...there is a thriving body of scholarship which demonstrates that slavery was practiced widely in various forms in Europe during the Middle Ages, alongside captivity, serfdom, and other types of unfreedom. Where then did the common knowledge come from? In the first instance, it derives from the late-18th- and 19th-century abolitionist assumption that as Christianity spread through Europe during the Middle Ages, it must surely have driven out slavery. [source]
Italy's was not the only economy that was boosted by the slave trade. Medieval Spain used slaves, especially under the Umayyads, and especially under the son of Hisham:
Al-Hakam was the first monarch of this family who surrounded his throne with a certain splendour and magnificence. He increased the number of mamelukes (slave soldiers) until they amounted to 5,000 horse and 1,000 foot. ... he increased the number of his slaves, eunuchs and servants; had a bodyguard of cavalry always stationed at the gate of his palace and surrounded his person with a guard of mamelukes...[Colins, Roger; Early Medieval Spain (1995)]
Spain, in turn, was raided for slaves. Al-Andalus in northern Spain was raided in the 9th and 10th centuries by Vikings. Slaves went out from Spain in other ways: Muslim and Jewish merchants exported slaves from Spain to other Islamic countries. Bishop Liutprand of Cremona wrote that Jewish merchants castrated slaves in order to create eunuchs, much in demand in Muslim Spain. He wasn't complaining, just reporting. In fact, on a visit to Constantinople he enjoyed the great hospitality he was shown, including being carried into the Emperor's presence on the shoulders of eunuchs who had their gentalia completely removed.
Speaking of the Vikings raiding Spain, we shall look at them next.
