Old Bulgaria broke up in the second half of the 7th century after a series of successful Khazar invasions. Some of its population fled north, to the Volga-Kama region, where they established Volga Bulgaria, which eventually became extremely wealthy: its capital then being the 4th-largest city in the world. Shortly after that, the Suar Duchy was forced to become a vassal state of Khazaria. About half a century later, the Suars took part in the Arab–Khazar wars of 732–737. The adoption of Islam in the early tenth century in Volga Bulgaria led to most of its people embracing that religion.