Early Population origins
Prior to the Roman Empire's conquests and expansion between 35 B.C. and 400 A.D., the Eastern European region was largely populated by Slavic and Baltic tribes in the north, and Celtic, Thracian and Illyrian tribes in the south. The Roman Empire conquered the Thracians in 46 A.D., but the Balts in the north managed to avoid falling under Rome's sphere of dominance.
The fate of the Illyrians is unclear, but some linguistic scholars believe the Albanian language may be a form of Illyrian or Thracian. Whether that means the Albanians are descended from the ancient Illyrians is a matter of debate. The fact remains that their origins cannot be conclusively determined and their language cannot be definitively classified, except to say that it is Indo-European and predates the Slavic migrations of the medieval period.
Post Roman Empire
Roman control of the East European region was relatively weak, partly because the population was largely rural. After the collapse of the Western Roman Empire, the southern area of the region, namely Bulgaria and Romania, remained part of the Byzantine Empire, while most of the remainder was overrun by invasions of Huns, Alans and other nomadic tribes from the Pontic steppe. Slavic tribes, possibly displaced by the invasions, spread south toward the Balkans.
The Avars and Bulgars, most likely Turkic tribes from Central Asia, arrived in the 7th century. These tribes established kingdoms called Khaganates in the south Balkans, pushing the Byzantine border south, almost to the Aegean Sea. Although subjugated by outsiders, the native Slavic tribes' culture persisted. The invaders were assimilated and “Slavicized,” creating new Slavic national identities.
In the area that now includes Belarus, Ukraine and western Russia, a confederation of Slavic tribes known as the Rus' established a kingdom with its capital in Kiev. Legend has it—though some scholars disagree—that the Rus' were ruled by a small group of Scandinavian warriors called the Varangians. Scandinavian or not, the Rus' were entirely Slavicized by the 10th century. Russia and Belarus are named after this kingdom, and both claim them as cultural ancestors.
The Magyar, a Uralic tribe from the northern part of the Asian steppe, settled in the Carpathian Basin around 900 A.D. and established the Kingdom of Hungary. However, unlike the Avars and Bulgars, the Hungarians resisted Slavic influence and maintained their language, which is closely related to Finnish and Estonian.
Led by two grandsons of Ghengis Khan, the Mongol raids and invasions of Eastern Europe were violent and fearsome. Medieval European warfare tactics were ill-suited to fight the mounted archers of the invading horde. The kingdoms of Rus' fell to the Mongols, who swept quickly across the steppe and into the Carpathian Mountains. Hungary was the main target of the Mongol campaign in Eastern Europe and was poorly prepared to defend itself after centuries of relative peace. Nearly half of the population was killed. In the terror and panic, refugees fled the Mongol armies in numbers never before seen. The Mongol Empire expanded to include Ukraine, Russia, Poland, Hungary and Bulgaria.