My stand has always been that Palestinians and Israelis both have equal claim on the land both for historical and legal reasons and they had better just learn to get along.

As for the Khazars being white caucasians, no...they were a Turkic tribe that migrated into the region, just like all the other Turkic tribes North and South of the Caucuses. Their language is similar to the Bulgar language that was found both in what is now Bulgaria and near the Volga river. So you even have THAT wrong.

As to Koestler, he lived in Palestine during the 1920s, on a kibbutz. At the University of Vienna he was President of a Zionist student fraternity. He always supported Israeli statehood. He also was no expert on his subject. I have read the Thirteeth Tribe and liked it. But found it ultimately little more than an interesting theory. And to date there has been no real supporting evidence of that interesting theory.

As to genetic studies, they clearly show a dominant Middle Eastern origin of Jews, except for Ashkenazi Levites, who seem to have a German origin. Here are citations:

M. F. Hammer, et al, "Jewish and Middle Eastern non-Jewish populations share a common pool of Y-chromosome biallelic haplotypes." PNAS, vol. 97, 6769–6774, (2000)

Thomas, et al, Y Chromosomes Traveling South: The Cohen Modal Haplotype and the Origins of the Lemba—the “Black Jews of Southern Africa.” Am. J. Hum. Genet. 66 674–686, (2000)

Picornell, et al, "Jewish population genetic data in 20 polymorphic loci." Forensic Science International 125 52–58, (2002)

Behar, et al, "Multiple Origins of Ashkenazi Levites: Y Chromosome Evidence for Both Near Eastern and European Ancestries." American Journal of Human Genetics 73 768–779 (2003)

Behar, et al, "The Matrilineal Ancestry of Ashkenazi Jewry: Portrait of a Recent Founder Event". The American Journal of Human Genetics 78 (3): 487-97 (2006)

That's what I've read, anyway. There are probably more out there.