In the eighth and ninth centuries, the south branches of East Slavic tribes had to pay tribute to the Khazars, a Turkic-speaking people who adopted Judaism in the late eighth or ninth century and lived in the southern Volga and Caucasus regions.
Among Shepard's theories is that the breakdown in Byzantine-Khazar relations and the shift in Byzantine foreign policy towards allying with the Pechenegs and the Rus against Khazaria was a result of the Khazar conversion to Judaism.