Interracial marriage
From AD 839, Viking Varangian mercenaries who were in the service of the Byzantine Empire, notably Harald Sigurdsson, campaigned in North Africa, Jerusalem and other places in the Middle East during the Byzantine-Arab Wars. They interbred with the local population as spoils of warfare or through eventual settling with many Scandinavian Viking men taking Arab or Anatolian women as wives. There is archaeological evidence the Vikings had established contact with the city of Baghdad, at the time the center of the Islamic Empire, and connected with the populace there. Regularly plying the Volga with their trade goods (furs, tusks, seal fat, seal boats and notably female slaves; the one period in the history of the slave-trade when females were priced higher than males), the Vikings were active in the Arab slave trade at the time. These slaves, most often Europeans that were captured from the coasts of Europe or during war periods, and sold to Arabic traders in Al-Andalus and the Emirate of Sicily.
Medieval warfare
The Vikings were a feared force in Europe because of their savagery and speed of their attacks. Whilst seaborne raids were nothing new at the time, the Vikings refined the practice to a science through their shipbuilding, tactics and training. Unlike other raiders, the Vikings made a lasting impact on the face of Europe. During the Viking age, their expeditions, frequently combining raiding and trading, penetrated most of the old Frankish empire, the British Isles, the Baltic region, Russia, and both Muslim and Christian Iberia. Many served as mercenaries, and the famed Varangian Guard, serving the Emperor of Constantinople, was drawn principally of Scandinavian warriors.