By EVAN R. GOLDSTEIN

This much is known: In the mid-eighth century, the ruling elite of the Khazars, a Turkic tribe in Eurasia, converted to Judaism. Their impetus was political, not spiritual. By embracing Judaism, the Khazars were able to maintain their independence from rival monotheistic states, the Muslim caliphate and the Christian Byzantine empire. Governed by a version of rabbinical law, the Khazar Jewish kingdom flourished along the Volga basin until the beginning of the second millennium, at which point it dissolved, leaving behind a mystery: Did the Khazar converts to Judaism remain Jews, and, if so, what became of them?

Enter Shlomo Sand. In a new book, “The Invention of the Jewish People,” the Tel Aviv University professor of history argues that large numbers of Khazar Jews migrated westward into Ukraine, Poland and Lithuania, where they played a decisive role in the establishment of Eastern European Jewry.

The implications are far-reaching: If the bulk of Eastern European Jews are the descendents of Khazars—not the ancient Israelites—then most Jews have no ancestral links to Palestine. Put differently: If most Jews are not Semites, then what justification is there for a Jewish state in the Middle East? By attempting to demonstrate the Khazar origins of Eastern European Jewry, Mr. Sand—a self-described post-Zionist who believes that Israel needs to shed its Jewish identity to become a democracy—aims to undermine the idea of a Jewish state.

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