 Complete the following phrase:  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 