourth and sixth centuries, most of the region fell to successive invasions of Huns, Ostrogoths, Lombards, Avars, and Slavs moving south of the Danube River.