a rivalry between the Khazars and the Bulgars over Turk patrimony and dominance in the Pontic–Caspian steppe. Some historians consider the war an extension of the Western Turks struggle, between the Nushibi tribes and Ashina clan, who led the Khazars, and the Duolu/Tu-lu tribes, which some scholars associated with the Dulo clan, from which Kubrat and many Bulgar rulers originated. The Khazars were ultimately victorious and parts of the Bulgar union broke up. Subsequent migrations It is unclear whether the parting ways by brothers was caused by the internal conflicts or strong Khazar pressure. The latter is considered more likely. The Bulgars led by the first two brothers Batbayan and Kotrag remained in the Pontic steppe zone, where they were known as Black Bulgars by Byzantine and Rus sources, and became Khazar vassals. The Bulgars led by Kotrag migrated to the middle Volga region during the 7th and 9th centuries, where they founded Volga Bulgaria, with Bolghar as its capital. According to Ahmad ibn Rustah (10th century), the Volga Bulgars were divided into three branches: "the first branch was called Bersula (Barsils), the second Esegel, and the third Bulgar". In 922 they accepted Islam as the official religion. They preserved their national identity well into the 13th century by repelling the first Mongol attacks in 1223. They were eventually subdued by the Mongols in 1237. They gradually lost their identity after 1431 when their towns and region were captured by the Russians. The third and most famous son, Asparukh, according to Nikephoros I: {{Quote|crossed the river Danapros and Danastros, lived in the locale around the Ister, having occupied a place suitable for settlement, called in their language ογγλον (ogglon; Slav. o(n)gl, "angle, corner"; Turk. agyl, "yard")... The people having been divided and scattered, the tribe of the Khazars, from within Berulia (Bessarabia), which neighbors with Sarmatia, attacked them with impunity. They overran all the lands lying behind the Pontos Euxeinos and penetrated to the sea. After this, having made Bayan a subject, they forced him to pay tribute.}} Asparukh, according to the Pseudo–Zacharias Rhetor, "fled from the Khazars out of the Bulgarian mountains". In the Khazar ruler Joseph's letter is recorded "in the country in which I live, there formerly lived the Vununtur (< Vunundur < Onoğundur). Our ancestors, the Khazars warred with them. The Vununtur were more numerous, as numerous as the sand by the sea, but they could