Bulgars were pastoralists originally from central Asia, who migrated westward to the steppe north of the Caspian Sea, as did the Huns and the Avars. They must have been Turkic in origin because nearly a hundred proto-Bulgarian (i.e., pre-Christian) inscriptions survive in characters known to be Turkic. Their history in the fifth and sixth centuries is obscure, although it has been suggested that they were related to the Cotrigurs and Utigurs. The establishment of the Avars in Pannonia by the late sixth century allowed the Bulgars to expand. One contingent aligned itself with the Lombards, helping them to conquer Italy. Another contingent participated with the Avars and Slavs in attacks on Thessalonike and Constantinople. However, most Bulgars lived north of the sea of Azov, where, in 632, one of their leaders, Kuvrat, formed a confederation of Bulgar tribes. After Kuvrat’s death, the confederation disintegrated under the growing power of the Khazars to the east. Some Bulgars, led by Asparuch, crossed the Danube River, resisting an attempt by Constantine VI to oust them by force. In 680 they signed a treaty with Byzantium, creating the first independent state that Byzantium recognized on its own soil. The region was already settled by Slavs, who were much more numerous, and who the Bulgars subjugated. By the ninth century, the Bulgars had virtually become Slavs, at which point this intermixed people are referred to as Bulgarians.
Bulgaria was established in 681 by Asparuch, Bulgaria became the first independent state on Byzantine soil to be recognized by Byzantium. The Bulgars, with their capital at Pliska, ruled over a large indigenous population of Slavs and Greeks. By the 10th century Bulgars and Slavs had intermixed into a single people, referred to as Bulgarians. Bulgaria always remained a kind of third-world country on Byzantium’s northern border, one that was dependent on the Byzantine economy, one subject to Byzantine cultural influences. It was also an intermittent military threat, especially under khan Krum, and tsars Symeon and Samuel. Byzantine cultural influence seemed to make headway when, in 864, khan Boris I received baptism from Byzantium. This provoked a revolt of the Bulgar nobility, which defended Bulgar paganism against Byzantine encroachment. Boris suppressed the revolt, but it made him hesitant to place the Bulgarian church under Byzantine ecclesiastical administration. However, in 870 he did just that, after negotiations with Pope Nicholas I broke down. In 885 Boris accepted four pupils of the brothers Cyril and Methodios who provided him with a Slavonic-speaking clergy and with necessary liturgical texts. Boris struggled to preserve Church Slavonic as the language of the Bulgarian church in the face of Byzantine insistence on Greek. Nevertheless, resistance to Byzantine cultural and ecclesiastical hegemony remained strong in Bulgaria, as Bogomilism demonstrated. To some extent these contradictory forces were never resolved. “Peace” meant Byzantine occupation of the country from 1018–1185, after Basil II ended a series of campaigns in 1014 with a decisive victory over the forces of Samuel of Bulgaria. Subsequently a revolt occurred in 1185, which by 1186 resulted in a new state referred to as the Second Bulgarian Empire. Its capital was at Turnovo and its early rulers were energetic men like Kalojan and John Asen II. The new state expanded into Thrace, and, after the battle of Klokotnitsa in 1230, into western Macedonia as well. However, the growing power of Serbia threatened Bulgaria, and at Velbuzd in 1330 the Serbs destroyed a Bulgarian army. Within several decades after Velbuzd, Bulgaria’s decline was made even more apparent by the Ottoman expansion into the Balkans. In 1373 Bulgaria became a vassal to the Ottomans, and in 1393 Murad I conquered Bulgaria outright, burning Turnovo.
(Source: «Historical Dictionary of Byzantium», by John H. Rosser)