The Thirteenth Tribe is a 1976 book by Arthur Koestler, in which the author advances the thesis that Ashkenazi Jews are not descended from the historical Israelites of antiquity, but from Khazars, a Turkic people. Koestler hypothesized that the Khazars (who may have converted to Judaism in the 8th century) migrated westwards into Eastern Europe in the 12th and 13th centuries when the Khazar Empire was collapsing. Koestler used previous works by Douglas Morton Dunlop, Raphael Patai and Abraham Polak as sources. His stated intent was to make antisemitism disappear by disproving its racial basis. Popular reviews of the book were mixed, academic critiques of its research were generally negative, and Koestler biographers David Cesarani and Michael Scammell panned it. Summary Contents Koestler advances the thesis that Ashkenazi Jews are not descended from the historical Israelites of antiquity, but from Khazars, a Turkic people originating in and populating an empire north of and between the Black Sea and Caspian Sea. Koestler's hypothesis is that the Khazars – who converted to Judaism in the 8th century – migrated westwards into current Eastern Europe (primarily Ukraine, Poland, Belarus, Lithuania, Hungary and Germany) in the 12th and 13th centuries when the Khazar Empire was collapsing. At the end of the book's last chapter, Koestler summarizes its content and his intentions as follows: "In Part One of this book I have attempted to trace the history of the Khazar Empire based on the scant existing sources. In Part Two, Chapters V-VII, I have compiled the historical evidence which indicates that the bulk of Eastern Jewry — and hence of world Jewry — is of Khazar-Turkish, rather than Semitic, origin. In the last chapter I have tried to show that the evidence from anthropology concurs with history in refuting the popular belief in a Jewish race descended from the biblical tribe." Sources Mattias Gardell writes that Koestler's thesis is "partly based on amateur anthropology", and its scientific arguments come from The Myth of a Jewish Race (1975) by Raphael Patai and his daughter Jennifer. It also relies on the work of earlier historians, particularly Russian-Israeli historian Abraham Poliak's Hebrew book Khazaria: Toledot mamlakhah yehudit (1951), and the History of the Jewish Khazars (1954) by Douglas Morton Dunlop, the author whom Koestler himself describes as a main source. Neil McInnes writes that Dunlop was, however, "much more tentative" in his conclusions, as were other historians of