Conditions during the 8th century prompted a growing shift in the Northern lands from farming and fishing, to raiding. We start to see “Viking” raids in the Irish, English and even Spanish records at the end of the 8th century.
Emir Hakam, grandson of Abd al-Rahman I, created an African slave army to insulate his power base from the rival powers around him. The use of slaves in al-Andalus grew during the next centuries, and eventually, at the end of the Middle Ages, the use of African slaves on sugar plantations came out of Portuguese and Spanish practice. But during the early Middle Ages, slavery was not at all about race.
Vikings raided remote Irish and English ports and monasteries, and they also acted as pirates on the open sea. They chained up able-bodied men and women, then took them off to the slave-trading markets. Some of these markets were along European rivers, where inland and sea-faring goods met. Since Vikings traveled on shallow-keeled ships, they could easily go up rivers to Cologne, Verdun or Mainz. They also carried their northern slaves south to al-Andalus. They took more slaves in unguarded spots along the southern coast and traded them at Italian ports.
Meanwhile, other slavers were capturing Slavs from all along the Baltic coast and inland from the Mediterranean. Eventually, the Latin-speaking town of Ragusa (now Slavic-speaking Dubrovnik) became a big slave market. The Slavs were easy prey because it took a long time before they had strong political organization. Slavs served in the cities of Italy, but many of them crossed the Sea to al-Andalus, where the Emirs were always looking to buy. Just as the Emirs didn’t pay attention to race in their dynastic marriages, neither did they care about it in building their armies.
Early Medieval Spain, Roger Collins.