Answer:
   
   The Khazars were a Turkic tribe that migrated to the steppes of what
   is today southern Russia and eastern Ukraine by the 5th century. They
   established a powerful kingdom that existed from the mid-7th century
   until the early-11th century. The Khazars had a two-king system,
   consisting of a military king (bek) and a sacral king (khaqan). The
   Khazar army, which took orders from the bek and the military commander
   (tarkhan), included tens of thousands of professional soldiers.
   
   The Khazars were a potent military force in eastern Europe till about
   the middle of the 11th century, their last power base being the
   Crimean peninsula. In the 7th and 8th centuries, they defeated the
   Eastern Caliphate in several key battles, thus halting the spread of
   Islam north of the Caucasus mountain range, much the same as what the
   Carolingian rulers did to the Western Caliphate at the Pyrenees.
   (Ironically, these Jewish converts made Eastern Europe safe for
   Christianity.) The Khazars gained control over major waterways such as
   the Caspian Sea, the Volga River, and the Dnieper River. The Khazar
   kings collected tribute from many of the East Slavic tribes as well as
   from traders traversing their country. Large garrisons were stationed
   at hill-forts located at strategic points throughout the kingdom
   (e.g., Kiev by the Dnieper, Sarkel by the Don, Samandar by the
   Caspian) to guard against enemy invaders such as the Rus.
   
   The king of the Khazars learned the Torah with the assistance of the
   Jewish preacher Isaac Sangari, whose existence has recently been
   verified (by the discovery of poems authored by Sangari in the
   Firkovitch collection of manuscripts). In the 9th century, the
   Khazarian kings and nobles officially converted to Judaism. Surrounded
   by the Islamic Eastern Caliphate of Persia and the Christian Byzantine
   Empire, the Khazars may have chosen Judaism as their state religion to
   avoid being religiously (and hence politically) dominated by either
   empire, so that they could avoid being labelled as heathens while at
   the same time remaining independent of their powerful neighbors. By
   the start of the 10th century, Judaism gained a stronghold among the
   common Khazar people, and the Hebrew script came into use in Khazaria.
   However, most of the soldiers in the Khazar army were Muslims, and the
   non-Khazar ethnic groups within the Khazar Empire (such as the Slavs,
   Bulgars, and Goths) did not adopt Judaism but rather remained pagans,
   Muslims, and Christians.
   
   Arab travelogues provide useful contemporary details about the life of
   the Khazars. Armenian, Slavic, and Hebrew sources also form the core
   of our knowledge about the Khazar people. Important Hebrew primary
   sources are:
    1. The Khazar Correspondence between Khaqan Joseph and Hasdai ibn
       Shaprut of Spain, now known to be authentic.
    2. The Schechter Letter, found in the Cairo Genizah, an account of
       the conversion of Khazars to Judaism, the migration of Jews to
       Khazaria, and the military victories of the Khazars.
    3. The Kievan Letter, found in the Cairo Genizah, written by the
       Khazar Jews of Kiev in the early 10th century.
       
   Within the past few decades, archaeological excavations in Russia and
   Ukraine have unearthed Khazar jewelry, pottery, gravesites, and
   tombstones containing engraved menorahs and Turkic tribe symbols. One
   of the most famous sites was Sarkel, which in 1952 was flooded for a
   dam by the Soviet government and is not available for further
   research. Other major Khazarian archaeological sites include Verkhneye
   Chiryurt (Balanjar, in Daghestan), Verkhneye Saltovo and Mayaki
   hill-fort (near the Don and Donets rivers), and Kerch and Sudak (on
   the Crimea). For several years, archaeologists have been trying to
   locate the precise site of the Khazar capital of Itil; some believe
   the wall which surrounded Itil has been found underwater, while others
   associate Itil with a hill in the Volga delta region called Samosdelka
   (south of Astrakhan).
   
