Excerpts from Books and Wikipedia
"Heraclius had recognized the kingship of the Bulgarian chief Kubrat, north of the Azov Sea. Kubrat had died in 669 . . . Like a Frankish chief, he left the kingdom to his five sons jointly. . . . The third son, Asparukh, settled between the Dniester and the Prut river with over thirty thousand followers of his own. . . . Constantine IV . . . marched against Asparukh’s thirty thousand Bulgarians, who lay just on Byzantium’s western border. He was at first successful. . . . The Bulgarian warleader Asparukh then made an alliance with the Slavic tribes nearby, fought his way down into Thracia, and generally caused a headache for the Byzantine armies on the northwestern border. By 681, Constantine IV had decided that he’d better make peace. He swore out a treaty with Asparukh, even agreeing to pay the Bulgars yearly tribute. The Byzantine tribute payments and the conquests gave Asparukh more than a temporary settlement: his group of refugees, funded by Constantinople’s tribute payments, became the First Bulgarian Empire." [Bauer: Medieval World, p. 321-2]