Re: Are most Jews really Jews?
Some more detail:
The theory that Ashkenazi Jews might be descended from Khazars dates back to the late nineteenth century, and is frequently cited to claim modern Jews are not Israelites and/or to question Israeli claims to territory also sought by Palestinians. It was first publicly proposed in lecture given by Ernest Renan on January 27, 1883, titled "Judaism as a Race and as Religion." It was popularized by racial theorist Lothrop Stoddard in a 1926 article in the Forum titled "The Pedigree of Judah", where he argued that Ashkenazi Jews were a mix of people, of which the Khazars were a primary element. Stoddard's views were "based on nineteenth and twentieth-century concepts of race, in which small variations on facial features as well as presumed accompanying character traits were deemed to pass from generation to generation, subject only to the corrupting effects of marriage with members of other groups, the result of which would lower the superior stock without raising the inferior partners." This theory was adopted by British Israelites, who saw it as a means of invalidating the claims of Jews (rather than themselves) to be the true descendants of the ancient Israelites, and was supported by early anti-Zionists.
In 1951 Southern Methodist University professor John O. Beaty published The Iron Curtain over America, a work which claimed that "Khazar Jews" were "responsible for all of America's - and the world's - ills beginning with World War I". The book repeated a number of familiar antisemitic claims, placing responsibility for U.S. involvement in World Wars I and II and the Bolshevik revolution on these Khazars, and insisting that Khazar Jews were attempting to subvert Western Christianity and establish communism throughout the world. The American millionaire J. Russell Maguire gave money towards its promotion, and it was met with enthusiasm by hate groups and the extreme right. By the 1960s the Khazar theory had become a "firm article of faith" amongst Christian Identity groups. In 1971 Glubb Pasha also took up this theme, insisting that Palestinians were more closely related to the ancient Judeans than were Jews.
The theory gained further support when the novelist Arthur Koestler devoted his popular book The Thirteenth Tribe (1976) to the topic. ,,,, His discussion of theories about Ashkenazi descent is largely unsupported; to the extent that Koestler referred to place-names and documentary evidence his analysis has been described as a mixture of flawed etymologies and misinterpreted primary sources. Commentors have also noted that Koestler mischaracterized the sources he cited, particularly D.M. Dunlop's History of the Jewish Khazars (1954).
In the 1970s and 80s the Khazar theory was also advanced used by some Russian chauvinist antisemites, particularly the historian Lev Gumilyov, who portrayed "Judeo-Khazars" as having repeatedly sabotaged Russia's development since the 7th century.
According to Bernard Lewis:
This theory… is supported by no evidence whatsoever. It has long since been abandoned by all serious scholars in the field, including those in Arab countries, where the Khazar theory is little used except in occasional political polemics.