In 19th century anthropology - not exactly the most enlightened field of study - it was a popular notion that Ashkenazi Judaism had its roots in the Khazar king

In 19th century anthropology - not exactly the most enlightened field of study - it was a popular notion that Ashkenazi Judaism had its roots in the Khazar kingdoms of this area, which themselves had deep Jewish roots. While that particular notion has aged out, it's indisputable that the Khazar kingdoms had Jewish kings and warriors among them, that they had familial roots reaching deep into antiquity, and that their numbers swelled greatly as the rift grew between Byzantium and its resident Jews. Indeed, this dynamic - Western Greeks and Eastern Jews - was to be instrumental in the story of Islam, a story perhaps for later.