In the middle Byzantine period, the khan of Khazaria in the northern Caucasus and his court converted to Judaism, partly in order to maintain neutrality between Christian Byzantium and the Islamic world... ...have recently (controversially) theorized that East-European Jews are more ethnically Khazar than they are Semitic.
It's better to be more specific.
https://en.m.wikipedia.org/wiki/Ashkenazi_Jews
https://en.m.wikipedia.org/wiki/Jewish_ethnic_divisions
https://en.m.wikipedia.org/wiki/Khazar_hypothesis_of_Ashkenazi_ancestry