Thracian control began to dissolve because of internal strife among the tribes as well as increasing conflicts with external powers looking to encroach on the region, including the Greeks, the Persians and the Macedonians. In the fourth century BCE, Philip of Macedonia extended his power across the Balkans. He pushed east and along the way established the city Philippopolous, now called Plovdiv. When he was assassinated shortly afterwards, his son Alexander the Great continued the family reign and Hellenic influence over the area by extending the territory north to the Danube in 336 BCE.
When Alexander died in his early thirties, three top generals inherited pieces of his huge empire, which encompassed the eastern portion of the Mediterranean, across Persia and all the way to the Indus River. The Balkans remained a place of conflict during the ensuing two centuries as groups continued to vie for strategic pieces of land. Eventually, the Macedonians came under the strengthening Roman power in the second century BCE, but the Thracians held out for another two centuries until 45 CE, when they fell under Roman control as well.
The area of present-day Bulgaria was divided into two Roman provinces. To the north of the Balkan Mountains (Stara Planina) was Moesia Inferior and the land south of that retained the name of Thrace. After many years of violent warfare in the region, with Roman control came relative peace and prosperity for approximately 200 years. As in the rest of the Empire at the time, major projects were carried out to improve the infrastructure. Craft, industrial and trade industries were fostered, and the fertile fields of Thracian valleys were prosperous. Roads, aqueducts and fortified, modernised cities were built, and many preserved ruins and artifacts remain in Bulgaria today to serve as reminders of that era. One of the best is the amphitheatre in the old city of Plovdiv.
Several northern forts were built on the shores of the Danube, which served more or less as the frontier of Roman lands in this region. Beginning in the third century, however, great numbers of people began to move out of central and eastern Asia in what has been called the Age of Migrations. These tribes, including such feared names as the Huns, the Avars and the Goths, began sweeping down into Europe. The emperor Diocletian even divided Roman lands into Eastern and Western halves in 286 to facilitate administration and protection against increased attacks by the influx of these 'barbarians'. Despite such efforts, the Western Roman Empire floundered and was eventually overthrown in the fifth century by invading groups such as the Visigoths and the Vandals.
During these times, the Balkans south of the Danube were part of the Eastern Roman Empire and ruled from the city of Byzantium (present-day Istanbul). The capital's name was changed to Constantinople in 330 by Constantine, the first emperor to convert to Christianity. Although the Byzantine Empire generally flourished while that in the West crumbled, during the following few centuries there were alternating skirmishes and treaties with invading groups still arriving in waves from central and eastern Asia.
One such group was the Slavs, who were part of the great migrations out of central Asia. They arrived in the Balkans during the sixth century, and although numerous enough to absorb (and possibly drive out) most of the Thracian-Roman population living on the Balkans at the time, in general they were peaceful people who lived in fairly democratic farming communities. The Slavic language and customs spread across the region and took root, making it the dominant culture in the area by the time the next invading group came along in the following century.
