The Jewish Kingdom of Khazaria

Khazars were a Central Asian people of Turkic, Hunnish and Iranian elements that arose in the Caucasus region. After converting to Judaism in the early Middle Ages, they moved westward into Russia and the Ukraine under pressure from Islam, eventually becoming a large component of Eastern and Central European Jewry. Many Ashkenazi Jews now find they have some Khazar (or intermingled Tatar) ancestry.

The Jews of Khazaria by Kevin Alan Brook, chronicles the history of the Khazars, a people who, in the early Middle Ages, founded a large empire in eastern Europe (located in present-day Ukraine and Russia).

The kingdom of Khazaria was one of the largest-sized political formations of its time, an economic and cultural superpower connected to several important trade routes.
Khazaria was especially notable for its religious tolerance, and in the 9th century, a large portion of the royal family converted to Judaism. Many of the nobles and commoners did likewise shortly thereafter.
After their conversion, the Khazars were ruled by a succession of Jewish Kings that began to adopt the hallmarks of Jewish civilization, including the Torah and Talmud, the Hebrew script, and the observance of Jewish holidays.
At its height, the Khazar state and its tributaries controlled much of what is now southern Russia, western Kazakhstan, eastern Ukraine, Azerbaijan and large parts of Russia's North Caucasus region.
Medieval Kingdom of Khazaria ( 652 CE -1016 CE )

Over a thousand years ago, the far east of Europe was ruled by Jewish kings who presided over numerous tribes, including their own tribe: the Turkic Khazars. After their conversion, the Khazar people used Jewish personal names, spoke and wrote in Hebrew, were circumcised, had synagogues and rabbis, studied the Torah and Talmud, and observed Hanukkah, Pesach, and the Sabbath. The Khazars were an advanced civilization with one of the most tolerant societies of the medieval period. It hosted merchants from all over Asia and Europe.

The fate of the Jewish Khazars is unclear. Jewish travellers of the 12th century continue to refer to them in passing. Khazar Jews are known to have lived in Kiev and even to have emigrated to Spain, the Byzantine Empire and Iraq.

According to some sources the majority may have gone to Hungary, Poland and the Crimea, mingling with Jews in those areas and with later waves of Jewish immigrants from the west.

Khazars - Wikipedia
List of Khazar Rulers - Wikipedia
The Jews of Khazaria Book
The Thirteenth Tribe, by Arthur Koestler
Three Khazar Tribes (they were the Kabars) Joined with the Seven Magyar Tribes and conquered the Carpathian Basin.
The link between the Khazars and Ashkenazi Jews has been controversial. According to the latest DNA study: "Our findings support the Khazarian Hypothesis and portray the European Jewish genome as a mosaic of Caucasus, European, and Semitic ancestries, thereby consolidating previous contradictory reports of Jewish ancestry."

Eran Israeli-Elhaik, The Missing Link of Jewish European Ancestry: Contrasting the Rhineland and the Khazarian Hypotheses (2012)