Area Handbook for Bulgaria

“In the seventh century A.D. the Bulgars in turn began to migrate into the Balkans. They had come originally from central Asia and were said to be related to the Huns. They were of the same stock as the Turks and spoke a language similar to Turkish. Before migrating to the Balkans, they had lived north of the Black Sea. Their social order was vastly different from that of the Slavs, although eventually the Slavic system became dominant. The Bulgars, unlike the Slavs who repudiated the concept of kingship, were governed autocratically by khans. The Bulgars were warriors who fought on horseback, and their customs and dress were Asiatic. When the Bulgars overran what is now northeastern Bulgaria, they found Slavic tribes already established and quickly made peace with them in order to strengthen themselves against the Byzantines. As the Slavs were far more numerous than the Bulgars, the latter were assimilated, and within two centuries the Bulgars had been completely slavicized. The Slavic language and culture were adopted, although the Bulgarian name and political structure were retained.””

— Eugene K. Keefe