In 1934 he received his MA in "Culture of Islam"[4] and went on to write his thesis on the subject of the History of Land Relationships in Egypt, Syria and Israel during the late Middle Ages and in Modern Times, under the guidance of Professor Leo Aryeh Mayer.
[9] Up until then, the culture of the Khazars was outside the scope of Polak's research but evidently, this article led him to delve deeper into this topic and in 1943 he published his book Khazaria: History of a Jewish Kingdom in Europe.
[10] In his book, Polak asserted that the Khazars were the demographic foundation of the Jews who inhabited Eastern Europe in the late Middle Ages and who later evolved into the Ashkenazi Jewry.
His superiors described Polak in various evaluations as being "...highly educated and with vast knowledge..." (1952), "an extraordinary man with special skills and talents" (1958), "...a remarkable person in many aspects and one who can not be defined in conventional terms, he possesses a deep intellect and amazing analytical capabilities" (1959).
Late in his life, Polak was to refer to the issue of the Khazars only once again, when in 1969 he gave a speech on that topic at the World Congress of Jewish Studies in Jerusalem.
His book Khazaria: History of a Jewish Kingdom in Europe was groundbreaking, not only due to the comprehensive approach to the issue of the Khazars, but specifically because for the first time, Islamic sources were used.
[13] In his book, Polak presented two important, albeit controversial assumptions - the first considered a non-Middle-Eastern origin for most of the Eastern-European Jews; the second, saw the birth of the Yiddish language in Crimea and not in Germany, as was previously believed.
It addresses the issue which everyone knows had a major influence on the formation of Polish and Eastern-European Jewry, but not many took the trouble to assemble the few surviving sources, to pursue them for many years until, behind the fog of myths and forgeries the true character of the Khazars' Kingdom, comes into light.
The book itself, the bibliography attached to it, prove the extent of undertaking by the author, who did not neglect any source, and with a unique sense of history knew to find the criterion in which to distinguish between truth and a lie, between fiction and fact, between tradition and folklore".
[25] Even Salo Wittmayer Baron and Ben-Zion Dinur, two of the leading Jewish historians in those years, accepted Polak's fundamental assumptions.