High Quality Content by WIKIPEDIA articles! The Thirteenth Tribe (1976) is a book by Arthur Koestler, which advances the thesis that Ashkenazi Jews are not descended from the historical Israelites of antiquity, but from Khazars, a Turkic people. Koestler`s hypothesis is that the Khazars converted to Judaism in the 8th century, and migrated westwards into Eastern Europe in the 12th and 13th centuries when the Khazar Empire was collapsing.