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.
Naval warfare
In the 8th century the Vikings appeared, although their usual style was to appear quickly, plunder, and disappear, preferably attacking undefended locations. The Vikings raided places along the coastline of England and France, with the greatest threats being in England. They would raid monasteries for their wealth and lack of formidable defenders. They also utilized rivers and other auxiliary waterways to work their way inland in the eventual invasion of Britain. They wreaked havoc in Northumbria and Mercia and the rest of Anglia before being halted by Wessex. King Alfred the Great of England was able to stay the Viking invasions with a pivotal victory at the Battle of Edington. Alfred defeated Guthrum, establishing the boundaries of Danelaw in an 884 treaty. The effectiveness of Alfred's 'fleet' has been debated; Dr. Kenneth Harl has pointed out that as few as eleven ships were sent to combat the Vikings, only two of which were not beaten back or captured.