Watch HDToday. In 651, Khan Kubrat died and the Khazars accelerated their raids upon Great Bulgaria. His five sons split the Bulgarian tribe and each led his to find new land where they could live in peace. Khan Asparukh, the youngest son, went west and, after an arduous journey lasting for years, southward across the River Danube, into Moesia. In 680-81, in alliance with the Slavs, he inflicted stunning defeats on the Roman legions and forced the Byzantine Empire to recognize the formation of the new state of Bulgaria in the lands where it still exists today.