The region that is now Bulgaria was at one time included in the Roman Empire as part of the provinces of Thrace and Moesia. Slavic and Turkic tribes settled in the area between about the 4th and 6th centuries ad. One branch of people known as Bulgars, who had established a large state near the Volga River on the east side of the Black Sea, invaded the Balkan Peninsula in the 7th century. They set up a state between the Danube River and the Balkan Mountains, an area that was then claimed by the Byzantine Empire. Byzantine armies failed repeatedly to dislodge the invaders during the 8th and early 9th centuries. By the end of the 9th century the Bulgarians had annexed considerable additional territory and laid the foundations for a strong state under Khan Krum, who reigned from 803 to 814. The Krum armies inflicted a devastating defeat on an invading Byzantine force in 811 and, assuming the offensive, nearly succeeded in 813 in taking Constantinople (present-day İstanbul, Turkey), the capital of the Byzantine Empire. Bulgarian-Byzantine relations were thereafter relatively peaceful and continued to be so during the first half of the 9th century. The immediate successors of Krum enlarged their dominions, mainly in the region of Serbia and Macedonia. In 860, however, during the reign of Boris I, Bulgaria suffered a severe military setback at the hands of the Serbs. Four years later Boris, responding to pressure from the Byzantine emperor Michael III, made Christianity the official religion. Boris accepted the primacy of the papacy in 866, but in 870, following the refusal of Pope Adrian II to make Bulgaria an archbishopric, he shifted his allegiance to the Eastern Orthodox Church.

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