The ʿAbbasids’ Turkish bodyguard at Baghdad [in the ninth and tenth centuries CE] had its counterpart, at the court of the ʿAbbasids’ Umayyad contemporaries and rivals at Cordova, in a bodyguard of European barbarians who were purchased by the Spanish Caliphs from their Frankish neighbours. The Franks supplied the Cordovan slave-markets by making slave-raids across the opposite frontier of the Frankish dominions [roughly the Iron Curtain]. The barbarians who were thus captured by the Franks in order to be sold to the Spanish Umayyads happened to be Slavs; and this is the origin of the word “slave” in the English language.
Were they already Christian? Did they all become Muslim in Spain? The Arabic word for their Slavic slaves was Saqaliba, a corruption of the Greek Sklavinoi, meaning Slavs. Their presence in the Islamic world was not confined to Spain.
Some Saqaliba became rulers of Muslim taifas after the collapse of the Caliphate of Cordoba (1031). Muyahid ibn Yusuf ibn Ali freed the Saqaliba of Dénia and established a taifa which extended its reach as far as Majorca.
Hitler conceived of Slavs as a slave population for an eastward-expanding German Reich.
A Study of History, Vol III, OUP, 1939 (footnote)