There is a somewhat widely held theory that the Khazars are the ancestors of the Jews of Northern and Eastern Europe, the so-called Ashkenazim
. This theory is demonstrably false.
The Khazar ancestry hypothesis has been a favorite of racial anti-Semites from the Nazi era to the present. It seeks to give the Jews of Europe a suspect ‘Asian’ racial origin. This fit with the Nazis’ desire to draw a racial divide themselves and the Jews that they had been living together with for 1500 years, and of late allows certain extreme Christian millenialists and others to believe that they are the ‘true’ descendants of ancient Israel, while those claiming to be Jews are ‘impostors.’
On the surface. the Khazar hypothesis does seem to solve the mystery of how the population of Ashkenazim got so big in the late middle ages from what must have been a rather small early medieval population. Plus there is the Russian connection. In recent centuries there have been a lot of Jews in Russia, and isn’t the Khazars’ territory now in Russia?
However, the theory doesn’t stand up to simple historical record and linguistic evidence. The language of the Ashkenazi people was Yiddish, which is a Germanic language augmented with Hebrew and Latin vocabulary. There are absolutely no Turkic or Caucasian elements in it.
The migration of Jews to Eastern Europe is a matter of historical record, and took place in the 16th and 17th centuries, as the emerging monarchies of Poland-Lithuania and Czarist Russia needed artisans and traders, and offered the Jews of western Europe relatively pogrom-free land. There is no historical record of mass movement of Jews into Eastern Europe from the East, and no record of substantial Jewish communities in Eastern Europe before this era. If the Khazars migrated to Eastern Europe, what happened to them between the 9th and 16th centuries?
Also one can just look at the Ashkenazim, a people with a wide variety of physical appearances, but one visibly without the influence of Central Asian horsemen.