How true is the assertion that most slaves in history were, in fact, white? Period: antiquity, location: middle east.
I'm not exactly sure what the claim really is - either that numerically most slaves were caucasian, either the enslavement was the most prolonged, or as a % of a population, it was the most significant.
Is there any truth to any variant on this claim?
In her excellent work on the formation of "whiteness", Nell Irvin Painter's The History of White People dispels this myth by pointing out that:
1.)"whiteness" didn't really exist before the ~1400s
and
2.)"whiteness" has very little to do with actual skin tone and more to do with sociopolitical institutions. Consider that the first "white people" could include Africans who adopted European culture, or exclude people who today are firmly within the sphere of "Caucasian", like Germans, Poles, Swedes, Finns, French, etc.
So no, not really. Anyone claiming to definitively know any variation of OP's claim is, intentionally or not, espousing beliefs associated with white supremacy apologetics.
What's your definition of "Caucasian"?