The Celts vanished by the end of that century, leaving a few place-names to mark their passage.  The city of Belgrade was known until the seventh century A.D. by its Celtic name of Singidunum.  Naissus, the modern Nish, is also possibly of Celtic origin.  It was towards 230 B.C. that Rome came into contact with Illyricum, owing to the piratical proclivities of its inhabitants, but for a long time it only controlled the Dalmatian coast, so called after the Delmati or Dalmati, an Illyrian tribe.  The reason for this was the formidable character of the mountains of Illyria, which run in several parallel and almost unbroken lines the whole length of the shore of the Adriatic and have always formed an effective barrier to invasion from the west.  The interior was only very gradually subdued by the Romans after Macedonia had been occupied by them in 146 B.C.  Throughout the first century B.C. conflicts raged with varying fortune between the invaders and all the native races living between the Adriatic and the Danube.  They were attacked both from Aquileia in the north and from Macedonia in the south, but it was not till the early years of our era that the Danube became the frontier of the Roman Empire.

In the year A.D. 6 Moesia, which included a large part of the modern kingdom of Serbia and the northern half of that of Bulgaria between the Danube and the Balkan range (the classical Haemus), became an imperial province, and twenty years later Thrace, the country between the Balkan range and the Aegean, was incorporated in the empire, and was made a province by the Emperor Claudius in A.D. 46.  The province of Illyricum or Dalmatia stretched between the Save and the Adriatic, and Pannonia lay between the Danube and the Save.  In 107 A.D. the Emperor Trajan conquered the Dacians beyond the lower Danube, and organized a province of Dacia out of territory roughly equivalent to the modern Wallachia and Transylvania, This trans-Danubian territory did not remain attached to the empire for more than a hundred and fifty years; but within the river line a vast belt of country, stretching from the head of the Adriatic to the mouths of the Danube on the Black Sea, was Romanized through and through.  The Emperor Trajan has been called the Charlemagne of the Balkan peninsula; all remains are attributed to him (he was nicknamed the Wallflower by Constantine the Great), and his reign marked the zenith of Roman power in this part of the world.  The Balkan peninsula enjoyed the benefits of Roman civilization for three centuries, from the first to the fourth, but from the second century onwards the attitude of the Romans was defensive rather than offensive.  The war against the Marcomanni under the Emperor Marcus Aurelius, in the second half of this century, was the turning-point.  Rome was still victorious, but no territory was added to the empire.  The third century saw the southward movement of the Germanic peoples, who took the place of the Celts.  The Goths invaded the peninsula,