Courtesy the tip from the Internet web site of Daniel Pipes and his article.
"Louvre Museum: Gabriel Martinez-Gros discusses military slavery"
MILITARY SLAVERY WITHIN THE CONTEXT OF THE ISLAMIC WORLD.
Military slavery a concept and practice particularly unique to the Islamic world?
Gabriel Martinez-Gros born in French Algeria and considered to be an authority on the Islamic world of al-Andalus [the Iberian peninsula].
Military slaves as preferred from among the people of the Caucasus mountains. Chechens, Circassians, etc.