   Secondary sources include:
     * The Kuzari by Yehuda HaLevi, a 12th century religious work using
       the story of the Khazars as justification for Judaism in the face
       of intense missionary pressure especially in Spain. The Kuzari was
       originally written in Arabic, but many excellent Hebrew and
       English translations have been published.
     * "The History of the Jewish Khazars" by Douglas M. Dunlop (New
       York: Schocken Books, 1967).
     * "The Thirteenth Tribe: The Khazar Empire and Its Heritage" by
       Arthur Koestler (New York: Random House, 1976).
     * "Khazar Studies: An Historico-Philological Inquiry into the
       Origins of the Khazars" by Peter Golden (Budapest: Akademiai
       Kiado, 1980).
     * "Khazarian Hebrew Documents of the Tenth Century" by Omeljan
       Pritsak (Ithaca, NY: Cornell Univ. Press, 1982).
     * "The Jews of Khazaria" by Kevin A. Brook (Northvale, NJ: Jason
       Aronson, 1999).
       
   Are Ashkenazi Jews descended from the Khazars? Some believe that they
   are, at least to a certain extent. An important Khazar community
   remained in Kiev, and family oral traditions indicate the persistence
   of Khazar Jewish communities in Hungary, Transylvania, Lithuania, and
   central Ukraine. Some Jews have features that might be considered
   almost Mongolian or Oriental. However, there is no remnant of Khazar
   custom among Ashkenazi Jews, and there are only a few Ashkenazi
   surnames (e.g., Balaban) that derive from Turkic. It is sometimes
   suggested that the surname Kogan derives from Khaqan, but the more
   likely derivation is from Kohen (meaning "Israelite priest"); the
   Ukrainians and Belarusians use the letter h, but in Russian h becomes
   g, as may be seen in such examples as Grodno-Hrodna and Girsch-Hirsch.
   
   It seems that after the fall of their kingdom, the Khazars adopted the
   Cyrillic script in place of Hebrew and began to speak East Slavic
   (sometimes called "Canaanic" because Benjamin of Tudela called Kievan
   Rus the "Land of Canaan"). These Slavic-speaking Jews are documented
   to have lived in Kievan Rus during the 11th-13th centuries. However,
   Yiddish-speaking Jewish immigrants from the west (especially Germany,
   Bohemia, and other areas of Central Europe) soon began to flood into
   Eastern Europe, and it is believed that these newer immigrants
   eventually outnumbered the Khazars. Thus, Eastern European Jews
   predominantly have ancestors who came from Central Europe rather than
   from the Khazar kingdom. The two groups (eastern and western Jews)
   intermarried over the centuries.
   
   The Ashkenazi Jews are also the direct descendants of the Israelites.
   Genetic tests seem to indicate some ancestry from the regions known
   today as Turkey, Armenia, Georgia, Iran, and Iraq. Mediterranean
   Fever, for example, is found among some Ashkenazi Jews as well as
   Armenians and Anatolian Turks. It is now asserted that many Ashkenazi
   men who belong to the priestly caste (Kohenim) possess a "Kohen"
   marker on the Y-chromosome. However, note that this provides no
   evidence of Khazar ancestry. Common genetic markers in people from
   these regions is expected for the following reasons, which alone could
   account for the common markers occurring in some Jews as well as
   non-Jews in Turkey, Armenia, Georgia, Iran, and Iraq:
    1. Archaeological evidence suggests that the some of the earliest
       ancestors of the ancient Levantine and Mesopotamian civilzations
       originated in the region of Armenia and moved southwards.
    2. The Tanach records extensive evidence of intermarriage between
       Jews and ancient peoples who originated in eastern Anatolia, viz.
       the Hittites and Hurrians (including the Jebusites of Jerusalem).
       The Edomites who were of mixed Hebrew and Hurrian ancestry were
       also absorbed into the Jewish people.
    3. The Armenians and Kurds are the descendants of people who remained
       in Eastern Anatolia / Armenia / Kurdistan and intermarried with
       the Turks and neighbouring peoples.
       
   Some descendants of the Khazars may still live in the north Caucasus
   among the Kumuks and the Balkars. These descendents include Crimean
   Jews called Krymchaks and Mountain Jews (a mix of Khazars and Iranian
   Caucasian Jews). Many Muslim Khazars settled in Azerbaijan and
   Kazakhstan and may have intermarried with Oghuz and Kipchak Turks.
   
   If you are interested in the subject of Khazar Jews, you can visit the
   Khazaria Information Center at <[5]http://www.khazaria.com>.