The First Bulgarian Empire (modern ) was a medieval Bulgarian state that existed in southeastern Europe between the 7th and 11th centuries AD. It was founded circa 681 when Bulgar tribes led by Asparukh moved to the northeastern Balkans and secured Byzantine recognition for their right to settle south of the Danube by defeating - possibly with the help of local South Slavic tribes - the Byzantine army led by Constantine IV. At the height of its power Bulgaria spread from the Danube Bend to the Black Sea and from the Dnieper River to the Adriatic Sea. As the state solidified its position in the Balkans, it entered on a centuries-long interaction, sometimes friendly and sometimes hostile, with the Byzantine Empire. Bulgaria emerged as Byzantium's chief antagonist to its north, resulting in several wars. The two powers also enjoyed periods of peace and alliance, most notably during the Second Arab siege of Constantinople, where the Bulgarian army broke the siege and destroyed the Arab army, thus preventing an Arab invasion of Southeastern Europe. Byzantium had a strong cultural influence on Bulgaria, which also led to the eventual adoption of Christianity in 864. After the disintegration of the Avar Khaganate, the country expanded its territory northwest to the Pannonian Plain. Later the Bulgarians confronted the advance of the Pechenegs and Cumans, and achieved a decisive victory over the Magyars, forcing them to establish themselves permanently in Pannonia.