Novelist and writer Arthur Koestler maintained that only Sephardi Jews were related to the Old Testament Jews, since most of the Jews in the Middle East were exterminated.
Ashkenazi Jews are only Jews by conversion, he claimed.
Seemed reasonable until DNA became widely available.
With the rise of Islam, most Jews in Islamic territory were separated from most Jews in Christian territory, with the expected result that various customs and family lines became increasingly separated.
There were even smaller groups that developed mostly isolated from both, such as Krymchaks.
Koestler claimed that Ashkenazi Jews were descended from the Kazars - a Turkic people. The Kazars had an 7th - 10th century empire in what is today European Russia, southern Ukraine, Crimea, and Kazakhstan. They interestingly enough converted in mass to Judaism. They felt it was a middle ground between Islam and Christianity. They were in imperial rivalry between the Byzantine Empire and various Muslim Caliphates It has been claimed that Ashkenazi Jews are the remnant of these people. DNA testing so far says no!
Koestler was a NOVELIST and 99& of those who subscribe to his stupid Khazar theory also advocate for the destruction of Jewish Israel.